# Theory Further_Topology

theory Further_Topology
imports Weierstrass_Theorems Polytope Complex_Transcendental
```section ‹Extending Continous Maps, Invariance of Domain, etc›

text‹Ported from HOL Light (moretop.ml) by L C Paulson›

theory Further_Topology
imports Equivalence_Lebesgue_Henstock_Integration Weierstrass_Theorems Polytope Complex_Transcendental
begin

subsection‹A map from a sphere to a higher dimensional sphere is nullhomotopic›

lemma spheremap_lemma1:
fixes f :: "'a::euclidean_space ⇒ 'a::euclidean_space"
assumes "subspace S" "subspace T" and dimST: "dim S < dim T"
and "S ⊆ T"
and diff_f: "f differentiable_on sphere 0 1 ∩ S"
shows "f ` (sphere 0 1 ∩ S) ≠ sphere 0 1 ∩ T"
proof
assume fim: "f ` (sphere 0 1 ∩ S) = sphere 0 1 ∩ T"
have inS: "⋀x. ⟦x ∈ S; x ≠ 0⟧ ⟹ (x /⇩R norm x) ∈ S"
using subspace_mul ‹subspace S› by blast
have subS01: "(λx. x /⇩R norm x) ` (S - {0}) ⊆ sphere 0 1 ∩ S"
using ‹subspace S› subspace_mul by fastforce
then have diff_f': "f differentiable_on (λx. x /⇩R norm x) ` (S - {0})"
by (rule differentiable_on_subset [OF diff_f])
define g where "g ≡ λx. norm x *⇩R f(inverse(norm x) *⇩R x)"
have gdiff: "g differentiable_on S - {0}"
unfolding g_def
by (rule diff_f' derivative_intros differentiable_on_compose [where f=f] | force)+
have geq: "g ` (S - {0}) = T - {0}"
proof
have "g ` (S - {0}) ⊆ T"
apply (auto simp: g_def subspace_mul [OF ‹subspace T›])
apply (metis (mono_tags, lifting) DiffI subS01 subspace_mul [OF ‹subspace T›] fim image_subset_iff inf_le2 singletonD)
done
moreover have "g ` (S - {0}) ⊆ UNIV - {0}"
proof (clarsimp simp: g_def)
fix y
assume "y ∈ S" and f0: "f (y /⇩R norm y) = 0"
then have "y ≠ 0 ⟹ y /⇩R norm y ∈ sphere 0 1 ∩ S"
by (auto simp: subspace_mul [OF ‹subspace S›])
then show "y = 0"
by (metis fim f0 Int_iff image_iff mem_sphere_0 norm_eq_zero zero_neq_one)
qed
ultimately show "g ` (S - {0}) ⊆ T - {0}"
by auto
next
have *: "sphere 0 1 ∩ T ⊆ f ` (sphere 0 1 ∩ S)"
using fim by (simp add: image_subset_iff)
have "x ∈ (λx. norm x *⇩R f (x /⇩R norm x)) ` (S - {0})"
if "x ∈ T" "x ≠ 0" for x
proof -
have "x /⇩R norm x ∈ T"
using ‹subspace T› subspace_mul that by blast
then show ?thesis
using * [THEN subsetD, of "x /⇩R norm x"] that apply clarsimp
apply (rule_tac x="norm x *⇩R xa" in image_eqI, simp)
apply (metis norm_eq_zero right_inverse scaleR_one scaleR_scaleR)
using ‹subspace S› subspace_mul apply force
done
qed
then have "T - {0} ⊆ (λx. norm x *⇩R f (x /⇩R norm x)) ` (S - {0})"
by force
then show "T - {0} ⊆ g ` (S - {0})"
qed
define T' where "T' ≡ {y. ∀x ∈ T. orthogonal x y}"
have "subspace T'"
have dim_eq: "dim T' + dim T = DIM('a)"
using dim_subspace_orthogonal_to_vectors [of T UNIV] ‹subspace T›
have "∃v1 v2. v1 ∈ span T ∧ (∀w ∈ span T. orthogonal v2 w) ∧ x = v1 + v2" for x
by (force intro: orthogonal_subspace_decomp_exists [of T x])
then obtain p1 p2 where p1span: "p1 x ∈ span T"
and "⋀w. w ∈ span T ⟹ orthogonal (p2 x) w"
and eq: "p1 x + p2 x = x" for x
by metis
then have p1: "⋀z. p1 z ∈ T" and ortho: "⋀w. w ∈ T ⟹ orthogonal (p2 x) w" for x
using span_eq_iff ‹subspace T› by blast+
then have p2: "⋀z. p2 z ∈ T'"
have p12_eq: "⋀x y. ⟦x ∈ T; y ∈ T'⟧ ⟹ p1(x + y) = x ∧ p2(x + y) = y"
proof (rule orthogonal_subspace_decomp_unique [OF eq p1span, where T=T'])
show "⋀x y. ⟦x ∈ T; y ∈ T'⟧ ⟹ p2 (x + y) ∈ span T'"
using span_eq_iff p2 ‹subspace T'› by blast
show "⋀a b. ⟦a ∈ T; b ∈ T'⟧ ⟹ orthogonal a b"
using T'_def by blast
qed (auto simp: span_base)
then have "⋀c x. p1 (c *⇩R x) = c *⇩R p1 x ∧ p2 (c *⇩R x) = c *⇩R p2 x"
proof -
fix c :: real and x :: 'a
have f1: "c *⇩R x = c *⇩R p1 x + c *⇩R p2 x"
by (metis eq pth_6)
have f2: "c *⇩R p2 x ∈ T'"
by (simp add: ‹subspace T'› p2 subspace_scale)
have "c *⇩R p1 x ∈ T"
by (metis (full_types) assms(2) p1span span_eq_iff subspace_scale)
then show "p1 (c *⇩R x) = c *⇩R p1 x ∧ p2 (c *⇩R x) = c *⇩R p2 x"
using f2 f1 p12_eq by presburger
qed
moreover have lin_add: "⋀x y. p1 (x + y) = p1 x + p1 y ∧ p2 (x + y) = p2 x + p2 y"
proof (rule orthogonal_subspace_decomp_unique [OF _ p1span, where T=T'])
show "⋀x y. p1 (x + y) + p2 (x + y) = p1 x + p1 y + (p2 x + p2 y)"
show  "⋀a b. ⟦a ∈ T; b ∈ T'⟧ ⟹ orthogonal a b"
using T'_def by blast
qed (auto simp: p1span p2 span_base span_add)
ultimately have "linear p1" "linear p2"
by unfold_locales auto
have "(λz. g (p1 z)) differentiable_on {x + y |x y. x ∈ S - {0} ∧ y ∈ T'}"
apply (rule differentiable_on_compose [where f=g])
apply (rule linear_imp_differentiable_on [OF ‹linear p1›])
apply (rule differentiable_on_subset [OF gdiff])
using p12_eq ‹S ⊆ T› apply auto
done
then have diff: "(λx. g (p1 x) + p2 x) differentiable_on {x + y |x y. x ∈ S - {0} ∧ y ∈ T'}"
by (intro derivative_intros linear_imp_differentiable_on [OF ‹linear p2›])
have "dim {x + y |x y. x ∈ S - {0} ∧ y ∈ T'} ≤ dim {x + y |x y. x ∈ S  ∧ y ∈ T'}"
by (blast intro: dim_subset)
also have "... = dim S + dim T' - dim (S ∩ T')"
using dim_sums_Int [OF ‹subspace S› ‹subspace T'›]
also have "... < DIM('a)"
using dimST dim_eq by auto
finally have neg: "negligible {x + y |x y. x ∈ S - {0} ∧ y ∈ T'}"
by (rule negligible_lowdim)
have "negligible ((λx. g (p1 x) + p2 x) ` {x + y |x y. x ∈ S - {0} ∧ y ∈ T'})"
by (rule negligible_differentiable_image_negligible [OF order_refl neg diff])
then have "negligible {x + y |x y. x ∈ g ` (S - {0}) ∧ y ∈ T'}"
proof (rule negligible_subset)
have "⟦t' ∈ T'; s ∈ S; s ≠ 0⟧
⟹ g s + t' ∈ (λx. g (p1 x) + p2 x) `
{x + t' |x t'. x ∈ S ∧ x ≠ 0 ∧ t' ∈ T'}" for t' s
apply (rule_tac x="s + t'" in image_eqI)
using ‹S ⊆ T› p12_eq by auto
then show "{x + y |x y. x ∈ g ` (S - {0}) ∧ y ∈ T'}
⊆ (λx. g (p1 x) + p2 x) ` {x + y |x y. x ∈ S - {0} ∧ y ∈ T'}"
by auto
qed
moreover have "- T' ⊆ {x + y |x y. x ∈ g ` (S - {0}) ∧ y ∈ T'}"
proof clarsimp
fix z assume "z ∉ T'"
show "∃x y. z = x + y ∧ x ∈ g ` (S - {0}) ∧ y ∈ T'"
apply (rule_tac x="p1 z" in exI)
apply (rule_tac x="p2 z" in exI)
apply (simp add: p1 eq p2 geq)
by (metis ‹z ∉ T'› add.left_neutral eq p2)
qed
ultimately have "negligible (-T')"
using negligible_subset by blast
moreover have "negligible T'"
using negligible_lowdim
ultimately have  "negligible (-T' ∪ T')"
by (metis negligible_Un_eq)
then show False
using negligible_Un_eq non_negligible_UNIV by simp
qed

lemma spheremap_lemma2:
fixes f :: "'a::euclidean_space ⇒ 'a::euclidean_space"
assumes ST: "subspace S" "subspace T" "dim S < dim T"
and "S ⊆ T"
and contf: "continuous_on (sphere 0 1 ∩ S) f"
and fim: "f ` (sphere 0 1 ∩ S) ⊆ sphere 0 1 ∩ T"
shows "∃c. homotopic_with (λx. True) (sphere 0 1 ∩ S) (sphere 0 1 ∩ T) f (λx. c)"
proof -
have [simp]: "⋀x. ⟦norm x = 1; x ∈ S⟧ ⟹ norm (f x) = 1"
using fim by (simp add: image_subset_iff)
have "compact (sphere 0 1 ∩ S)"
by (simp add: ‹subspace S› closed_subspace compact_Int_closed)
then obtain g where pfg: "polynomial_function g" and gim: "g ` (sphere 0 1 ∩ S) ⊆ T"
and g12: "⋀x. x ∈ sphere 0 1 ∩ S ⟹ norm(f x - g x) < 1/2"
apply (rule Stone_Weierstrass_polynomial_function_subspace [OF _ contf _ ‹subspace T›, of "1/2"])
using fim apply auto
done
have gnz: "g x ≠ 0" if "x ∈ sphere 0 1 ∩ S" for x
proof -
have "norm (f x) = 1"
using fim that by (simp add: image_subset_iff)
then show ?thesis
using g12 [OF that] by auto
qed
have diffg: "g differentiable_on sphere 0 1 ∩ S"
by (metis pfg differentiable_on_polynomial_function)
define h where "h ≡ λx. inverse(norm(g x)) *⇩R g x"
have h: "x ∈ sphere 0 1 ∩ S ⟹ h x ∈ sphere 0 1 ∩ T" for x
unfolding h_def
using gnz [of x]
by (auto simp: subspace_mul [OF ‹subspace T›] subsetD [OF gim])
have diffh: "h differentiable_on sphere 0 1 ∩ S"
unfolding h_def
apply (intro derivative_intros diffg differentiable_on_compose [OF diffg])
using gnz apply auto
done
have homfg: "homotopic_with (λz. True) (sphere 0 1 ∩ S) (T - {0}) f g"
proof (rule homotopic_with_linear [OF contf])
show "continuous_on (sphere 0 1 ∩ S) g"
using pfg by (simp add: differentiable_imp_continuous_on diffg)
next
have non0fg: "0 ∉ closed_segment (f x) (g x)" if "norm x = 1" "x ∈ S" for x
proof -
have "f x ∈ sphere 0 1"
using fim that by (simp add: image_subset_iff)
moreover have "norm(f x - g x) < 1/2"
apply (rule g12)
using that by force
ultimately show ?thesis
by (auto simp: norm_minus_commute dest: segment_bound)
qed
show "⋀x. x ∈ sphere 0 1 ∩ S ⟹ closed_segment (f x) (g x) ⊆ T - {0}"
apply (rule hull_minimal)
using fim image_eqI gim apply force
apply (rule subspace_imp_convex [OF ‹subspace T›])
done
qed
obtain d where d: "d ∈ (sphere 0 1 ∩ T) - h ` (sphere 0 1 ∩ S)"
using h spheremap_lemma1 [OF ST ‹S ⊆ T› diffh] by force
then have non0hd: "0 ∉ closed_segment (h x) (- d)" if "norm x = 1" "x ∈ S" for x
using midpoint_between [of 0 "h x" "-d"] that h [of x]
by (auto simp: between_mem_segment midpoint_def)
have conth: "continuous_on (sphere 0 1 ∩ S) h"
using differentiable_imp_continuous_on diffh by blast
have hom_hd: "homotopic_with (λz. True) (sphere 0 1 ∩ S) (T - {0}) h (λx. -d)"
apply (rule homotopic_with_linear [OF conth continuous_on_const])
apply (rule hull_minimal)
using h d apply (force simp: subspace_neg [OF ‹subspace T›])
apply (rule subspace_imp_convex [OF ‹subspace T›])
done
have conT0: "continuous_on (T - {0}) (λy. inverse(norm y) *⇩R y)"
by (intro continuous_intros) auto
have sub0T: "(λy. y /⇩R norm y) ` (T - {0}) ⊆ sphere 0 1 ∩ T"
by (fastforce simp: assms(2) subspace_mul)
obtain c where homhc: "homotopic_with (λz. True) (sphere 0 1 ∩ S) (sphere 0 1 ∩ T) h (λx. c)"
apply (rule_tac c="-d" in that)
apply (rule homotopic_with_eq)
apply (rule homotopic_compose_continuous_left [OF hom_hd conT0 sub0T])
using d apply (auto simp: h_def)
done
show ?thesis
apply (rule_tac x=c in exI)
apply (rule homotopic_with_trans [OF _ homhc])
apply (rule homotopic_with_eq)
apply (rule homotopic_compose_continuous_left [OF homfg conT0 sub0T])
apply (auto simp: h_def)
done
qed

lemma spheremap_lemma3:
assumes "bounded S" "convex S" "subspace U" and affSU: "aff_dim S ≤ dim U"
obtains T where "subspace T" "T ⊆ U" "S ≠ {} ⟹ aff_dim T = aff_dim S"
"(rel_frontier S) homeomorphic (sphere 0 1 ∩ T)"
proof (cases "S = {}")
case True
with ‹subspace U› subspace_0 show ?thesis
by (rule_tac T = "{0}" in that) auto
next
case False
then obtain a where "a ∈ S"
by auto
then have affS: "aff_dim S = int (dim ((λx. -a+x) ` S))"
by (metis hull_inc aff_dim_eq_dim)
with affSU have "dim ((λx. -a+x) ` S) ≤ dim U"
by linarith
with choose_subspace_of_subspace
obtain T where "subspace T" "T ⊆ span U" and dimT: "dim T = dim ((λx. -a+x) ` S)" .
show ?thesis
proof (rule that [OF ‹subspace T›])
show "T ⊆ U"
using span_eq_iff ‹subspace U› ‹T ⊆ span U› by blast
show "aff_dim T = aff_dim S"
using dimT ‹subspace T› affS aff_dim_subspace by fastforce
show "rel_frontier S homeomorphic sphere 0 1 ∩ T"
proof -
have "aff_dim (ball 0 1 ∩ T) = aff_dim (T)"
by (metis IntI interior_ball ‹subspace T› aff_dim_convex_Int_nonempty_interior centre_in_ball empty_iff inf_commute subspace_0 subspace_imp_convex zero_less_one)
then have affS_eq: "aff_dim S = aff_dim (ball 0 1 ∩ T)"
using ‹aff_dim T = aff_dim S› by simp
have "rel_frontier S homeomorphic rel_frontier(ball 0 1 ∩ T)"
apply (rule homeomorphic_rel_frontiers_convex_bounded_sets [OF ‹convex S› ‹bounded S›])
apply (simp add: ‹subspace T› convex_Int subspace_imp_convex)
apply (rule affS_eq)
done
also have "... = frontier (ball 0 1) ∩ T"
apply (rule convex_affine_rel_frontier_Int [OF convex_ball])
apply (simp add: ‹subspace T› subspace_imp_affine)
using ‹subspace T› subspace_0 by force
also have "... = sphere 0 1 ∩ T"
by auto
finally show ?thesis .
qed
qed
qed

proposition inessential_spheremap_lowdim_gen:
fixes f :: "'M::euclidean_space ⇒ 'a::euclidean_space"
assumes "convex S" "bounded S" "convex T" "bounded T"
and affST: "aff_dim S < aff_dim T"
and contf: "continuous_on (rel_frontier S) f"
and fim: "f ` (rel_frontier S) ⊆ rel_frontier T"
obtains c where "homotopic_with (λz. True) (rel_frontier S) (rel_frontier T) f (λx. c)"
proof (cases "S = {}")
case True
then show ?thesis
next
case False
then show ?thesis
proof (cases "T = {}")
case True
then show ?thesis
using fim that by auto
next
case False
obtain T':: "'a set"
where "subspace T'" and affT': "aff_dim T' = aff_dim T"
and homT: "rel_frontier T homeomorphic sphere 0 1 ∩ T'"
apply (rule spheremap_lemma3 [OF ‹bounded T› ‹convex T› subspace_UNIV, where 'b='a])
using ‹T ≠ {}› by blast
with homeomorphic_imp_homotopy_eqv
have relT: "sphere 0 1 ∩ T'  homotopy_eqv rel_frontier T"
using homotopy_eqv_sym by blast
have "aff_dim S ≤ int (dim T')"
using affT' ‹subspace T'› affST aff_dim_subspace by force
with spheremap_lemma3 [OF ‹bounded S› ‹convex S› ‹subspace T'›] ‹S ≠ {}›
obtain S':: "'a set" where "subspace S'" "S' ⊆ T'"
and affS': "aff_dim S' = aff_dim S"
and homT: "rel_frontier S homeomorphic sphere 0 1 ∩ S'"
by metis
with homeomorphic_imp_homotopy_eqv
have relS: "sphere 0 1 ∩ S'  homotopy_eqv rel_frontier S"
using homotopy_eqv_sym by blast
have dimST': "dim S' < dim T'"
by (metis ‹S' ⊆ T'› ‹subspace S'› ‹subspace T'› affS' affST affT' less_irrefl not_le subspace_dim_equal)
have "∃c. homotopic_with (λz. True) (rel_frontier S) (rel_frontier T) f (λx. c)"
apply (rule homotopy_eqv_homotopic_triviality_null_imp [OF relT contf fim])
apply (rule homotopy_eqv_cohomotopic_triviality_null[OF relS, THEN iffD1, rule_format])
apply (metis dimST' ‹subspace S'›  ‹subspace T'›  ‹S' ⊆ T'› spheremap_lemma2, blast)
done
with that show ?thesis by blast
qed
qed

lemma inessential_spheremap_lowdim:
fixes f :: "'M::euclidean_space ⇒ 'a::euclidean_space"
assumes
"DIM('M) < DIM('a)" and f: "continuous_on (sphere a r) f" "f ` (sphere a r) ⊆ (sphere b s)"
obtains c where "homotopic_with (λz. True) (sphere a r) (sphere b s) f (λx. c)"
proof (cases "s ≤ 0")
case True then show ?thesis
by (meson nullhomotopic_into_contractible f contractible_sphere that)
next
case False
show ?thesis
proof (cases "r ≤ 0")
case True then show ?thesis
by (meson f nullhomotopic_from_contractible contractible_sphere that)
next
case False
with ‹~ s ≤ 0› have "r > 0" "s > 0" by auto
show ?thesis
apply (rule inessential_spheremap_lowdim_gen [of "cball a r" "cball b s" f])
using  ‹0 < r› ‹0 < s› assms(1)
using that by blast
qed
qed

subsection‹ Some technical lemmas about extending maps from cell complexes›

lemma extending_maps_Union_aux:
assumes fin: "finite ℱ"
and "⋀S. S ∈ ℱ ⟹ closed S"
and "⋀S T. ⟦S ∈ ℱ; T ∈ ℱ; S ≠ T⟧ ⟹ S ∩ T ⊆ K"
and "⋀S. S ∈ ℱ ⟹ ∃g. continuous_on S g ∧ g ` S ⊆ T ∧ (∀x ∈ S ∩ K. g x = h x)"
shows "∃g. continuous_on (⋃ℱ) g ∧ g ` (⋃ℱ) ⊆ T ∧ (∀x ∈ ⋃ℱ ∩ K. g x = h x)"
using assms
proof (induction ℱ)
case empty show ?case by simp
next
case (insert S ℱ)
then obtain f where contf: "continuous_on (S) f" and fim: "f ` S ⊆ T" and feq: "∀x ∈ S ∩ K. f x = h x"
by (meson insertI1)
obtain g where contg: "continuous_on (⋃ℱ) g" and gim: "g ` ⋃ℱ ⊆ T" and geq: "∀x ∈ ⋃ℱ ∩ K. g x = h x"
using insert by auto
have fg: "f x = g x" if "x ∈ T" "T ∈ ℱ" "x ∈ S" for x T
proof -
have "T ∩ S ⊆ K ∨ S = T"
using that by (metis (no_types) insert.prems(2) insertCI)
then show ?thesis
using UnionI feq geq ‹S ∉ ℱ› subsetD that by fastforce
qed
show ?case
apply (rule_tac x="λx. if x ∈ S then f x else g x" in exI, simp)
apply (intro conjI continuous_on_cases)
apply (simp_all add: insert closed_Union contf contg)
using fim gim feq geq
apply (force simp: insert closed_Union contf contg inf_commute intro: fg)+
done
qed

lemma extending_maps_Union:
assumes fin: "finite ℱ"
and "⋀S. S ∈ ℱ ⟹ ∃g. continuous_on S g ∧ g ` S ⊆ T ∧ (∀x ∈ S ∩ K. g x = h x)"
and "⋀S. S ∈ ℱ ⟹ closed S"
and K: "⋀X Y. ⟦X ∈ ℱ; Y ∈ ℱ; ~ X ⊆ Y; ~ Y ⊆ X⟧ ⟹ X ∩ Y ⊆ K"
shows "∃g. continuous_on (⋃ℱ) g ∧ g ` (⋃ℱ) ⊆ T ∧ (∀x ∈ ⋃ℱ ∩ K. g x = h x)"
apply (simp add: Union_maximal_sets [OF fin, symmetric])
apply (rule extending_maps_Union_aux)
apply (simp_all add: Union_maximal_sets [OF fin] assms)
by (metis K psubsetI)

lemma extend_map_lemma:
assumes "finite ℱ" "𝒢 ⊆ ℱ" "convex T" "bounded T"
and poly: "⋀X. X ∈ ℱ ⟹ polytope X"
and aff: "⋀X. X ∈ ℱ - 𝒢 ⟹ aff_dim X < aff_dim T"
and face: "⋀S T. ⟦S ∈ ℱ; T ∈ ℱ⟧ ⟹ (S ∩ T) face_of S ∧ (S ∩ T) face_of T"
and contf: "continuous_on (⋃𝒢) f" and fim: "f ` (⋃𝒢) ⊆ rel_frontier T"
obtains g where "continuous_on (⋃ℱ) g" "g ` (⋃ℱ) ⊆ rel_frontier T" "⋀x. x ∈ ⋃𝒢 ⟹ g x = f x"
proof (cases "ℱ - 𝒢 = {}")
case True
then have "⋃ℱ ⊆ ⋃𝒢"
then show ?thesis
apply (rule_tac g=f in that)
using contf continuous_on_subset apply blast
using fim apply blast
by simp
next
case False
then have "0 ≤ aff_dim T"
by (metis aff aff_dim_empty aff_dim_geq aff_dim_negative_iff all_not_in_conv not_less)
then obtain i::nat where i: "int i = aff_dim T"
by (metis nonneg_eq_int)
have Union_empty_eq: "⋃{D. D = {} ∧ P D} = {}" for P :: "'a set ⇒ bool"
by auto
have extendf: "∃g. continuous_on (⋃(𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < i})) g ∧
g ` (⋃ (𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < i})) ⊆ rel_frontier T ∧
(∀x ∈ ⋃𝒢. g x = f x)"
if "i ≤ aff_dim T" for i::nat
using that
proof (induction i)
case 0 then show ?case
apply (rule_tac x=f in exI)
apply (intro conjI)
using contf continuous_on_subset apply blast
using fim apply blast
by simp
next
case (Suc p)
with ‹bounded T› have "rel_frontier T ≠ {}"
by (auto simp: rel_frontier_eq_empty affine_bounded_eq_lowdim [of T])
then obtain t where t: "t ∈ rel_frontier T" by auto
have ple: "int p ≤ aff_dim T" using Suc.prems by force
obtain h where conth: "continuous_on (⋃(𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p})) h"
and him: "h ` (⋃ (𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p}))
⊆ rel_frontier T"
and heq: "⋀x. x ∈ ⋃𝒢 ⟹ h x = f x"
using Suc.IH [OF ple] by auto
let ?Faces = "{D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D ≤ p}"
have extendh: "∃g. continuous_on D g ∧
g ` D ⊆ rel_frontier T ∧
(∀x ∈ D ∩ ⋃(𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p}). g x = h x)"
if D: "D ∈ 𝒢 ∪ ?Faces" for D
proof (cases "D ⊆ ⋃(𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p})")
case True
then show ?thesis
apply (rule_tac x=h in exI)
apply (intro conjI)
apply (blast intro: continuous_on_subset [OF conth])
using him apply blast
by simp
next
case False
note notDsub = False
show ?thesis
proof (cases "∃a. D = {a}")
case True
then obtain a where "D = {a}" by auto
with notDsub t show ?thesis
by (rule_tac x="λx. t" in exI) simp
next
case False
have "D ≠ {}" using notDsub by auto
have Dnotin: "D ∉ 𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p}"
using notDsub by auto
then have "D ∉ 𝒢" by simp
have "D ∈ ?Faces - {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < p}"
using Dnotin that by auto
then obtain C where "C ∈ ℱ" "D face_of C" and affD: "aff_dim D = int p"
by auto
then have "bounded D"
using face_of_polytope_polytope poly polytope_imp_bounded by blast
then have [simp]: "¬ affine D"
using affine_bounded_eq_trivial False ‹D ≠ {}› ‹bounded D› by blast
have "{F. F facet_of D} ⊆ {E. E face_of C ∧ aff_dim E < int p}"
apply clarify
apply (metis ‹D face_of C› affD eq_iff face_of_trans facet_of_def zle_diff1_eq)
done
moreover have "polyhedron D"
using ‹C ∈ ℱ› ‹D face_of C› face_of_polytope_polytope poly polytope_imp_polyhedron by auto
ultimately have relf_sub: "rel_frontier D ⊆ ⋃ {E. E face_of C ∧ aff_dim E < p}"
then have him_relf: "h ` rel_frontier D ⊆ rel_frontier T"
using ‹C ∈ ℱ› him by blast
have "convex D"
by (simp add: ‹polyhedron D› polyhedron_imp_convex)
have affD_lessT: "aff_dim D < aff_dim T"
using Suc.prems affD by linarith
have contDh: "continuous_on (rel_frontier D) h"
using ‹C ∈ ℱ› relf_sub by (blast intro: continuous_on_subset [OF conth])
then have *: "(∃c. homotopic_with (λx. True) (rel_frontier D) (rel_frontier T) h (λx. c)) =
(∃g. continuous_on UNIV g ∧  range g ⊆ rel_frontier T ∧
(∀x∈rel_frontier D. g x = h x))"
apply (rule nullhomotopic_into_rel_frontier_extension [OF closed_rel_frontier])
apply (simp_all add: assms rel_frontier_eq_empty him_relf)
done
have "(∃c. homotopic_with (λx. True) (rel_frontier D)
(rel_frontier T) h (λx. c))"
by (metis inessential_spheremap_lowdim_gen
[OF ‹convex D› ‹bounded D› ‹convex T› ‹bounded T› affD_lessT contDh him_relf])
then obtain g where contg: "continuous_on UNIV g"
and gim: "range g ⊆ rel_frontier T"
and gh: "⋀x. x ∈ rel_frontier D ⟹ g x = h x"
by (metis *)
have "D ∩ E ⊆ rel_frontier D"
if "E ∈ 𝒢 ∪ {D. Bex ℱ ((face_of) D) ∧ aff_dim D < int p}" for E
proof (rule face_of_subset_rel_frontier)
show "D ∩ E face_of D"
using that ‹C ∈ ℱ› ‹D face_of C› face
apply auto
apply (meson face_of_Int_subface ‹𝒢 ⊆ ℱ› face_of_refl_eq poly polytope_imp_convex subsetD)
using face_of_Int_subface apply blast
done
show "D ∩ E ≠ D"
using that notDsub by auto
qed
then show ?thesis
apply (rule_tac x=g in exI)
apply (intro conjI ballI)
using continuous_on_subset contg apply blast
using gim apply blast
using gh by fastforce
qed
qed
have intle: "i < 1 + int j ⟷ i ≤ int j" for i j
by auto
have "finite 𝒢"
using ‹finite ℱ› ‹𝒢 ⊆ ℱ› rev_finite_subset by blast
then have fin: "finite (𝒢 ∪ ?Faces)"
apply simp
apply (rule_tac B = "⋃{{D. D face_of C}| C. C ∈ ℱ}" in finite_subset)
by (auto simp: ‹finite ℱ› finite_polytope_faces poly)
have clo: "closed S" if "S ∈ 𝒢 ∪ ?Faces" for S
using that ‹𝒢 ⊆ ℱ› face_of_polytope_polytope poly polytope_imp_closed by blast
have K: "X ∩ Y ⊆ ⋃(𝒢 ∪ {D. ∃C∈ℱ. D face_of C ∧ aff_dim D < int p})"
if "X ∈ 𝒢 ∪ ?Faces" "Y ∈ 𝒢 ∪ ?Faces" "~ Y ⊆ X" for X Y
proof -
have ff: "X ∩ Y face_of X ∧ X ∩ Y face_of Y"
if XY: "X face_of D" "Y face_of E" and DE: "D ∈ ℱ" "E ∈ ℱ" for D E
apply (rule face_of_Int_subface [OF _ _ XY])
apply (auto simp: face DE)
done
show ?thesis
using that
apply auto
apply (drule_tac x="X ∩ Y" in spec, safe)
using ff face_of_imp_convex [of X] face_of_imp_convex [of Y]
apply (fastforce dest: face_of_aff_dim_lt)
by (meson face_of_trans ff)
qed
obtain g where "continuous_on (⋃(𝒢 ∪ ?Faces)) g"
"g ` ⋃(𝒢 ∪ ?Faces) ⊆ rel_frontier T"
"(∀x ∈ ⋃(𝒢 ∪ ?Faces) ∩
⋃(𝒢 ∪ {D. ∃C∈ℱ. D face_of C ∧ aff_dim D < p}). g x = h x)"
apply (rule exE [OF extending_maps_Union [OF fin extendh clo K]], blast+)
done
then show ?case
apply (simp add: intle local.heq [symmetric], blast)
done
qed
have eq: "⋃(𝒢 ∪ {D. ∃C ∈ ℱ. D face_of C ∧ aff_dim D < i}) = ⋃ℱ"
proof
show "⋃(𝒢 ∪ {D. ∃C∈ℱ. D face_of C ∧ aff_dim D < int i}) ⊆ ⋃ℱ"
apply (rule Union_subsetI)
using ‹𝒢 ⊆ ℱ› face_of_imp_subset  apply force
done
show "⋃ℱ ⊆ ⋃(𝒢 ∪ {D. ∃C∈ℱ. D face_of C ∧ aff_dim D < i})"
apply (rule Union_mono)
using face  apply (fastforce simp: aff i)
done
qed
have "int i ≤ aff_dim T" by (simp add: i)
then show ?thesis
using extendf [of i] unfolding eq by (metis that)
qed

lemma extend_map_lemma_cofinite0:
assumes "finite ℱ"
and "pairwise (λS T. S ∩ T ⊆ K) ℱ"
and "⋀S. S ∈ ℱ ⟹ ∃a g. a ∉ U ∧ continuous_on (S - {a}) g ∧ g ` (S - {a}) ⊆ T ∧ (∀x ∈ S ∩ K. g x = h x)"
and "⋀S. S ∈ ℱ ⟹ closed S"
shows "∃C g. finite C ∧ disjnt C U ∧ card C ≤ card ℱ ∧
continuous_on (⋃ℱ - C) g ∧ g ` (⋃ℱ - C) ⊆ T
∧ (∀x ∈ (⋃ℱ - C) ∩ K. g x = h x)"
using assms
proof induction
case empty then show ?case
by force
next
case (insert X ℱ)
then have "closed X" and clo: "⋀X. X ∈ ℱ ⟹ closed X"
and ℱ: "⋀S. S ∈ ℱ ⟹ ∃a g. a ∉ U ∧ continuous_on (S - {a}) g ∧ g ` (S - {a}) ⊆ T ∧ (∀x ∈ S ∩ K. g x = h x)"
and pwX: "⋀Y. Y ∈ ℱ ∧ Y ≠ X ⟶ X ∩ Y ⊆ K ∧ Y ∩ X ⊆ K"
and pwF: "pairwise (λ S T. S ∩ T ⊆ K) ℱ"
obtain C g where C: "finite C" "disjnt C U" "card C ≤ card ℱ"
and contg: "continuous_on (⋃ℱ - C) g"
and gim: "g ` (⋃ℱ - C) ⊆ T"
and gh:  "⋀x. x ∈ (⋃ℱ - C) ∩ K ⟹ g x = h x"
using insert.IH [OF pwF ℱ clo] by auto
obtain a f where "a ∉ U"
and contf: "continuous_on (X - {a}) f"
and fim: "f ` (X - {a}) ⊆ T"
and fh: "(∀x ∈ X ∩ K. f x = h x)"
using insert.prems by (meson insertI1)
show ?case
proof (intro exI conjI)
show "finite (insert a C)"
show "disjnt (insert a C) U"
using C ‹a ∉ U› by simp
show "card (insert a C) ≤ card (insert X ℱ)"
by (simp add: C card_insert_if insert.hyps le_SucI)
have "closed (⋃ℱ)"
using clo insert.hyps by blast
have "continuous_on (X - insert a C ∪ (⋃ℱ - insert a C)) (λx. if x ∈ X then f x else g x)"
apply (rule continuous_on_cases_local)
using ‹closed X› apply blast
using ‹closed (⋃ℱ)› apply blast
using contf apply (force simp: elim: continuous_on_subset)
using contg apply (force simp: elim: continuous_on_subset)
using fh gh insert.hyps pwX by fastforce
then show "continuous_on (⋃insert X ℱ - insert a C) (λa. if a ∈ X then f a else g a)"
by (blast intro: continuous_on_subset)
show "∀x∈(⋃insert X ℱ - insert a C) ∩ K. (if x ∈ X then f x else g x) = h x"
using gh by (auto simp: fh)
show "(λa. if a ∈ X then f a else g a) ` (⋃insert X ℱ - insert a C) ⊆ T"
using fim gim by auto force
qed
qed

lemma extend_map_lemma_cofinite1:
assumes "finite ℱ"
and ℱ: "⋀X. X ∈ ℱ ⟹ ∃a g. a ∉ U ∧ continuous_on (X - {a}) g ∧ g ` (X - {a}) ⊆ T ∧ (∀x ∈ X ∩ K. g x = h x)"
and clo: "⋀X. X ∈ ℱ ⟹ closed X"
and K: "⋀X Y. ⟦X ∈ ℱ; Y ∈ ℱ; ~(X ⊆ Y); ~(Y ⊆ X)⟧ ⟹ X ∩ Y ⊆ K"
obtains C g where "finite C" "disjnt C U" "card C ≤ card ℱ" "continuous_on (⋃ℱ - C) g"
"g ` (⋃ℱ - C) ⊆ T"
"⋀x. x ∈ (⋃ℱ - C) ∩ K ⟹ g x = h x"
proof -
let ?ℱ = "{X ∈ ℱ. ∀Y∈ℱ. ¬ X ⊂ Y}"
have [simp]: "⋃?ℱ = ⋃ℱ"
have fin: "finite ?ℱ"
by (force intro: finite_subset [OF _ ‹finite ℱ›])
have pw: "pairwise (λ S T. S ∩ T ⊆ K) ?ℱ"
by (simp add: pairwise_def) (metis K psubsetI)
have "card {X ∈ ℱ. ∀Y∈ℱ. ¬ X ⊂ Y} ≤ card ℱ"
by (simp add: ‹finite ℱ› card_mono)
moreover
obtain C g where "finite C ∧ disjnt C U ∧ card C ≤ card ?ℱ ∧
continuous_on (⋃?ℱ - C) g ∧ g ` (⋃?ℱ - C) ⊆ T
∧ (∀x ∈ (⋃?ℱ - C) ∩ K. g x = h x)"
apply (rule exE [OF extend_map_lemma_cofinite0 [OF fin pw, of U T h]])
apply (fastforce intro!:  clo ℱ)+
done
ultimately show ?thesis
by (rule_tac C=C and g=g in that) auto
qed

lemma extend_map_lemma_cofinite:
assumes "finite ℱ" "𝒢 ⊆ ℱ" and T: "convex T" "bounded T"
and poly: "⋀X. X ∈ ℱ ⟹ polytope X"
and contf: "continuous_on (⋃𝒢) f" and fim: "f ` (⋃𝒢) ⊆ rel_frontier T"
and face: "⋀X Y. ⟦X ∈ ℱ; Y ∈ ℱ⟧ ⟹ (X ∩ Y) face_of X ∧ (X ∩ Y) face_of Y"
and aff: "⋀X. X ∈ ℱ - 𝒢 ⟹ aff_dim X ≤ aff_dim T"
obtains C g where
"finite C" "disjnt C (⋃𝒢)" "card C ≤ card ℱ" "continuous_on (⋃ℱ - C) g"
"g ` (⋃ ℱ - C) ⊆ rel_frontier T" "⋀x. x ∈ ⋃𝒢 ⟹ g x = f x"
proof -
define ℋ where "ℋ ≡ 𝒢 ∪ {D. ∃C ∈ ℱ - 𝒢. D face_of C ∧ aff_dim D < aff_dim T}"
have "finite 𝒢"
using assms finite_subset by blast
moreover have "finite (⋃{{D. D face_of C} |C. C ∈ ℱ})"
apply (rule finite_Union)
using finite_polytope_faces poly by auto
ultimately have "finite ℋ"
apply (rule finite_subset [of _ "⋃ {{D. D face_of C} | C. C ∈ ℱ}"], auto)
done
have *: "⋀X Y. ⟦X ∈ ℋ; Y ∈ ℋ⟧ ⟹ X ∩ Y face_of X ∧ X ∩ Y face_of Y"
unfolding ℋ_def
apply (elim UnE bexE CollectE DiffE)
using subsetD [OF ‹𝒢 ⊆ ℱ›] apply (simp_all add: face)
apply (meson subsetD [OF ‹𝒢 ⊆ ℱ›] face face_of_Int_subface face_of_imp_subset face_of_refl poly polytope_imp_convex)+
done
obtain h where conth: "continuous_on (⋃ℋ) h" and him: "h ` (⋃ℋ) ⊆ rel_frontier T"
and hf: "⋀x. x ∈ ⋃𝒢 ⟹ h x = f x"
using ‹finite ℋ›
unfolding ℋ_def
apply (rule extend_map_lemma [OF _ Un_upper1 T _ _ _ contf fim])
using ‹𝒢 ⊆ ℱ› face_of_polytope_polytope poly apply fastforce
using * apply (auto simp: ℋ_def)
done
have "bounded (⋃𝒢)"
using ‹finite 𝒢› ‹𝒢 ⊆ ℱ› poly polytope_imp_bounded by blast
then have "⋃𝒢 ≠ UNIV"
by auto
then obtain a where a: "a ∉ ⋃𝒢"
by blast
have ℱ: "∃a g. a ∉ ⋃𝒢 ∧ continuous_on (D - {a}) g ∧
g ` (D - {a}) ⊆ rel_frontier T ∧ (∀x ∈ D ∩ ⋃ℋ. g x = h x)"
if "D ∈ ℱ" for D
proof (cases "D ⊆ ⋃ℋ")
case True
then show ?thesis
apply (rule_tac x=a in exI)
apply (rule_tac x=h in exI)
using him apply (blast intro!: ‹a ∉ ⋃𝒢› continuous_on_subset [OF conth]) +
done
next
case False
note D_not_subset = False
show ?thesis
proof (cases "D ∈ 𝒢")
case True
with D_not_subset show ?thesis
by (auto simp: ℋ_def)
next
case False
then have affD: "aff_dim D ≤ aff_dim T"
by (simp add: ‹D ∈ ℱ› aff)
show ?thesis
proof (cases "rel_interior D = {}")
case True
with ‹D ∈ ℱ› poly a show ?thesis
by (force simp: rel_interior_eq_empty polytope_imp_convex)
next
case False
then obtain b where brelD: "b ∈ rel_interior D"
by blast
have "polyhedron D"
by (simp add: poly polytope_imp_polyhedron that)
have "rel_frontier D retract_of affine hull D - {b}"
by (simp add: rel_frontier_retract_of_punctured_affine_hull poly polytope_imp_bounded polytope_imp_convex that brelD)
then obtain r where relfD: "rel_frontier D ⊆ affine hull D - {b}"
and contr: "continuous_on (affine hull D - {b}) r"
and rim: "r ` (affine hull D - {b}) ⊆ rel_frontier D"
and rid: "⋀x. x ∈ rel_frontier D ⟹ r x = x"
by (auto simp: retract_of_def retraction_def)
show ?thesis
proof (intro exI conjI ballI)
show "b ∉ ⋃𝒢"
proof clarify
fix E
assume "b ∈ E" "E ∈ 𝒢"
then have "E ∩ D face_of E ∧ E ∩ D face_of D"
using ‹𝒢 ⊆ ℱ› face that by auto
with face_of_subset_rel_frontier ‹E ∈ 𝒢› ‹b ∈ E› brelD rel_interior_subset [of D]
D_not_subset rel_frontier_def ℋ_def
show False
by blast
qed
have "r ` (D - {b}) ⊆ r ` (affine hull D - {b})"
by (simp add: Diff_mono hull_subset image_mono)
also have "... ⊆ rel_frontier D"
by (rule rim)
also have "... ⊆ ⋃{E. E face_of D ∧ aff_dim E < aff_dim T}"
using affD
by (force simp: rel_frontier_of_polyhedron [OF ‹polyhedron D›] facet_of_def)
also have "... ⊆ ⋃(ℋ)"
using D_not_subset ℋ_def that by fastforce
finally have rsub: "r ` (D - {b}) ⊆ ⋃(ℋ)" .
show "continuous_on (D - {b}) (h ∘ r)"
apply (intro conjI ‹b ∉ ⋃𝒢› continuous_on_compose)
apply (rule continuous_on_subset [OF contr])
apply (rule continuous_on_subset [OF conth rsub])
done
show "(h ∘ r) ` (D - {b}) ⊆ rel_frontier T"
using brelD him rsub by fastforce
show "(h ∘ r) x = h x" if x: "x ∈ D ∩ ⋃ℋ" for x
proof -
consider A where "x ∈ D" "A ∈ 𝒢" "x ∈ A"
| A B where "x ∈ D" "A face_of B" "B ∈ ℱ" "B ∉ 𝒢" "aff_dim A < aff_dim T" "x ∈ A"
using x by (auto simp: ℋ_def)
then have xrel: "x ∈ rel_frontier D"
proof cases
case 1 show ?thesis
proof (rule face_of_subset_rel_frontier [THEN subsetD])
show "D ∩ A face_of D"
using ‹A ∈ 𝒢› ‹𝒢 ⊆ ℱ› face ‹D ∈ ℱ› by blast
show "D ∩ A ≠ D"
using ‹A ∈ 𝒢› D_not_subset ℋ_def by blast
qed (auto simp: 1)
next
case 2 show ?thesis
proof (rule face_of_subset_rel_frontier [THEN subsetD])
show "D ∩ A face_of D"
apply (rule face_of_Int_subface [of D B _ A, THEN conjunct1])
apply (simp_all add: 2 ‹D ∈ ℱ› face)
apply (simp add: ‹polyhedron D› polyhedron_imp_convex face_of_refl)
done
show "D ∩ A ≠ D"
using "2" D_not_subset ℋ_def by blast
qed (auto simp: 2)
qed
show ?thesis
qed
qed
qed
qed
qed
have clo: "⋀S. S ∈ ℱ ⟹ closed S"
obtain C g where "finite C" "disjnt C (⋃𝒢)" "card C ≤ card ℱ" "continuous_on (⋃ℱ - C) g"
"g ` (⋃ℱ - C) ⊆ rel_frontier T"
and gh: "⋀x. x ∈ (⋃ℱ - C) ∩ ⋃ℋ ⟹ g x = h x"
proof (rule extend_map_lemma_cofinite1 [OF ‹finite ℱ› ℱ clo])
show "X ∩ Y ⊆ ⋃ℋ" if XY: "X ∈ ℱ" "Y ∈ ℱ" and "¬ X ⊆ Y" "¬ Y ⊆ X" for X Y
proof (cases "X ∈ 𝒢")
case True
then show ?thesis
by (auto simp: ℋ_def)
next
case False
have "X ∩ Y ≠ X"
using ‹¬ X ⊆ Y› by blast
with XY
show ?thesis
by (clarsimp simp: ℋ_def)
(metis Diff_iff Int_iff aff antisym_conv face face_of_aff_dim_lt face_of_refl
not_le poly polytope_imp_convex)
qed
qed (blast)+
with ‹𝒢 ⊆ ℱ› show ?thesis
apply (rule_tac C=C and g=g in that)
apply (auto simp: disjnt_def hf [symmetric] ℋ_def intro!: gh)
done
qed

text‹The next two proofs are similar›
theorem extend_map_cell_complex_to_sphere:
assumes "finite ℱ" and S: "S ⊆ ⋃ℱ" "closed S" and T: "convex T" "bounded T"
and poly: "⋀X. X ∈ ℱ ⟹ polytope X"
and aff: "⋀X. X ∈ ℱ ⟹ aff_dim X < aff_dim T"
and face: "⋀X Y. ⟦X ∈ ℱ; Y ∈ ℱ⟧ ⟹ (X ∩ Y) face_of X ∧ (X ∩ Y) face_of Y"
and contf: "continuous_on S f" and fim: "f ` S ⊆ rel_frontier T"
obtains g where "continuous_on (⋃ℱ) g"
"g ` (⋃ℱ) ⊆ rel_frontier T" "⋀x. x ∈ S ⟹ g x = f x"
proof -
obtain V g where "S ⊆ V" "open V" "continuous_on V g" and gim: "g ` V ⊆ rel_frontier T" and gf: "⋀x. x ∈ S ⟹ g x = f x"
using neighbourhood_extension_into_ANR [OF contf fim _ ‹closed S›] ANR_rel_frontier_convex T by blast
have "compact S"
by (meson assms compact_Union poly polytope_imp_compact seq_compact_closed_subset seq_compact_eq_compact)
then obtain d where "d > 0" and d: "⋀x y. ⟦x ∈ S; y ∈ - V⟧ ⟹ d ≤ dist x y"
using separate_compact_closed [of S "-V"] ‹open V› ‹S ⊆ V› by force
obtain 𝒢 where "finite 𝒢" "⋃𝒢 = ⋃ℱ"
and diaG: "⋀X. X ∈ 𝒢 ⟹ diameter X < d"
and polyG: "⋀X. X ∈ 𝒢 ⟹ polytope X"
and affG: "⋀X. X ∈ 𝒢 ⟹ aff_dim X ≤ aff_dim T - 1"
and faceG: "⋀X Y. ⟦X ∈ 𝒢; Y ∈ 𝒢⟧ ⟹ X ∩ Y face_of X ∧ X ∩ Y face_of Y"
proof (rule cell_complex_subdivision_exists [OF ‹d>0› ‹finite ℱ› poly _ face])
show "⋀X. X ∈ ℱ ⟹ aff_dim X ≤ aff_dim T - 1"
qed auto
obtain h where conth: "continuous_on (⋃𝒢) h" and him: "h ` ⋃𝒢 ⊆ rel_frontier T" and hg: "⋀x. x ∈ ⋃(𝒢 ∩ Pow V) ⟹ h x = g x"
proof (rule extend_map_lemma [of 𝒢 "𝒢 ∩ Pow V" T g])
show "continuous_on (⋃(𝒢 ∩ Pow V)) g"
by (metis Union_Int_subset Union_Pow_eq ‹continuous_on V g› continuous_on_subset le_inf_iff)
qed (use ‹finite 𝒢› T polyG affG faceG gim in fastforce)+
show ?thesis
proof
show "continuous_on (⋃ℱ) h"
using ‹⋃𝒢 = ⋃ℱ› conth by auto
show "h ` ⋃ℱ ⊆ rel_frontier T"
using ‹⋃𝒢 = ⋃ℱ› him by auto
show "h x = f x" if "x ∈ S" for x
proof -
have "x ∈ ⋃𝒢"
using ‹⋃𝒢 = ⋃ℱ› ‹S ⊆ ⋃ℱ› that by auto
then obtain X where "x ∈ X" "X ∈ 𝒢" by blast
then have "diameter X < d" "bounded X"
by (auto simp: diaG ‹X ∈ 𝒢› polyG polytope_imp_bounded)
then have "X ⊆ V" using d [OF ‹x ∈ S›] diameter_bounded_bound [OF ‹bounded X› ‹x ∈ X›]
by fastforce
have "h x = g x"
apply (rule hg)
using ‹X ∈ 𝒢› ‹X ⊆ V› ‹x ∈ X› by blast
also have "... = f x"
finally show "h x = f x" .
qed
qed
qed

theorem extend_map_cell_complex_to_sphere_cofinite:
assumes "finite ℱ" and S: "S ⊆ ⋃ℱ" "closed S" and T: "convex T" "bounded T"
and poly: "⋀X. X ∈ ℱ ⟹ polytope X"
and aff: "⋀X. X ∈ ℱ ⟹ aff_dim X ≤ aff_dim T"
and face: "⋀X Y. ⟦X ∈ ℱ; Y ∈ ℱ⟧ ⟹ (X ∩ Y) face_of X ∧ (X ∩ Y) face_of Y"
and contf: "continuous_on S f" and fim: "f ` S ⊆ rel_frontier T"
obtains C g where "finite C" "disjnt C S" "continuous_on (⋃ℱ - C) g"
"g ` (⋃ℱ - C) ⊆ rel_frontier T" "⋀x. x ∈ S ⟹ g x = f x"
proof -
obtain V g where "S ⊆ V" "open V" "continuous_on V g" and gim: "g ` V ⊆ rel_frontier T" and gf: "⋀x. x ∈ S ⟹ g x = f x"
using neighbourhood_extension_into_ANR [OF contf fim _ ‹closed S›] ANR_rel_frontier_convex T by blast
have "compact S"
by (meson assms compact_Union poly polytope_imp_compact seq_compact_closed_subset seq_compact_eq_compact)
then obtain d where "d > 0" and d: "⋀x y. ⟦x ∈ S; y ∈ - V⟧ ⟹ d ≤ dist x y"
using separate_compact_closed [of S "-V"] ‹open V› ‹S ⊆ V› by force
obtain 𝒢 where "finite 𝒢" "⋃𝒢 = ⋃ℱ"
and diaG: "⋀X. X ∈ 𝒢 ⟹ diameter X < d"
and polyG: "⋀X. X ∈ 𝒢 ⟹ polytope X"
and affG: "⋀X. X ∈ 𝒢 ⟹ aff_dim X ≤ aff_dim T"
and faceG: "⋀X Y. ⟦X ∈ 𝒢; Y ∈ 𝒢⟧ ⟹ X ∩ Y face_of X ∧ X ∩ Y face_of Y"
by (rule cell_complex_subdivision_exists [OF ‹d>0› ‹finite ℱ› poly aff face]) auto
obtain C h where "finite C" and dis: "disjnt C (⋃(𝒢 ∩ Pow V))"
and card: "card C ≤ card 𝒢" and conth: "continuous_on (⋃𝒢 - C) h"
and him: "h ` (⋃𝒢 - C) ⊆ rel_frontier T"
and hg: "⋀x. x ∈ ⋃(𝒢 ∩ Pow V) ⟹ h x = g x"
proof (rule extend_map_lemma_cofinite [of 𝒢 "𝒢 ∩ Pow V" T g])
show "continuous_on (⋃(𝒢 ∩ Pow V)) g"
by (metis Union_Int_subset Union_Pow_eq ‹continuous_on V g› continuous_on_subset le_inf_iff)
show "g ` ⋃(𝒢 ∩ Pow V) ⊆ rel_frontier T"
using gim by force
qed (auto intro: ‹finite 𝒢› T polyG affG dest: faceG)
have Ssub: "S ⊆ ⋃(𝒢 ∩ Pow V)"
proof
fix x
assume "x ∈ S"
then have "x ∈ ⋃𝒢"
using ‹⋃𝒢 = ⋃ℱ› ‹S ⊆ ⋃ℱ› by auto
then obtain X where "x ∈ X" "X ∈ 𝒢" by blast
then have "diameter X < d" "bounded X"
by (auto simp: diaG ‹X ∈ 𝒢› polyG polytope_imp_bounded)
then have "X ⊆ V" using d [OF ‹x ∈ S›] diameter_bounded_bound [OF ‹bounded X› ‹x ∈ X›]
by fastforce
then show "x ∈ ⋃(𝒢 ∩ Pow V)"
using ‹X ∈ 𝒢› ‹x ∈ X› by blast
qed
show ?thesis
proof
show "continuous_on (⋃ℱ-C) h"
using ‹⋃𝒢 = ⋃ℱ› conth by auto
show "h ` (⋃ℱ - C) ⊆ rel_frontier T"
using ‹⋃𝒢 = ⋃ℱ› him by auto
show "h x = f x" if "x ∈ S" for x
proof -
have "h x = g x"
apply (rule hg)
using Ssub that by blast
also have "... = f x"
finally show "h x = f x" .
qed
show "disjnt C S"
using dis Ssub  by (meson disjnt_iff subset_eq)
qed (intro ‹finite C›)
qed

subsection‹ Special cases and corollaries involving spheres›

lemma disjnt_Diff1: "X ⊆ Y' ⟹ disjnt (X - Y) (X' - Y')"
by (auto simp: disjnt_def)

proposition extend_map_affine_to_sphere_cofinite_simple:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "compact S" "convex U" "bounded U"
and aff: "aff_dim T ≤ aff_dim U"
and "S ⊆ T" and contf: "continuous_on S f"
and fim: "f ` S ⊆ rel_frontier U"
obtains K g where "finite K" "K ⊆ T" "disjnt K S" "continuous_on (T - K) g"
"g ` (T - K) ⊆ rel_frontier U"
"⋀x. x ∈ S ⟹ g x = f x"
proof -
have "∃K g. finite K ∧ disjnt K S ∧ continuous_on (T - K) g ∧
g ` (T - K) ⊆ rel_frontier U ∧ (∀x ∈ S. g x = f x)"
if "affine T" "S ⊆ T" and aff: "aff_dim T ≤ aff_dim U"  for T
proof (cases "S = {}")
case True
show ?thesis
proof (cases "rel_frontier U = {}")
case True
with ‹bounded U› have "aff_dim U ≤ 0"
using affine_bounded_eq_lowdim rel_frontier_eq_empty by auto
with aff have "aff_dim T ≤ 0" by auto
then obtain a where "T ⊆ {a}"
using ‹affine T› affine_bounded_eq_lowdim affine_bounded_eq_trivial by auto
then show ?thesis
using ‹S = {}› fim
by (metis Diff_cancel contf disjnt_empty2 finite.emptyI finite_insert finite_subset)
next
case False
then obtain a where "a ∈ rel_frontier U"
by auto
then show ?thesis
using continuous_on_const [of _ a] ‹S = {}› by force
qed
next
case False
have "bounded S"
by (simp add: ‹compact S› compact_imp_bounded)
then obtain b where b: "S ⊆ cbox (-b) b"
using bounded_subset_cbox_symmetric by blast
define bbox where "bbox ≡ cbox (-(b+One)) (b+One)"
have "cbox (-b) b ⊆ bbox"
by (auto simp: bbox_def algebra_simps intro!: subset_box_imp)
with b ‹S ⊆ T› have "S ⊆ bbox ∩ T"
by auto
then have Ssub: "S ⊆ ⋃{bbox ∩ T}"
by auto
then have "aff_dim (bbox ∩ T) ≤ aff_dim U"
by (metis aff aff_dim_subset inf_commute inf_le1 order_trans)
obtain K g where K: "finite K" "disjnt K S"
and contg: "continuous_on (⋃{bbox ∩ T} - K) g"
and gim: "g ` (⋃{bbox ∩ T} - K) ⊆ rel_frontier U"
and gf: "⋀x. x ∈ S ⟹ g x = f x"
proof (rule extend_map_cell_complex_to_sphere_cofinite
[OF _ Ssub _ ‹convex U› ‹bounded U› _ _ _ contf fim])
show "closed S"
using ‹compact S› compact_eq_bounded_closed by auto
show poly: "⋀X. X ∈ {bbox ∩ T} ⟹ polytope X"
by (simp add: polytope_Int_polyhedron bbox_def polytope_interval affine_imp_polyhedron ‹affine T›)
show "⋀X Y. ⟦X ∈ {bbox ∩ T}; Y ∈ {bbox ∩ T}⟧ ⟹ X ∩ Y face_of X ∧ X ∩ Y face_of Y"
show "⋀X. X ∈ {bbox ∩ T} ⟹ aff_dim X ≤ aff_dim U"
by (simp add: ‹aff_dim (bbox ∩ T) ≤ aff_dim U›)
qed auto
define fro where "fro ≡ λd. frontier(cbox (-(b + d *⇩R One)) (b + d *⇩R One))"
obtain d where d12: "1/2 ≤ d" "d ≤ 1" and dd: "disjnt K (fro d)"
proof (rule disjoint_family_elem_disjnt [OF _ ‹finite K›])
show "infinite {1/2..1::real}"
have dis1: "disjnt (fro x) (fro y)" if "x<y" for x y
by (auto simp: algebra_simps that subset_box_imp disjnt_Diff1 frontier_def fro_def)
then show "disjoint_family_on fro {1/2..1}"
by (auto simp: disjoint_family_on_def disjnt_def neq_iff)
qed auto
define c where "c ≡ b + d *⇩R One"
have cbsub: "cbox (-b) b ⊆ box (-c) c"  "cbox (-b) b ⊆ cbox (-c) c"  "cbox (-c) c ⊆ bbox"
using d12 by (auto simp: algebra_simps subset_box_imp c_def bbox_def)
have clo_cbT: "closed (cbox (- c) c ∩ T)"
by (simp add: affine_closed closed_Int closed_cbox ‹affine T›)
have cpT_ne: "cbox (- c) c ∩ T ≠ {}"
using ‹S ≠ {}› b cbsub(2) ‹S ⊆ T› by fastforce
have "closest_point (cbox (- c) c ∩ T) x ∉ K" if "x ∈ T" "x ∉ K" for x
proof (cases "x ∈ cbox (-c) c")
case True with that show ?thesis
next
case False
have int_ne: "interior (cbox (-c) c) ∩ T ≠ {}"
using ‹S ≠ {}› ‹S ⊆ T› b ‹cbox (- b) b ⊆ box (- c) c› by force
have "convex T"
by (meson ‹affine T› affine_imp_convex)
then have "x ∈ affine hull (cbox (- c) c ∩ T)"
by (metis Int_commute Int_iff ‹S ≠ {}› ‹S ⊆ T› cbsub(1) ‹x ∈ T› affine_hull_convex_Int_nonempty_interior all_not_in_conv b hull_inc inf.orderE interior_cbox)
then have "x ∈ affine hull (cbox (- c) c ∩ T) - rel_interior (cbox (- c) c ∩ T)"
by (meson DiffI False Int_iff rel_interior_subset subsetCE)
then have "closest_point (cbox (- c) c ∩ T) x ∈ rel_frontier (cbox (- c) c ∩ T)"
by (rule closest_point_in_rel_frontier [OF clo_cbT cpT_ne])
moreover have "(rel_frontier (cbox (- c) c ∩ T)) ⊆ fro d"
apply (subst convex_affine_rel_frontier_Int [OF _  ‹affine T› int_ne])
apply (auto simp: fro_def c_def)
done
ultimately show ?thesis
using dd  by (force simp: disjnt_def)
qed
then have cpt_subset: "closest_point (cbox (- c) c ∩ T) ` (T - K) ⊆ ⋃{bbox ∩ T} - K"
using closest_point_in_set [OF clo_cbT cpT_ne] cbsub(3) by force
show ?thesis
proof (intro conjI ballI exI)
have "continuous_on (T - K) (closest_point (cbox (- c) c ∩ T))"
apply (rule continuous_on_closest_point)
using ‹S ≠ {}› cbsub(2) b that
by (auto simp: affine_imp_convex convex_Int affine_closed closed_Int closed_cbox ‹affine T›)
then show "continuous_on (T - K) (g ∘ closest_point (cbox (- c) c ∩ T))"
by (metis continuous_on_compose continuous_on_subset [OF contg cpt_subset])
have "(g ∘ closest_point (cbox (- c) c ∩ T)) ` (T - K) ⊆ g ` (⋃{bbox ∩ T} - K)"
by (metis image_comp image_mono cpt_subset)
also have "... ⊆ rel_frontier U"
by (rule gim)
finally show "(g ∘ closest_point (cbox (- c) c ∩ T)) ` (T - K) ⊆ rel_frontier U" .
show "(g ∘ closest_point (cbox (- c) c ∩ T)) x = f x" if "x ∈ S" for x
proof -
have "(g ∘ closest_point (cbox (- c) c ∩ T)) x = g x"
unfolding o_def
by (metis IntI ‹S ⊆ T› b cbsub(2) closest_point_self subset_eq that)
also have "... = f x"
finally show ?thesis .
qed
qed (auto simp: K)
qed
then obtain K g where "finite K" "disjnt K S"
and contg: "continuous_on (affine hull T - K) g"
and gim:  "g ` (affine hull T - K) ⊆ rel_frontier U"
and gf:   "⋀x. x ∈ S ⟹ g x = f x"
by (metis aff affine_affine_hull aff_dim_affine_hull
order_trans [OF ‹S ⊆ T› hull_subset [of T affine]])
then obtain K g where "finite K" "disjnt K S"
and contg: "continuous_on (T - K) g"
and gim:  "g ` (T - K) ⊆ rel_frontier U"
and gf:   "⋀x. x ∈ S ⟹ g x = f x"
by (rule_tac K=K and g=g in that) (auto simp: hull_inc elim: continuous_on_subset)
then show ?thesis
by (rule_tac K="K ∩ T" and g=g in that) (auto simp: disjnt_iff Diff_Int contg)
qed

subsection‹Extending maps to spheres›

(*Up to extend_map_affine_to_sphere_cofinite_gen*)

lemma extend_map_affine_to_sphere1:
fixes f :: "'a::euclidean_space ⇒ 'b::topological_space"
assumes "finite K" "affine U" and contf: "continuous_on (U - K) f"
and fim: "f ` (U - K) ⊆ T"
and comps: "⋀C. ⟦C ∈ components(U - S); C ∩ K ≠ {}⟧ ⟹ C ∩ L ≠ {}"
and clo: "closedin (subtopology euclidean U) S" and K: "disjnt K S" "K ⊆ U"
obtains g where "continuous_on (U - L) g" "g ` (U - L) ⊆ T" "⋀x. x ∈ S ⟹ g x = f x"
proof (cases "K = {}")
case True
then show ?thesis
by (metis Diff_empty Diff_subset contf fim continuous_on_subset image_subsetI rev_image_eqI subset_iff that)
next
case False
have "S ⊆ U"
using clo closedin_limpt by blast
then have "(U - S) ∩ K ≠ {}"
by (metis Diff_triv False Int_Diff K disjnt_def inf.absorb_iff2 inf_commute)
then have "⋃(components (U - S)) ∩ K ≠ {}"
using Union_components by simp
then obtain C0 where C0: "C0 ∈ components (U - S)" "C0 ∩ K ≠ {}"
by blast
have "convex U"
by (simp add: affine_imp_convex ‹affine U›)
then have "locally connected U"
by (rule convex_imp_locally_connected)
have "∃a g. a ∈ C ∧ a ∈ L ∧ continuous_on (S ∪ (C - {a})) g ∧
g ` (S ∪ (C - {a})) ⊆ T ∧ (∀x ∈ S. g x = f x)"
if C: "C ∈ components (U - S)" and CK: "C ∩ K ≠ {}" for C
proof -
have "C ⊆ U-S" "C ∩ L ≠ {}"
by (simp_all add: in_components_subset comps that)
then obtain a where a: "a ∈ C" "a ∈ L" by auto
have opeUC: "openin (subtopology euclidean U) C"
proof (rule openin_trans)
show "openin (subtopology euclidean (U-S)) C"
by (simp add: ‹locally connected U› clo locally_diff_closed openin_components_locally_connected [OF _ C])
show "openin (subtopology euclidean U) (U - S)"
qed
then obtain d where "C ⊆ U" "0 < d" and d: "cball a d ∩ U ⊆ C"
using openin_contains_cball by (metis ‹a ∈ C›)
then have "ball a d ∩ U ⊆ C"
by auto
obtain h k where homhk: "homeomorphism (S ∪ C) (S ∪ C) h k"
and subC: "{x. (~ (h x = x ∧ k x = x))} ⊆ C"
and bou: "bounded {x. (~ (h x = x ∧ k x = x))}"
and hin: "⋀x. x ∈ C ∩ K ⟹ h x ∈ ball a d ∩ U"
proof (rule homeomorphism_grouping_points_exists_gen [of C "ball a d ∩ U" "C ∩ K" "S ∪ C"])
show "openin (subtopology euclidean C) (ball a d ∩ U)"
by (metis open_ball ‹C ⊆ U› ‹ball a d ∩ U ⊆ C› inf.absorb_iff2 inf.orderE inf_assoc open_openin openin_subtopology)
show "openin (subtopology euclidean (affine hull C)) C"
by (metis ‹a ∈ C› ‹openin (subtopology euclidean U) C› affine_hull_eq affine_hull_openin all_not_in_conv ‹affine U›)
show "ball a d ∩ U ≠ {}"
using ‹0 < d› ‹C ⊆ U› ‹a ∈ C› by force
show "finite (C ∩ K)"
show "S ∪ C ⊆ affine hull C"
by (metis ‹C ⊆ U› ‹S ⊆ U› ‹a ∈ C› opeUC affine_hull_eq affine_hull_openin all_not_in_conv assms(2) sup.bounded_iff)
show "connected C"
by (metis C in_components_connected)
qed auto
have a_BU: "a ∈ ball a d ∩ U"
using ‹0 < d› ‹C ⊆ U› ‹a ∈ C› by auto
have "rel_frontier (cball a d ∩ U) retract_of (affine hull (cball a d ∩ U) - {a})"
apply (rule rel_frontier_retract_of_punctured_affine_hull)
apply (auto simp: ‹convex U› convex_Int)
by (metis ‹affine U› convex_cball empty_iff interior_cball a_BU rel_interior_convex_Int_affine)
moreover have "rel_frontier (cball a d ∩ U) = frontier (cball a d) ∩ U"
apply (rule convex_affine_rel_frontier_Int)
using a_BU by (force simp: ‹affine U›)+
moreover have "affine hull (cball a d ∩ U) = U"
by (metis ‹convex U› a_BU affine_hull_convex_Int_nonempty_interior affine_hull_eq ‹affine U› equals0D inf.commute interior_cball)
ultimately have "frontier (cball a d) ∩ U retract_of (U - {a})"
by metis
then obtain r where contr: "continuous_on (U - {a}) r"
and rim: "r ` (U - {a}) ⊆ sphere a d"  "r ` (U - {a}) ⊆ U"
and req: "⋀x. x ∈ sphere a d ∩ U ⟹ r x = x"
using ‹affine U› by (auto simp: retract_of_def retraction_def hull_same)
define j where "j ≡ λx. if x ∈ ball a d then r x else x"
have kj: "⋀x. x ∈ S ⟹ k (j x) = x"
using ‹C ⊆ U - S› ‹S ⊆ U› ‹ball a d ∩ U ⊆ C› j_def subC by auto
have Uaeq: "U - {a} = (cball a d - {a}) ∩ U ∪ (U - ball a d)"
using ‹0 < d› by auto
have jim: "j ` (S ∪ (C - {a})) ⊆ (S ∪ C) - ball a d"
proof clarify
fix y  assume "y ∈ S ∪ (C - {a})"
then have "y ∈ U - {a}"
using ‹C ⊆ U - S› ‹S ⊆ U› ‹a ∈ C› by auto
then have "r y ∈ sphere a d"
using rim by auto
then show "j y ∈ S ∪ C - ball a d"
using ‹r y ∈ sphere a d› ‹y ∈ U - {a}› ‹y ∈ S ∪ (C - {a})› d rim by fastforce
qed
have contj: "continuous_on (U - {a}) j"
unfolding j_def Uaeq
proof (intro continuous_on_cases_local continuous_on_id, simp_all add: req closedin_closed Uaeq [symmetric])
show "∃T. closed T ∧ (cball a d - {a}) ∩ U = (U - {a}) ∩ T"
apply (rule_tac x="(cball a d) ∩ U" in exI)
using affine_closed ‹affine U› by blast
show "∃T. closed T ∧ U - ball a d = (U - {a}) ∩ T"
apply (rule_tac x="U - ball a d" in exI)
using ‹0 < d›  by (force simp: affine_closed ‹affine U› closed_Diff)
show "continuous_on ((cball a d - {a}) ∩ U) r"
by (force intro: continuous_on_subset [OF contr])
qed
have fT: "x ∈ U - K ⟹ f x ∈ T" for x
using fim by blast
show ?thesis
proof (intro conjI exI)
show "continuous_on (S ∪ (C - {a})) (f ∘ k ∘ j)"
proof (intro continuous_on_compose)
show "continuous_on (S ∪ (C - {a})) j"
apply (rule continuous_on_subset [OF contj])
using ‹C ⊆ U - S› ‹S ⊆ U› ‹a ∈ C› by force
show "continuous_on (j ` (S ∪ (C - {a}))) k"
apply (rule continuous_on_subset [OF homeomorphism_cont2 [OF homhk]])
using jim ‹C ⊆ U - S› ‹S ⊆ U› ‹ball a d ∩ U ⊆ C› j_def by fastforce
show "continuous_on (k ` j ` (S ∪ (C - {a}))) f"
proof (clarify intro!: continuous_on_subset [OF contf])
fix y  assume "y ∈ S ∪ (C - {a})"
have ky: "k y ∈ S ∪ C"
using homeomorphism_image2 [OF homhk] ‹y ∈ S ∪ (C - {a})› by blast
have jy: "j y ∈ S ∪ C - ball a d"
using Un_iff ‹y ∈ S ∪ (C - {a})› jim by auto
show "k (j y) ∈ U - K"
apply safe
using ‹C ⊆ U› ‹S ⊆ U›  homeomorphism_image2 [OF homhk] jy apply blast
by (metis DiffD1 DiffD2 Int_iff Un_iff ‹disjnt K S› disjnt_def empty_iff hin homeomorphism_apply2 homeomorphism_image2 homhk imageI jy)
qed
qed
have ST: "⋀x. x ∈ S ⟹ (f ∘ k ∘ j) x ∈ T"
apply (metis DiffI ‹S ⊆ U› ‹disjnt K S› subsetD disjnt_iff fim image_subset_iff)
done
moreover have "(f ∘ k ∘ j) x ∈ T" if "x ∈ C" "x ≠ a" "x ∉ S" for x
proof -
have rx: "r x ∈ sphere a d"
using ‹C ⊆ U› rim that by fastforce
have jj: "j x ∈ S ∪ C - ball a d"
using jim that by blast
have "k (j x) = j x ⟶ k (j x) ∈ C ∨ j x ∈ C"
by (metis Diff_iff Int_iff Un_iff ‹S ⊆ U› subsetD d j_def jj rx sphere_cball that(1))
then have "k (j x) ∈ C"
using homeomorphism_apply2 [OF homhk, of "j x"]   ‹C ⊆ U› ‹S ⊆ U› a rx
by (metis (mono_tags, lifting) Diff_iff subsetD jj mem_Collect_eq subC)
with jj ‹C ⊆ U› show ?thesis
apply safe
using ST j_def apply fastforce
apply (auto simp: not_less intro!: fT)
by (metis DiffD1 DiffD2 Int_iff hin homeomorphism_apply2 [OF homhk] jj)
qed
ultimately show "(f ∘ k ∘ j) ` (S ∪ (C - {a})) ⊆ T"
by force
show "∀x∈S. (f ∘ k ∘ j) x = f x" using kj by simp
qed (auto simp: a)
qed
then obtain a h where
ah: "⋀C. ⟦C ∈ components (U - S); C ∩ K ≠ {}⟧
⟹ a C ∈ C ∧ a C ∈ L ∧ continuous_on (S ∪ (C - {a C})) (h C) ∧
h C ` (S ∪ (C - {a C})) ⊆ T ∧ (∀x ∈ S. h C x = f x)"
using that by metis
define F where "F ≡ {C ∈ components (U - S). C ∩ K ≠ {}}"
define G where "G ≡ {C ∈ components (U - S). C ∩ K = {}}"
define UF where "UF ≡ (⋃C∈F. C - {a C})"
have "C0 ∈ F"
by (auto simp: F_def C0)
have "finite F"
proof (subst finite_image_iff [of "λC. C ∩ K" F, symmetric])
show "inj_on (λC. C ∩ K) F"
unfolding F_def inj_on_def
using components_nonoverlap by blast
show "finite ((λC. C ∩ K) ` F)"
unfolding F_def
by (rule finite_subset [of _ "Pow K"]) (auto simp: ‹finite K›)
qed
obtain g where contg: "continuous_on (S ∪ UF) g"
and gh: "⋀x i. ⟦i ∈ F; x ∈ (S ∪ UF) ∩ (S ∪ (i - {a i}))⟧
⟹ g x = h i x"
proof (rule pasting_lemma_exists_closed [OF ‹finite F›, of "S ∪ UF" "λC. S ∪ (C - {a C})" h])
show "S ∪ UF ⊆ (⋃C∈F. S ∪ (C - {a C}))"
using ‹C0 ∈ F› by (force simp: UF_def)
show "closedin (subtopology euclidean (S ∪ UF)) (S ∪ (C - {a C}))"
if "C ∈ F" for C
proof (rule closedin_closed_subset [of U "S ∪ C"])
show "closedin (subtopology euclidean U) (S ∪ C)"
apply (rule closedin_Un_complement_component [OF ‹locally connected U› clo])
using F_def that by blast
next
have "x = a C'" if "C' ∈ F"  "x ∈ C'" "x ∉ U" for x C'
proof -
have "∀A. x ∈ ⋃A ∨ C' ∉ A"
using ‹x ∈ C'› by blast
with that show "x = a C'"
by (metis (lifting) DiffD1 F_def Union_components mem_Collect_eq)
qed
then show "S ∪ UF ⊆ U"
using ‹S ⊆ U› by (force simp: UF_def)
next
show "S ∪ (C - {a C}) = (S ∪ C) ∩ (S ∪ UF)"
using F_def UF_def components_nonoverlap that by auto
qed
next
show "continuous_on (S ∪ (C' - {a C'})) (h C')" if "C' ∈ F" for C'
using ah F_def that by blast
show "⋀i j x. ⟦i ∈ F; j ∈ F;
x ∈ (S ∪ UF) ∩ (S ∪ (i - {a i})) ∩ (S ∪ (j - {a j}))⟧
⟹ h i x = h j x"
using components_eq by (fastforce simp: components_eq F_def ah)
qed blast
have SU': "S ∪ ⋃G ∪ (S ∪ UF) ⊆ U"
using ‹S ⊆ U› in_components_subset by (auto simp: F_def G_def UF_def)
have clo1: "closedin (subtopology euclidean (S ∪ ⋃G ∪ (S ∪ UF))) (S ∪ ⋃G)"
proof (rule closedin_closed_subset [OF _ SU'])
have *: "⋀C. C ∈ F ⟹ openin (subtopology euclidean U) C"
unfolding F_def
by clarify (metis (no_types, lifting) ‹locally connected U› clo closedin_def locally_diff_closed openin_components_locally_connected openin_trans topspace_euclidean_subtopology)
show "closedin (subtopology euclidean U) (U - UF)"
unfolding UF_def
by (force intro: openin_delete *)
show "S ∪ ⋃G = (U - UF) ∩ (S ∪ ⋃G ∪ (S ∪ UF))"
using ‹S ⊆ U› apply (auto simp: F_def G_def UF_def)
apply (metis Diff_iff UnionI Union_components)
apply (metis DiffD1 UnionI Union_components)
by (metis (no_types, lifting) IntI components_nonoverlap empty_iff)
qed
have clo2: "closedin (subtopology euclidean (S ∪ ⋃G ∪ (S ∪ UF))) (S ∪ UF)"
proof (rule closedin_closed_subset [OF _ SU'])
show "closedin (subtopology euclidean U) (⋃C∈F. S ∪ C)"
apply (rule closedin_Union)
using F_def ‹locally connected U› clo closedin_Un_complement_component by blast
show "S ∪ UF = (⋃C∈F. S ∪ C) ∩ (S ∪ ⋃G ∪ (S ∪ UF))"
using ‹S ⊆ U› apply (auto simp: F_def G_def UF_def)
using C0 apply blast
by (metis components_nonoverlap disjnt_def disjnt_iff)
qed
have SUG: "S ∪ ⋃G ⊆ U - K"
using ‹S ⊆ U› K apply (auto simp: G_def disjnt_iff)
by (meson Diff_iff subsetD in_components_subset)
then have contf': "continuous_on (S ∪ ⋃G) f"
by (rule continuous_on_subset [OF contf])
have contg': "continuous_on (S ∪ UF) g"
apply (rule continuous_on_subset [OF contg])
using ‹S ⊆ U› by (auto simp: F_def G_def)
have  "⋀x. ⟦S ⊆ U; x ∈ S⟧ ⟹ f x = g x"
by (subst gh) (auto simp: ah C0 intro: ‹C0 ∈ F›)
then have f_eq_g: "⋀x. x ∈ S ∪ UF ∧ x ∈ S ∪ ⋃G ⟹ f x = g x"
using ‹S ⊆ U› apply (auto simp: F_def G_def UF_def dest: in_components_subset)
using components_eq by blast
have cont: "continuous_on (S ∪ ⋃G ∪ (S ∪ UF)) (λx. if x ∈ S ∪ ⋃G then f x else g x)"
by (blast intro: continuous_on_cases_local [OF clo1 clo2 contf' contg' f_eq_g, of "λx. x ∈ S ∪ ⋃G"])
show ?thesis
proof
have UF: "⋃F - L ⊆ UF"
unfolding F_def UF_def using ah by blast
have "U - S - L = ⋃(components (U - S)) - L"
by simp
also have "... = ⋃F ∪ ⋃G - L"
unfolding F_def G_def by blast
also have "... ⊆ UF ∪ ⋃G"
using UF by blast
finally have "U - L ⊆ S ∪ ⋃G ∪ (S ∪ UF)"
by blast
then show "continuous_on (U - L) (λx. if x ∈ S ∪ ⋃G then f x else g x)"
by (rule continuous_on_subset [OF cont])
have "((U - L) ∩ {x. x ∉ S ∧ (∀xa∈G. x ∉ xa)}) ⊆  ((U - L) ∩ (-S ∩ UF))"
using ‹U - L ⊆ S ∪ ⋃G ∪ (S ∪ UF)› by auto
moreover have "g ` ((U - L) ∩ (-S ∩ UF)) ⊆ T"
proof -
have "g x ∈ T" if "x ∈ U" "x ∉ L" "x ∉ S" "C ∈ F" "x ∈ C" "x ≠ a C" for x C
proof (subst gh)
show "x ∈ (S ∪ UF) ∩ (S ∪ (C - {a C}))"
using that by (auto simp: UF_def)
show "h C x ∈ T"
using ah that by (fastforce simp add: F_def)
qed (rule that)
then show ?thesis
by (force simp: UF_def)
qed
ultimately have "g ` ((U - L) ∩ {x. x ∉ S ∧ (∀xa∈G. x ∉ xa)}) ⊆ T"
using image_mono order_trans by blast
moreover have "f ` ((U - L) ∩ (S ∪ ⋃G)) ⊆ T"
using fim SUG by blast
ultimately show "(λx. if x ∈ S ∪ ⋃G then f x else g x) ` (U - L) ⊆ T"
by force
show "⋀x. x ∈ S ⟹ (if x ∈ S ∪ ⋃G then f x else g x) = f x"
qed
qed

lemma extend_map_affine_to_sphere2:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "compact S" "convex U" "bounded U" "affine T" "S ⊆ T"
and affTU: "aff_dim T ≤ aff_dim U"
and contf: "continuous_on S f"
and fim: "f ` S ⊆ rel_frontier U"
and ovlap: "⋀C. C ∈ components(T - S) ⟹ C ∩ L ≠ {}"
obtains K g where "finite K" "K ⊆ L" "K ⊆ T" "disjnt K S"
"continuous_on (T - K) g" "g ` (T - K) ⊆ rel_frontier U"
"⋀x. x ∈ S ⟹ g x = f x"
proof -
obtain K g where K: "finite K" "K ⊆ T" "disjnt K S"
and contg: "continuous_on (T - K) g"
and gim: "g ` (T - K) ⊆ rel_frontier U"
and gf: "⋀x. x ∈ S ⟹ g x = f x"
using assms extend_map_affine_to_sphere_cofinite_simple by metis
have "(∃y C. C ∈ components (T - S) ∧ x ∈ C ∧ y ∈ C ∧ y ∈ L)" if "x ∈ K" for x
proof -
have "x ∈ T-S"
using ‹K ⊆ T› ‹disjnt K S› disjnt_def that by fastforce
then obtain C where "C ∈ components(T - S)" "x ∈ C"
by (metis UnionE Union_components)
with ovlap [of C] show ?thesis
by blast
qed
then obtain ξ where ξ: "⋀x. x ∈ K ⟹ ∃C. C ∈ components (T - S) ∧ x ∈ C ∧ ξ x ∈ C ∧ ξ x ∈ L"
by metis
obtain h where conth: "continuous_on (T - ξ ` K) h"
and him: "h ` (T - ξ ` K) ⊆ rel_frontier U"
and hg: "⋀x. x ∈ S ⟹ h x = g x"
proof (rule extend_map_affine_to_sphere1 [OF ‹finite K› ‹affine T› contg gim, of S "ξ ` K"])
show cloTS: "closedin (subtopology euclidean T) S"
by (simp add: ‹compact S› ‹S ⊆ T› closed_subset compact_imp_closed)
show "⋀C. ⟦C ∈ components (T - S); C ∩ K ≠ {}⟧ ⟹ C ∩ ξ ` K ≠ {}"
using ξ components_eq by blast
qed (use K in auto)
show ?thesis
proof
show *: "ξ ` K ⊆ L"
using ξ by blast
show "finite (ξ ` K)"
show "ξ ` K ⊆ T"
by clarify (meson ξ Diff_iff contra_subsetD in_components_subset)
show "continuous_on (T - ξ ` K) h"
by (rule conth)
show "disjnt (ξ ` K) S"
using K
apply (auto simp: disjnt_def)
by (metis ξ DiffD2 UnionI Union_components)
qed (simp_all add: him hg gf)
qed

proposition extend_map_affine_to_sphere_cofinite_gen:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes SUT: "compact S" "convex U" "bounded U" "affine T" "S ⊆ T"
and aff: "aff_dim T ≤ aff_dim U"
and contf: "continuous_on S f"
and fim: "f ` S ⊆ rel_frontier U"
and dis: "⋀C. ⟦C ∈ components(T - S); bounded C⟧ ⟹ C ∩ L ≠ {}"
obtains K g where "finite K" "K ⊆ L" "K ⊆ T" "disjnt K S" "continuous_on (T - K) g"
"g ` (T - K) ⊆ rel_frontier U"
"⋀x. x ∈ S ⟹ g x = f x"
proof (cases "S = {}")
case True
show ?thesis
proof (cases "rel_frontier U = {}")
case True
with aff have "aff_dim T ≤ 0"
using affine_bounded_eq_lowdim ‹bounded U› order_trans by auto
with aff_dim_geq [of T] consider "aff_dim T = -1" |  "aff_dim T = 0"
by linarith
then show ?thesis
proof cases
assume "aff_dim T = -1"
then have "T = {}"
then show ?thesis
by (rule_tac K="{}" in that) auto
next
assume "aff_dim T = 0"
then obtain a where "T = {a}"
using aff_dim_eq_0 by blast
then have "a ∈ L"
using dis [of "{a}"] ‹S = {}› by (auto simp: in_components_self)
with ‹S = {}› ‹T = {a}› show ?thesis
by (rule_tac K="{a}" and g=f in that) auto
qed
next
case False
then obtain y where "y ∈ rel_frontier U"
by auto
with ‹S = {}› show ?thesis
by (rule_tac K="{}" and g="λx. y" in that)  (auto simp: continuous_on_const)
qed
next
case False
have "bounded S"
then obtain b where b: "S ⊆ cbox (-b) b"
using bounded_subset_cbox_symmetric by blast
define LU where "LU ≡ L ∪ (⋃ {C ∈ components (T - S). ~bounded C} - cbox (-(b+One)) (b+One))"
obtain K g where "finite K" "K ⊆ LU" "K ⊆ T" "disjnt K S"
and contg: "continuous_on (T - K) g"
and gim: "g ` (T - K) ⊆ rel_frontier U"
and gf:  "⋀x. x ∈ S ⟹ g x = f x"
proof (rule extend_map_affine_to_sphere2 [OF SUT aff contf fim])
show "C ∩ LU ≠ {}" if "C ∈ components (T - S)" for C
proof (cases "bounded C")
case True
with dis that show ?thesis
unfolding LU_def by fastforce
next
case False
then have "¬ bounded (⋃{C ∈ components (T - S). ¬ bounded C})"
by (metis (no_types, lifting) Sup_upper bounded_subset mem_Collect_eq that)
then show ?thesis
apply (clarsimp simp: LU_def Int_Un_distrib Diff_Int_distrib Int_UN_distrib)
by (metis (no_types, lifting) False Sup_upper bounded_cbox bounded_subset inf.orderE mem_Collect_eq that)
qed
qed blast
have *: False if "x ∈ cbox (- b - m *⇩R One) (b + m *⇩R One)"
"x ∉ box (- b - n *⇩R One) (b + n *⇩R One)"
"0 ≤ m" "m < n" "n ≤ 1" for m n x
using that by (auto simp: mem_box algebra_simps)
have "disjoint_family_on (λd. frontier (cbox (- b - d *⇩R One) (b + d *⇩R One))) {1 / 2..1}"
by (auto simp: disjoint_family_on_def neq_iff frontier_def dest: *)
then obtain d where d12: "1/2 ≤ d" "d ≤ 1"
and ddis: "disjnt K (frontier (cbox (-(b + d *⇩R One)) (b + d *⇩R One)))"
using disjoint_family_elem_disjnt [of "{1/2..1::real}" K "λd. frontier (cbox (-(b + d *⇩R One)) (b + d *⇩R One))"]
by (auto simp: ‹finite K›)
define c where "c ≡ b + d *⇩R One"
have cbsub: "cbox (-b) b ⊆ box (-c) c"
"cbox (-b) b ⊆ cbox (-c) c"
"cbox (-c) c ⊆ cbox (-(b+One)) (b+One)"
using d12 by (simp_all add: subset_box c_def inner_diff_left inner_left_distrib)
have clo_cT: "closed (cbox (- c) c ∩ T)"
using affine_closed ‹affine T› by blast
have cT_ne: "cbox (- c) c ∩ T ≠ {}"
using ‹S ≠ {}› ‹S ⊆ T› b cbsub by fastforce
have S_sub_cc: "S ⊆ cbox (- c) c"
using ‹cbox (- b) b ⊆ cbox (- c) c› b by auto
show ?thesis
proof
show "finite (K ∩ cbox (-(b+One)) (b+One))"
using ‹finite K› by blast
show "K ∩ cbox (- (b + One)) (b + One) ⊆ L"
using ‹K ⊆ LU› by (auto simp: LU_def)
show "K ∩ cbox (- (b + One)) (b + One) ⊆ T"
using ‹K ⊆ T› by auto
show "disjnt (K ∩ cbox (- (b + One)) (b + One)) S"
using ‹disjnt K S›  by (simp add: disjnt_def disjoint_eq_subset_Compl inf.coboundedI1)
have cloTK: "closest_point (cbox (- c) c ∩ T) x ∈ T - K"
if "x ∈ T" and Knot: "x ∈ K ⟶ x ∉ cbox (- b - One) (b + One)" for x
proof (cases "x ∈ cbox (- c) c")
case True
with ‹x ∈ T› show ?thesis
using cbsub(3) Knot  by (force simp: closest_point_self)
next
case False
have clo_in_rf: "closest_point (cbox (- c) c ∩ T) x ∈ rel_frontier (cbox (- c) c ∩ T)"
proof (intro closest_point_in_rel_frontier [OF clo_cT cT_ne] DiffI notI)
have "T ∩ interior (cbox (- c) c) ≠ {}"
using ‹S ≠ {}› ‹S ⊆ T› b cbsub(1) by fastforce
then show "x ∈ affine hull (cbox (- c) c ∩ T)"
by (simp add: Int_commute affine_hull_affine_Int_nonempty_interior ‹affine T› hull_inc that(1))
next
show "False" if "x ∈ rel_interior (cbox (- c) c ∩ T)"
proof -
have "interior (cbox (- c) c) ∩ T ≠ {}"
using ‹S ≠ {}› ‹S ⊆ T› b cbsub(1) by fastforce
then have "affine hull (T ∩ cbox (- c) c) = T"
using affine_hull_convex_Int_nonempty_interior [of T "cbox (- c) c"]
by (simp add: affine_imp_convex ‹affine T› inf_commute)
then show ?thesis
by (meson subsetD le_inf_iff rel_interior_subset that False)
qed
qed
have "closest_point (cbox (- c) c ∩ T) x ∉ K"
proof
assume inK: "closest_point (cbox (- c) c ∩ T) x ∈ K"
have "⋀x. x ∈ K ⟹ x ∉ frontier (cbox (- (b + d *⇩R One)) (b + d *⇩R One))"
by (metis ddis disjnt_iff)
then show False
by (metis DiffI Int_iff ‹affine T› cT_ne c_def clo_cT clo_in_rf closest_point_in_set
convex_affine_rel_frontier_Int convex_box(1) empty_iff frontier_cbox inK interior_cbox)
qed
then show ?thesis
using cT_ne clo_cT closest_point_in_set by blast
qed
show "continuous_on (T - K ∩ cbox (- (b + One)) (b + One)) (g ∘ closest_point (cbox (-c) c ∩ T))"
apply (intro continuous_on_compose continuous_on_closest_point continuous_on_subset [OF contg])
apply (simp_all add: clo_cT affine_imp_convex ‹affine T› convex_Int cT_ne)
using cloTK by blast
have "g (closest_point (cbox (- c) c ∩ T) x) ∈ rel_frontier U"
if "x ∈ T" "x ∈ K ⟶ x ∉ cbox (- b - One) (b + One)" for x
apply (rule gim [THEN subsetD])
using that cloTK by blast
then show "(g ∘ closest_point (cbox (- c) c ∩ T)) ` (T - K ∩ cbox (- (b + One)) (b + One))
⊆ rel_frontier U"
by force
show "⋀x. x ∈ S ⟹ (g ∘ closest_point (cbox (- c) c ∩ T)) x = f x"
by simp (metis (mono_tags, lifting) IntI ‹S ⊆ T› cT_ne clo_cT closest_point_refl gf subsetD S_sub_cc)
qed
qed

corollary extend_map_affine_to_sphere_cofinite:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes SUT: "compact S" "affine T" "S ⊆ T"
and aff: "aff_dim T ≤ DIM('b)" and "0 ≤ r"
and contf: "continuous_on S f"
and fim: "f ` S ⊆ sphere a r"
and dis: "⋀C. ⟦C ∈ components(T - S); bounded C⟧ ⟹ C ∩ L ≠ {}"
obtains K g where "finite K" "K ⊆ L" "K ⊆ T" "disjnt K S" "continuous_on (T - K) g"
"g ` (T - K) ⊆ sphere a r" "⋀x. x ∈ S ⟹ g x = f x"
proof (cases "r = 0")
case True
with fim show ?thesis
by (rule_tac K="{}" and g = "λx. a" in that) (auto simp: continuous_on_const)
next
case False
with assms have "0 < r" by auto
then have "aff_dim T ≤ aff_dim (cball a r)"
then show ?thesis
apply (rule extend_map_affine_to_sphere_cofinite_gen
[OF ‹compact S› convex_cball bounded_cball ‹affine T› ‹S ⊆ T› _ contf])
using fim apply (auto simp: assms False that dest: dis)
done
qed

corollary extend_map_UNIV_to_sphere_cofinite:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes aff: "DIM('a) ≤ DIM('b)" and "0 ≤ r"
and SUT: "compact S"
and contf: "continuous_on S f"
and fim: "f ` S ⊆ sphere a r"
and dis: "⋀C. ⟦C ∈ components(- S); bounded C⟧ ⟹ C ∩ L ≠ {}"
obtains K g where "finite K" "K ⊆ L" "disjnt K S" "continuous_on (- K) g"
"g ` (- K) ⊆ sphere a r" "⋀x. x ∈ S ⟹ g x = f x"
apply (rule extend_map_affine_to_sphere_cofinite
[OF ‹compact S› affine_UNIV subset_UNIV _ ‹0 ≤ r› contf fim dis])
apply (auto simp: assms that Compl_eq_Diff_UNIV [symmetric])
done

corollary extend_map_UNIV_to_sphere_no_bounded_component:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes aff: "DIM('a) ≤ DIM('b)" and "0 ≤ r"
and SUT: "compact S"
and contf: "continuous_on S f"
and fim: "f ` S ⊆ sphere a r"
and dis: "⋀C. C ∈ components(- S) ⟹ ¬ bounded C"
obtains g where "continuous_on UNIV g" "g ` UNIV ⊆ sphere a r" "⋀x. x ∈ S ⟹ g x = f x"
apply (rule extend_map_UNIV_to_sphere_cofinite [OF aff ‹0 ≤ r› ‹compact S› contf fim, of "{}"])
apply (auto simp: that dest: dis)
done

theorem Borsuk_separation_theorem_gen:
fixes S :: "'a::euclidean_space set"
assumes "compact S"
shows "(∀c ∈ components(- S). ~bounded c) ⟷
(∀f. continuous_on S f ∧ f ` S ⊆ sphere (0::'a) 1
⟶ (∃c. homotopic_with (λx. True) S (sphere 0 1) f (λx. c)))"
(is "?lhs = ?rhs")
proof
assume L [rule_format]: ?lhs
show ?rhs
proof clarify
fix f :: "'a ⇒ 'a"
assume contf: "continuous_on S f" and fim: "f ` S ⊆ sphere 0 1"
obtain g where contg: "continuous_on UNIV g" and gim: "range g ⊆ sphere 0 1"
and gf: "⋀x. x ∈ S ⟹ g x = f x"
by (rule extend_map_UNIV_to_sphere_no_bounded_component [OF _ _ ‹compact S› contf fim L]) auto
then show "∃c. homotopic_with (λx. True) S (sphere 0 1) f (λx. c)"
using nullhomotopic_from_contractible [OF contg gim]
by (metis assms compact_imp_closed contf empty_iff fim homotopic_with_equal nullhomotopic_into_sphere_extension)
qed
next
assume R [rule_format]: ?rhs
show ?lhs
unfolding components_def
proof clarify
fix a
assume "a ∉ S" and a: "bounded (connected_component_set (- S) a)"
have cont: "continuous_on S (λx. inverse(norm(x - a)) *⇩R (x - a))"
apply (intro continuous_intros)
using ‹a ∉ S› by auto
have im: "(λx. inverse(norm(x - a)) *⇩R (x - a)) ` S ⊆ sphere 0 1"
by clarsimp (metis ‹a ∉ S› eq_iff_diff_eq_0 left_inverse norm_eq_zero)
show False
using R cont im Borsuk_map_essential_bounded_component [OF ‹compact S› ‹a ∉ S›] a by blast
qed
qed

corollary Borsuk_separation_theorem:
fixes S :: "'a::euclidean_space set"
assumes "compact S" and 2: "2 ≤ DIM('a)"
shows "connected(- S) ⟷
(∀f. continuous_on S f ∧ f ` S ⊆ sphere (0::'a) 1
⟶ (∃c. homotopic_with (λx. True) S (sphere 0 1) f (λx. c)))"
(is "?lhs = ?rhs")
proof
assume L: ?lhs
show ?rhs
proof (cases "S = {}")
case True
then show ?thesis by auto
next
case False
then have "(∀c∈components (- S). ¬ bounded c)"
by (metis L assms(1) bounded_empty cobounded_imp_unbounded compact_imp_bounded in_components_maximal order_refl)
then show ?thesis
by (simp add: Borsuk_separation_theorem_gen [OF ‹compact S›])
qed
next
assume R: ?rhs
then show ?lhs
apply (simp add: Borsuk_separation_theorem_gen [OF ‹compact S›, symmetric])
apply (auto simp: components_def connected_iff_eq_connected_component_set)
using connected_component_in apply fastforce
using cobounded_unique_unbounded_component [OF _ 2, of "-S"] ‹compact S› compact_eq_bounded_closed by fastforce
qed

lemma homotopy_eqv_separation:
fixes S :: "'a::euclidean_space set" and T :: "'a set"
assumes "S homotopy_eqv T" and "compact S" and "compact T"
shows "connected(- S) ⟷ connected(- T)"
proof -
consider "DIM('a) = 1" | "2 ≤ DIM('a)"
by (metis DIM_ge_Suc0 One_nat_def Suc_1 dual_order.antisym not_less_eq_eq)
then show ?thesis
proof cases
case 1
then show ?thesis
using bounded_connected_Compl_1 compact_imp_bounded homotopy_eqv_empty1 homotopy_eqv_empty2 assms by metis
next
case 2
with assms show ?thesis
qed
qed

lemma Jordan_Brouwer_separation:
fixes S :: "'a::euclidean_space set" and a::'a
assumes hom: "S homeomorphic sphere a r" and "0 < r"
shows "¬ connected(- S)"
proof -
have "- sphere a r ∩ ball a r ≠ {}"
using ‹0 < r› by (simp add: Int_absorb1 subset_eq)
moreover
have eq: "- sphere a r - ball a r = - cball a r"
by auto
have "- cball a r ≠ {}"
proof -
have "frontier (cball a r) ≠ {}"
using ‹0 < r› by auto
then show ?thesis
by (metis frontier_complement frontier_empty)
qed
with eq have "- sphere a r - ball a r ≠ {}"
by auto
moreover
have "connected (- S) = connected (- sphere a r)"
proof (rule homotopy_eqv_separation)
show "S homotopy_eqv sphere a r"
using hom homeomorphic_imp_homotopy_eqv by blast
show "compact (sphere a r)"
by simp
then show " compact S"
using hom homeomorphic_compactness by blast
qed
ultimately show ?thesis
using connected_Int_frontier [of "- sphere a r" "ball a r"] by (auto simp: ‹0 < r›)
qed

lemma Jordan_Brouwer_frontier:
fixes S :: "'a::euclidean_space set" and a::'a
assumes S: "S homeomorphic sphere a r" and T: "T ∈ components(- S)" and 2: "2 ≤ DIM('a)"
shows "frontier T = S"
proof (cases r rule: linorder_cases)
assume "r < 0"
with S T show ?thesis by auto
next
assume "r = 0"
with S T card_eq_SucD obtain b where "S = {b}"
by (auto simp: homeomorphic_finite [of "{a}" S])
have "components (- {b}) = { -{b}}"
using T ‹S = {b}› by (auto simp: components_eq_sing_iff connected_punctured_universe 2)
with T show ?thesis
by (metis ‹S = {b}› cball_trivial frontier_cball frontier_complement singletonD sphere_trivial)
next
assume "r > 0"
have "compact S"
using homeomorphic_compactness compact_sphere S by blast
show ?thesis
proof (rule frontier_minimal_separating_closed)
show "closed S"
using ‹compact S› compact_eq_bounded_closed by blast
show "¬ connected (- S)"
using Jordan_Brouwer_separation S ‹0 < r› by blast
obtain f g where hom: "homeomorphism S (sphere a r) f g"
using S by (auto simp: homeomorphic_def)
show "connected (- T)" if "closed T" "T ⊂ S" for T
proof -
have "f ` T ⊆ sphere a r"
using ‹T ⊂ S› hom homeomorphism_image1 by blast
moreover have "f ` T ≠ sphere a r"
using ‹T ⊂ S› hom
by (metis homeomorphism_image2 homeomorphism_of_subsets order_refl psubsetE)
ultimately have "f ` T ⊂ sphere a r" by blast
then have "connected (- f ` T)"
by (rule psubset_sphere_Compl_connected [OF _ ‹0 < r› 2])
moreover have "compact T"
using ‹compact S› bounded_subset compact_eq_bounded_closed that by blast
moreover then have "compact (f ` T)"
by (meson compact_continuous_image continuous_on_subset hom homeomorphism_def psubsetE ‹T ⊂ S›)
moreover have "T homotopy_eqv f ` T"
by (meson ‹f ` T ⊆ sphere a r› dual_order.strict_implies_order hom homeomorphic_def homeomorphic_imp_homotopy_eqv homeomorphism_of_subsets ‹T ⊂ S›)
ultimately show ?thesis
using homotopy_eqv_separation [of T "f`T"] by blast
qed
qed (rule T)
qed

lemma Jordan_Brouwer_nonseparation:
fixes S :: "'a::euclidean_space set" and a::'a
assumes S: "S homeomorphic sphere a r" and "T ⊂ S" and 2: "2 ≤ DIM('a)"
shows "connected(- T)"
proof -
have *: "connected(C ∪ (S - T))" if "C ∈ components(- S)" for C
proof (rule connected_intermediate_closure)
show "connected C"
using in_components_connected that by auto
have "S = frontier C"
using "2" Jordan_Brouwer_frontier S that by blast
with closure_subset show "C ∪ (S - T) ⊆ closure C"
by (auto simp: frontier_def)
qed auto
have "components(- S) ≠ {}"
by (metis S bounded_empty cobounded_imp_unbounded compact_eq_bounded_closed compact_sphere
components_eq_empty homeomorphic_compactness)
then have "- T = (⋃C ∈ components(- S). C ∪ (S - T))"
using Union_components [of "-S"] ‹T ⊂ S› by auto
then show ?thesis
apply (rule ssubst)
apply (rule connected_Union)
using ‹T ⊂ S› apply (auto simp: *)
done
qed

subsection‹ Invariance of domain and corollaries›

lemma invariance_of_domain_ball:
fixes f :: "'a ⇒ 'a::euclidean_space"
assumes contf: "continuous_on (cball a r) f" and "0 < r"
and inj: "inj_on f (cball a r)"
shows "open(f ` ball a r)"
proof (cases "DIM('a) = 1")
case True
obtain h::"'a⇒real" and k
where "linear h" "linear k" "h ` UNIV = UNIV" "k ` UNIV = UNIV"
"⋀x. norm(h x) = norm x" "⋀x. norm(k x) = norm x"
"⋀x. k(h x) = x" "⋀x. h(k x) = x"
apply (rule isomorphisms_UNIV_UNIV [where 'M='a and 'N=real])
using True
apply force
by (metis UNIV_I UNIV_eq_I imageI)
have cont: "continuous_on S h"  "continuous_on T k" for S T
by (simp_all add: ‹linear h› ‹linear k› linear_continuous_on linear_linear)
have "continuous_on (h ` cball a r) (h ∘ f ∘ k)"
apply (intro continuous_on_compose cont continuous_on_subset [OF contf])
apply (auto simp: ‹⋀x. k (h x) = x›)
done
moreover have "is_interval (h ` cball a r)"
by (simp add: is_interval_connected_1 ‹linear h› linear_continuous_on linear_linear connected_continuous_image)
moreover have "inj_on (h ∘ f ∘ k) (h ` cball a r)"
using inj by (simp add: inj_on_def) (metis ‹⋀x. k (h x) = x›)
ultimately have *: "⋀T. ⟦open T; T ⊆ h ` cball a r⟧ ⟹ open ((h ∘ f ∘ k) ` T)"
using injective_eq_1d_open_map_UNIV by blast
have "open ((h ∘ f ∘ k) ` (h ` ball a r))"
by (rule *) (auto simp: ‹linear h› ‹range h = UNIV› open_surjective_linear_image)
then have "open ((h ∘ f) ` ball a r)"
by (simp add: image_comp ‹⋀x. k (h x) = x› cong: image_cong)
then show ?thesis
apply (metis open_bijective_linear_image_eq ‹linear h› ‹⋀x. k (h x) = x› ‹range h = UNIV› bijI inj_on_def)
done
next
case False
then have 2: "DIM('a) ≥ 2"
by (metis DIM_ge_Suc0 One_nat_def Suc_1 antisym not_less_eq_eq)
have fimsub: "f ` ball a r ⊆ - f ` sphere a r"
using inj  by clarsimp (metis inj_onD less_eq_real_def mem_cball order_less_irrefl)
have hom: "f ` sphere a r homeomorphic sphere a r"
by (meson compact_sphere contf continuous_on_subset homeomorphic_compact homeomorphic_sym inj inj_on_subset sphere_cball)
then have nconn: "¬ connected (- f ` sphere a r)"
by (rule Jordan_Brouwer_separation) (auto simp: ‹0 < r›)
obtain C where C: "C ∈ components (- f ` sphere a r)" and "bounded C"
apply (rule cobounded_has_bounded_component [OF _ nconn])
by (meson compact_imp_bounded compact_continuous_image_eq compact_sphere contf inj sphere_cball)
moreover have "f ` (ball a r) = C"
proof
have "C ≠ {}"
by (rule in_components_nonempty [OF C])
show "C ⊆ f ` ball a r"
proof (rule ccontr)
assume nonsub: "¬ C ⊆ f ` ball a r"
have "- f ` cball a r ⊆ C"
proof (rule components_maximal [OF C])
have "f ` cball a r homeomorphic cball a r"
using compact_cball contf homeomorphic_compact homeomorphic_sym inj by blast
then show "connected (- f ` cball a r)"
by (auto intro: connected_complement_homeomorphic_convex_compact 2)
show "- f ` cball a r ⊆ - f ` sphere a r"
by auto
then show "C ∩ - f ` cball a r ≠ {}"
using ‹C ≠ {}› in_components_subset [OF C] nonsub
using image_iff by fastforce
qed
then have "bounded (- f ` cball a r)"
using bounded_subset ‹bounded C› by auto
then have "¬ bounded (f ` cball a r)"
using cobounded_imp_unbounded by blast
then show "False"
using compact_continuous_image [OF contf] compact_cball compact_imp_bounded by blast
qed
with ‹C ≠ {}› have "C ∩ f ` ball a r ≠ {}"
then show "f ` ball a r ⊆ C"
by (metis components_maximal [OF C _ fimsub] connected_continuous_image ball_subset_cball connected_ball contf continuous_on_subset)
qed
moreover have "open (- f ` sphere a r)"
using hom compact_eq_bounded_closed compact_sphere homeomorphic_compactness by blast
ultimately show ?thesis
using open_components by blast
qed

text‹Proved by L. E. J. Brouwer (1912)›
theorem invariance_of_domain:
fixes f :: "'a ⇒ 'a::euclidean_space"
assumes "continuous_on S f" "open S" "inj_on f S"
shows "open(f ` S)"
unfolding open_subopen [of "f`S"]
proof clarify
fix a
assume "a ∈ S"
obtain δ where "δ > 0" and δ: "cball a δ ⊆ S"
using ‹open S› ‹a ∈ S› open_contains_cball_eq by blast
show "∃T. open T ∧ f a ∈ T ∧ T ⊆ f ` S"
proof (intro exI conjI)
show "open (f ` (ball a δ))"
by (meson δ ‹0 < δ› assms continuous_on_subset inj_on_subset invariance_of_domain_ball)
show "f a ∈ f ` ball a δ"
by (simp add: ‹0 < δ›)
show "f ` ball a δ ⊆ f ` S"
using δ ball_subset_cball by blast
qed
qed

lemma inv_of_domain_ss0:
fixes f :: "'a ⇒ 'a::euclidean_space"
assumes contf: "continuous_on U f" and injf: "inj_on f U" and fim: "f ` U ⊆ S"
and "subspace S" and dimS: "dim S = DIM('b::euclidean_space)"
and ope: "openin (subtopology euclidean S) U"
shows "openin (subtopology euclidean S) (f ` U)"
proof -
have "U ⊆ S"
using ope openin_imp_subset by blast
have "(UNIV::'b set) homeomorphic S"
by (simp add: ‹subspace S› dimS homeomorphic_subspaces)
then obtain h k where homhk: "homeomorphism (UNIV::'b set) S h k"
using homeomorphic_def by blast
have homkh: "homeomorphism S (k ` S) k h"
using homhk homeomorphism_image2 homeomorphism_sym by fastforce
have "open ((k ∘ f ∘ h) ` k ` U)"
proof (rule invariance_of_domain)
show "continuous_on (k ` U) (k ∘ f ∘ h)"
proof (intro continuous_intros)
show "continuous_on (k ` U) h"
by (meson continuous_on_subset [OF homeomorphism_cont1 [OF homhk]] top_greatest)
show "continuous_on (h ` k ` U) f"
apply (rule continuous_on_subset [OF contf], clarify)
apply (metis homhk homeomorphism_def ope openin_imp_subset rev_subsetD)
done
show "continuous_on (f ` h ` k ` U) k"
apply (rule continuous_on_subset [OF homeomorphism_cont2 [OF homhk]])
using fim homhk homeomorphism_apply2 ope openin_subset by fastforce
qed
have ope_iff: "⋀T. open T ⟷ openin (subtopology euclidean (k ` S)) T"
using homhk homeomorphism_image2 open_openin by fastforce
show "open (k ` U)"
by (simp add: ope_iff homeomorphism_imp_open_map [OF homkh ope])
show "inj_on (k ∘ f ∘ h) (k ` U)"
apply (clarsimp simp: inj_on_def)
by (metis subsetD fim homeomorphism_apply2 [OF homhk] image_subset_iff inj_on_eq_iff injf ‹U ⊆ S›)
qed
moreover
have eq: "f ` U = h ` (k ∘ f ∘ h ∘ k) ` U"
apply (auto simp: image_comp [symmetric])
apply (metis homkh ‹U ⊆ S› fim homeomorphism_image2 homeomorphism_of_subsets homhk imageI subset_UNIV)
by (metis ‹U ⊆ S› subsetD fim homeomorphism_def homhk image_eqI)
ultimately show ?thesis
by (metis (no_types, hide_lams) homeomorphism_imp_open_map homhk image_comp open_openin subtopology_UNIV)
qed

lemma inv_of_domain_ss1:
fixes f :: "'a ⇒ 'a::euclidean_space"
assumes contf: "continuous_on U f" and injf: "inj_on f U" and fim: "f ` U ⊆ S"
and "subspace S"
and ope: "openin (subtopology euclidean S) U"
shows "openin (subtopology euclidean S) (f ` U)"
proof -
define S' where "S' ≡ {y. ∀x ∈ S. orthogonal x y}"
have "subspace S'"
define g where "g ≡ λz::'a*'a. ((f ∘ fst)z, snd z)"
have "openin (subtopology euclidean (S × S')) (g ` (U × S'))"
proof (rule inv_of_domain_ss0)
show "continuous_on (U × S') g"
apply (intro continuous_intros continuous_on_compose2 [OF contf continuous_on_fst], auto)
done
show "g ` (U × S') ⊆ S × S'"
using fim  by (auto simp: g_def)
show "inj_on g (U × S')"
using injf by (auto simp: g_def inj_on_def)
show "subspace (S × S')"
by (simp add: ‹subspace S'› ‹subspace S› subspace_Times)
show "openin (subtopology euclidean (S × S')) (U × S')"
by (simp add: openin_Times [OF ope])
have "dim (S × S') = dim S + dim S'"
by (simp add: ‹subspace S'› ‹subspace S› dim_Times)
also have "... = DIM('a)"
using dim_subspace_orthogonal_to_vectors [OF ‹subspace S› subspace_UNIV]
finally show "dim (S × S') = DIM('a)" .
qed
moreover have "g ` (U × S') = f ` U × S'"
by (auto simp: g_def image_iff)
moreover have "0 ∈ S'"
using ‹subspace S'› subspace_affine by blast
ultimately show ?thesis
by (auto simp: openin_Times_eq)
qed

corollary invariance_of_domain_subspaces:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes ope: "openin (subtopology euclidean U) S"
and "subspace U" "subspace V" and VU: "dim V ≤ dim U"
and contf: "continuous_on S f" and fim: "f ` S ⊆ V"
and injf: "inj_on f S"
shows "openin (subtopology euclidean V) (f ` S)"
proof -
obtain V' where "subspace V'" "V' ⊆ U" "dim V' = dim V"
using choose_subspace_of_subspace [OF VU]
by (metis span_eq_iff ‹subspace U›)
then have "V homeomorphic V'"
by (simp add: ‹subspace V› homeomorphic_subspaces)
then obtain h k where homhk: "homeomorphism V V' h k"
using homeomorphic_def by blast
have eq: "f ` S = k ` (h ∘ f) ` S"
proof -
have "k ` h ` f ` S = f ` S"
by (meson fim homeomorphism_def homeomorphism_of_subsets homhk subset_refl)
then show ?thesis
qed
show ?thesis
unfolding eq
proof (rule homeomorphism_imp_open_map)
show homkh: "homeomorphism V' V k h"
have hfV': "(h ∘ f) ` S ⊆ V'"
using fim homeomorphism_image1 homhk by fastforce
moreover have "openin (subtopology euclidean U) ((h ∘ f) ` S)"
proof (rule inv_of_domain_ss1)
show "continuous_on S (h ∘ f)"
by (meson contf continuous_on_compose continuous_on_subset fim homeomorphism_cont1 homhk)
show "inj_on (h ∘ f) S"
apply (clarsimp simp: inj_on_def)
by (metis fim homeomorphism_apply2 [OF homkh] image_subset_iff inj_onD injf)
show "(h ∘ f) ` S ⊆ U"
using ‹V' ⊆ U› hfV' by auto
qed (auto simp: assms)
ultimately show "openin (subtopology euclidean V') ((h ∘ f) ` S)"
using openin_subset_trans ‹V' ⊆ U› by force
qed
qed

corollary invariance_of_dimension_subspaces:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes ope: "openin (subtopology euclidean U) S"
and "subspace U" "subspace V"
and contf: "continuous_on S f" and fim: "f ` S ⊆ V"
and injf: "inj_on f S" and "S ≠ {}"
shows "dim U ≤ dim V"
proof -
have "False" if "dim V < dim U"
proof -
obtain T where "subspace T" "T ⊆ U" "dim T = dim V"
using choose_subspace_of_subspace [of "dim V" U]
by (metis ‹dim V < dim U› assms(2) order.strict_implies_order span_eq_iff)
then have "V homeomorphic T"
by (simp add: ‹subspace V› homeomorphic_subspaces)
then obtain h k where homhk: "homeomorphism V T h k"
using homeomorphic_def  by blast
have "continuous_on S (h ∘ f)"
by (meson contf continuous_on_compose continuous_on_subset fim homeomorphism_cont1 homhk)
moreover have "(h ∘ f) ` S ⊆ U"
using ‹T ⊆ U› fim homeomorphism_image1 homhk by fastforce
moreover have "inj_on (h ∘ f) S"
apply (clarsimp simp: inj_on_def)
by (metis fim homeomorphism_apply1 homhk image_subset_iff inj_onD injf)
ultimately have ope_hf: "openin (subtopology euclidean U) ((h ∘ f) ` S)"
using invariance_of_domain_subspaces [OF ope ‹subspace U› ‹subspace U›] by auto
have "(h ∘ f) ` S ⊆ T"
using fim homeomorphism_image1 homhk by fastforce
then show ?thesis
by (metis dim_openin ‹dim T = dim V› ope_hf ‹subspace U› ‹S ≠ {}› dim_subset image_is_empty not_le that)
qed
then show ?thesis
using not_less by blast
qed

corollary invariance_of_domain_affine_sets:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes ope: "openin (subtopology euclidean U) S"
and aff: "affine U" "affine V" "aff_dim V ≤ aff_dim U"
and contf: "continuous_on S f" and fim: "f ` S ⊆ V"
and injf: "inj_on f S"
shows "openin (subtopology euclidean V) (f ` S)"
proof (cases "S = {}")
case True
then show ?thesis by auto
next
case False
obtain a b where "a ∈ S" "a ∈ U" "b ∈ V"
using False fim ope openin_contains_cball by fastforce
have "openin (subtopology euclidean ((+) (- b) ` V)) (((+) (- b) ∘ f ∘ (+) a) ` (+) (- a) ` S)"
proof (rule invariance_of_domain_subspaces)
show "openin (subtopology euclidean ((+) (- a) ` U)) ((+) (- a) ` S)"
by (metis ope homeomorphism_imp_open_map homeomorphism_translation translation_galois)
show "subspace ((+) (- a) ` U)"
by (simp add: ‹a ∈ U› affine_diffs_subspace ‹affine U›)
show "subspace ((+) (- b) ` V)"
by (simp add: ‹b ∈ V› affine_diffs_subspace ‹affine V›)
show "dim ((+) (- b) ` V) ≤ dim ((+) (- a) ` U)"
by (metis ‹a ∈ U› ‹b ∈ V› aff_dim_eq_dim affine_hull_eq aff of_nat_le_iff)
show "continuous_on ((+) (- a) ` S) ((+) (- b) ∘ f ∘ (+) a)"
by (metis contf continuous_on_compose homeomorphism_cont2 homeomorphism_translation translation_galois)
show "((+) (- b) ∘ f ∘ (+) a) ` (+) (- a) ` S ⊆ (+) (- b) ` V"
using fim by auto
show "inj_on ((+) (- b) ∘ f ∘ (+) a) ((+) (- a) ` S)"
by (auto simp: inj_on_def) (meson inj_onD injf)
qed
then show ?thesis
by (metis (no_types, lifting) homeomorphism_imp_open_map homeomorphism_translation image_comp translation_galois)
qed

corollary invariance_of_dimension_affine_sets:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes ope: "openin (subtopology euclidean U) S"
and aff: "affine U" "affine V"
and contf: "continuous_on S f" and fim: "f ` S ⊆ V"
and injf: "inj_on f S" and "S ≠ {}"
shows "aff_dim U ≤ aff_dim V"
proof -
obtain a b where "a ∈ S" "a ∈ U" "b ∈ V"
using ‹S ≠ {}› fim ope openin_contains_cball by fastforce
have "dim ((+) (- a) ` U) ≤ dim ((+) (- b) ` V)"
proof (rule invariance_of_dimension_subspaces)
show "openin (subtopology euclidean ((+) (- a) ` U)) ((+) (- a) ` S)"
by (metis ope homeomorphism_imp_open_map homeomorphism_translation translation_galois)
show "subspace ((+) (- a) ` U)"
by (simp add: ‹a ∈ U› affine_diffs_subspace ‹affine U›)
show "subspace ((+) (- b) ` V)"
by (simp add: ‹b ∈ V› affine_diffs_subspace ‹affine V›)
show "continuous_on ((+) (- a) ` S) ((+) (- b) ∘ f ∘ (+) a)"
by (metis contf continuous_on_compose homeomorphism_cont2 homeomorphism_translation translation_galois)
show "((+) (- b) ∘ f ∘ (+) a) ` (+) (- a) ` S ⊆ (+) (- b) ` V"
using fim by auto
show "inj_on ((+) (- b) ∘ f ∘ (+) a) ((+) (- a) ` S)"
by (auto simp: inj_on_def) (meson inj_onD injf)
qed (use ‹S ≠ {}› in auto)
then show ?thesis
by (metis ‹a ∈ U› ‹b ∈ V› aff_dim_eq_dim affine_hull_eq aff of_nat_le_iff)
qed

corollary invariance_of_dimension:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes contf: "continuous_on S f" and "open S"
and injf: "inj_on f S" and "S ≠ {}"
shows "DIM('a) ≤ DIM('b)"
using invariance_of_dimension_subspaces [of UNIV S UNIV f] assms
by auto

corollary continuous_injective_image_subspace_dim_le:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "subspace S" "subspace T"
and contf: "continuous_on S f" and fim: "f ` S ⊆ T"
and injf: "inj_on f S"
shows "dim S ≤ dim T"
apply (rule invariance_of_dimension_subspaces [of S S _ f])
using assms by (auto simp: subspace_affine)

lemma invariance_of_dimension_convex_domain:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "convex S"
and contf: "continuous_on S f" and fim: "f ` S ⊆ affine hull T"
and injf: "inj_on f S"
shows "aff_dim S ≤ aff_dim T"
proof (cases "S = {}")
case True
then show ?thesis by (simp add: aff_dim_geq)
next
case False
have "aff_dim (affine hull S) ≤ aff_dim (affine hull T)"
proof (rule invariance_of_dimension_affine_sets)
show "openin (subtopology euclidean (affine hull S)) (rel_interior S)"
show "continuous_on (rel_interior S) f"
using contf continuous_on_subset rel_interior_subset by blast
show "f ` rel_interior S ⊆ affine hull T"
using fim rel_interior_subset by blast
show "inj_on f (rel_interior S)"
using inj_on_subset injf rel_interior_subset by blast
show "rel_interior S ≠ {}"
by (simp add: False ‹convex S› rel_interior_eq_empty)
qed auto
then show ?thesis
by simp
qed

lemma homeomorphic_convex_sets_le:
assumes "convex S" "S homeomorphic T"
shows "aff_dim S ≤ aff_dim T"
proof -
obtain h k where homhk: "homeomorphism S T h k"
using homeomorphic_def assms  by blast
show ?thesis
proof (rule invariance_of_dimension_convex_domain [OF ‹convex S›])
show "continuous_on S h"
using homeomorphism_def homhk by blast
show "h ` S ⊆ affine hull T"
by (metis homeomorphism_def homhk hull_subset)
show "inj_on h S"
by (meson homeomorphism_apply1 homhk inj_on_inverseI)
qed
qed

lemma homeomorphic_convex_sets:
assumes "convex S" "convex T" "S homeomorphic T"
shows "aff_dim S = aff_dim T"
by (meson assms dual_order.antisym homeomorphic_convex_sets_le homeomorphic_sym)

lemma homeomorphic_convex_compact_sets_eq:
assumes "convex S" "compact S" "convex T" "compact T"
shows "S homeomorphic T ⟷ aff_dim S = aff_dim T"
by (meson assms homeomorphic_convex_compact_sets homeomorphic_convex_sets)

lemma invariance_of_domain_gen:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "open S" "continuous_on S f" "inj_on f S" "DIM('b) ≤ DIM('a)"
shows "open(f ` S)"
using invariance_of_domain_subspaces [of UNIV S UNIV f] assms by auto

lemma injective_into_1d_imp_open_map_UNIV:
fixes f :: "'a::euclidean_space ⇒ real"
assumes "open T" "continuous_on S f" "inj_on f S" "T ⊆ S"
shows "open (f ` T)"
apply (rule invariance_of_domain_gen [OF ‹open T›])
using assms apply (auto simp: elim: continuous_on_subset subset_inj_on)
done

lemma continuous_on_inverse_open:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "open S" "continuous_on S f" "DIM('b) ≤ DIM('a)" and gf: "⋀x. x ∈ S ⟹ g(f x) = x"
shows "continuous_on (f ` S) g"
fix T :: "'a set"
assume "open T"
have eq: "f ` S ∩ g -` T = f ` (S ∩ T)"
by (auto simp: gf)
show "openin (subtopology euclidean (f ` S)) (f ` S ∩ g -` T)"
apply (subst eq)
apply (rule open_openin_trans)
apply (rule invariance_of_domain_gen)
using assms
apply auto
using inj_on_inverseI apply auto[1]
by (metis ‹open T› continuous_on_subset inj_onI inj_on_subset invariance_of_domain_gen openin_open openin_open_eq)
qed

lemma invariance_of_domain_homeomorphism:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "open S" "continuous_on S f" "DIM('b) ≤ DIM('a)" "inj_on f S"
obtains g where "homeomorphism S (f ` S) f g"
proof
show "homeomorphism S (f ` S) f (inv_into S f)"
by (simp add: assms continuous_on_inverse_open homeomorphism_def)
qed

corollary invariance_of_domain_homeomorphic:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "open S" "continuous_on S f" "DIM('b) ≤ DIM('a)" "inj_on f S"
shows "S homeomorphic (f ` S)"
using invariance_of_domain_homeomorphism [OF assms]
by (meson homeomorphic_def)

lemma continuous_image_subset_interior:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes "continuous_on S f" "inj_on f S" "DIM('b) ≤ DIM('a)"
shows "f ` (interior S) ⊆ interior(f ` S)"
apply (rule interior_maximal)
apply (rule invariance_of_domain_gen)
using assms
apply (auto simp: subset_inj_on interior_subset continuous_on_subset)
done

lemma homeomorphic_interiors_same_dimension:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" and dimeq: "DIM('a) = DIM('b)"
shows "(interior S) homeomorphic (interior T)"
using assms [unfolded homeomorphic_minimal]
unfolding homeomorphic_def
proof (clarify elim!: ex_forward)
fix f g
assume S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
then have fST: "f ` S = T" and gTS: "g ` T = S" and "inj_on f S" "inj_on g T"
by (auto simp: inj_on_def intro: rev_image_eqI) metis+
have fim: "f ` interior S ⊆ interior T"
using continuous_image_subset_interior [OF contf ‹inj_on f S›] dimeq fST by simp
have gim: "g ` interior T ⊆ interior S"
using continuous_image_subset_interior [OF contg ‹inj_on g T›] dimeq gTS by simp
show "homeomorphism (interior S) (interior T) f g"
unfolding homeomorphism_def
proof (intro conjI ballI)
show "⋀x. x ∈ interior S ⟹ g (f x) = x"
by (meson ‹∀x∈S. f x ∈ T ∧ g (f x) = x› subsetD interior_subset)
have "interior T ⊆ f ` interior S"
proof
fix x assume "x ∈ interior T"
then have "g x ∈ interior S"
using gim by blast
then show "x ∈ f ` interior S"
by (metis T ‹x ∈ interior T› image_iff interior_subset subsetCE)
qed
then show "f ` interior S = interior T"
using fim by blast
show "continuous_on (interior S) f"
by (metis interior_subset continuous_on_subset contf)
show "⋀y. y ∈ interior T ⟹ f (g y) = y"
by (meson T subsetD interior_subset)
have "interior S ⊆ g ` interior T"
proof
fix x assume "x ∈ interior S"
then have "f x ∈ interior T"
using fim by blast
then show "x ∈ g ` interior T"
by (metis S ‹x ∈ interior S› image_iff interior_subset subsetCE)
qed
then show "g ` interior T = interior S"
using gim by blast
show "continuous_on (interior T) g"
by (metis interior_subset continuous_on_subset contg)
qed
qed

lemma homeomorphic_open_imp_same_dimension:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "open S" "S ≠ {}" "open T" "T ≠ {}"
shows "DIM('a) = DIM('b)"
using assms
apply (rule order_antisym; metis inj_onI invariance_of_dimension)
done

lemma homeomorphic_interiors:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "interior S = {} ⟷ interior T = {}"
shows "(interior S) homeomorphic (interior T)"
proof (cases "interior T = {}")
case True
with assms show ?thesis by auto
next
case False
then have "DIM('a) = DIM('b)"
using assms
apply (rule order_antisym; metis continuous_on_subset inj_onI inj_on_subset interior_subset invariance_of_dimension open_interior)
done
then show ?thesis
by (rule homeomorphic_interiors_same_dimension [OF ‹S homeomorphic T›])
qed

lemma homeomorphic_frontiers_same_dimension:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "closed S" "closed T" and dimeq: "DIM('a) = DIM('b)"
shows "(frontier S) homeomorphic (frontier T)"
using assms [unfolded homeomorphic_minimal]
unfolding homeomorphic_def
proof (clarify elim!: ex_forward)
fix f g
assume S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
then have fST: "f ` S = T" and gTS: "g ` T = S" and "inj_on f S" "inj_on g T"
by (auto simp: inj_on_def intro: rev_image_eqI) metis+
have "g ` interior T ⊆ interior S"
using continuous_image_subset_interior [OF contg ‹inj_on g T›] dimeq gTS by simp
then have fim: "f ` frontier S ⊆ frontier T"
using continuous_image_subset_interior assms(2) assms(3) S by auto
have "f ` interior S ⊆ interior T"
using continuous_image_subset_interior [OF contf ‹inj_on f S›] dimeq fST by simp
then have gim: "g ` frontier T ⊆ frontier S"
using continuous_image_subset_interior T assms(2) assms(3) by auto
show "homeomorphism (frontier S) (frontier T) f g"
unfolding homeomorphism_def
proof (intro conjI ballI)
show gf: "⋀x. x ∈ frontier S ⟹ g (f x) = x"
by (simp add: S assms(2) frontier_def)
show fg: "⋀y. y ∈ frontier T ⟹ f (g y) = y"
by (simp add: T assms(3) frontier_def)
have "frontier T ⊆ f ` frontier S"
proof
fix x assume "x ∈ frontier T"
then have "g x ∈ frontier S"
using gim by blast
then show "x ∈ f ` frontier S"
by (metis fg ‹x ∈ frontier T› imageI)
qed
then show "f ` frontier S = frontier T"
using fim by blast
show "continuous_on (frontier S) f"
by (metis Diff_subset assms(2) closure_eq contf continuous_on_subset frontier_def)
have "frontier S ⊆ g ` frontier T"
proof
fix x assume "x ∈ frontier S"
then have "f x ∈ frontier T"
using fim by blast
then show "x ∈ g ` frontier T"
by (metis gf ‹x ∈ frontier S› imageI)
qed
then show "g ` frontier T = frontier S"
using gim by blast
show "continuous_on (frontier T) g"
by (metis Diff_subset assms(3) closure_closed contg continuous_on_subset frontier_def)
qed
qed

lemma homeomorphic_frontiers:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "closed S" "closed T"
"interior S = {} ⟷ interior T = {}"
shows "(frontier S) homeomorphic (frontier T)"
proof (cases "interior T = {}")
case True
then show ?thesis
by (metis Diff_empty assms closure_eq frontier_def)
next
case False
show ?thesis
apply (rule homeomorphic_frontiers_same_dimension)
using False assms homeomorphic_interiors homeomorphic_open_imp_same_dimension by blast
qed

lemma continuous_image_subset_rel_interior:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes contf: "continuous_on S f" and injf: "inj_on f S" and fim: "f ` S ⊆ T"
and TS: "aff_dim T ≤ aff_dim S"
shows "f ` (rel_interior S) ⊆ rel_interior(f ` S)"
proof (rule rel_interior_maximal)
show "f ` rel_interior S ⊆ f ` S"
show "openin (subtopology euclidean (affine hull f ` S)) (f ` rel_interior S)"
proof (rule invariance_of_domain_affine_sets)
show "openin (subtopology euclidean (affine hull S)) (rel_interior S)"
show "aff_dim (affine hull f ` S) ≤ aff_dim (affine hull S)"
by (metis aff_dim_affine_hull aff_dim_subset fim TS order_trans)
show "f ` rel_interior S ⊆ affine hull f ` S"
by (meson ‹f ` rel_interior S ⊆ f ` S› hull_subset order_trans)
show "continuous_on (rel_interior S) f"
using contf continuous_on_subset rel_interior_subset by blast
show "inj_on f (rel_interior S)"
using inj_on_subset injf rel_interior_subset by blast
qed auto
qed

lemma homeomorphic_rel_interiors_same_dimension:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" and aff: "aff_dim S = aff_dim T"
shows "(rel_interior S) homeomorphic (rel_interior T)"
using assms [unfolded homeomorphic_minimal]
unfolding homeomorphic_def
proof (clarify elim!: ex_forward)
fix f g
assume S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
then have fST: "f ` S = T" and gTS: "g ` T = S" and "inj_on f S" "inj_on g T"
by (auto simp: inj_on_def intro: rev_image_eqI) metis+
have fim: "f ` rel_interior S ⊆ rel_interior T"
by (metis ‹inj_on f S› aff contf continuous_image_subset_rel_interior fST order_refl)
have gim: "g ` rel_interior T ⊆ rel_interior S"
by (metis ‹inj_on g T› aff contg continuous_image_subset_rel_interior gTS order_refl)
show "homeomorphism (rel_interior S) (rel_interior T) f g"
unfolding homeomorphism_def
proof (intro conjI ballI)
show gf: "⋀x. x ∈ rel_interior S ⟹ g (f x) = x"
using S rel_interior_subset by blast
show fg: "⋀y. y ∈ rel_interior T ⟹ f (g y) = y"
using T mem_rel_interior_ball by blast
have "rel_interior T ⊆ f ` rel_interior S"
proof
fix x assume "x ∈ rel_interior T"
then have "g x ∈ rel_interior S"
using gim by blast
then show "x ∈ f ` rel_interior S"
by (metis fg ‹x ∈ rel_interior T› imageI)
qed
moreover have "f ` rel_interior S ⊆ rel_interior T"
by (metis ‹inj_on f S› aff contf continuous_image_subset_rel_interior fST order_refl)
ultimately show "f ` rel_interior S = rel_interior T"
by blast
show "continuous_on (rel_interior S) f"
using contf continuous_on_subset rel_interior_subset by blast
have "rel_interior S ⊆ g ` rel_interior T"
proof
fix x assume "x ∈ rel_interior S"
then have "f x ∈ rel_interior T"
using fim by blast
then show "x ∈ g ` rel_interior T"
by (metis gf ‹x ∈ rel_interior S› imageI)
qed
then show "g ` rel_interior T = rel_interior S"
using gim by blast
show "continuous_on (rel_interior T) g"
using contg continuous_on_subset rel_interior_subset by blast
qed
qed

lemma homeomorphic_rel_interiors:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "rel_interior S = {} ⟷ rel_interior T = {}"
shows "(rel_interior S) homeomorphic (rel_interior T)"
proof (cases "rel_interior T = {}")
case True
with assms show ?thesis by auto
next
case False
obtain f g
where S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
using  assms [unfolded homeomorphic_minimal] by auto
have "aff_dim (affine hull S) ≤ aff_dim (affine hull T)"
apply (rule invariance_of_dimension_affine_sets [of _ "rel_interior S" _ f])
apply (simp_all add: openin_rel_interior False assms)
using contf continuous_on_subset rel_interior_subset apply blast
apply (meson S hull_subset image_subsetI rel_interior_subset rev_subsetD)
apply (metis S inj_on_inverseI inj_on_subset rel_interior_subset)
done
moreover have "aff_dim (affine hull T) ≤ aff_dim (affine hull S)"
apply (rule invariance_of_dimension_affine_sets [of _ "rel_interior T" _ g])
apply (simp_all add: openin_rel_interior False assms)
using contg continuous_on_subset rel_interior_subset apply blast
apply (meson T hull_subset image_subsetI rel_interior_subset rev_subsetD)
apply (metis T inj_on_inverseI inj_on_subset rel_interior_subset)
done
ultimately have "aff_dim S = aff_dim T" by force
then show ?thesis
by (rule homeomorphic_rel_interiors_same_dimension [OF ‹S homeomorphic T›])
qed

lemma homeomorphic_rel_boundaries_same_dimension:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" and aff: "aff_dim S = aff_dim T"
shows "(S - rel_interior S) homeomorphic (T - rel_interior T)"
using assms [unfolded homeomorphic_minimal]
unfolding homeomorphic_def
proof (clarify elim!: ex_forward)
fix f g
assume S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
then have fST: "f ` S = T" and gTS: "g ` T = S" and "inj_on f S" "inj_on g T"
by (auto simp: inj_on_def intro: rev_image_eqI) metis+
have fim: "f ` rel_interior S ⊆ rel_interior T"
by (metis ‹inj_on f S› aff contf continuous_image_subset_rel_interior fST order_refl)
have gim: "g ` rel_interior T ⊆ rel_interior S"
by (metis ‹inj_on g T› aff contg continuous_image_subset_rel_interior gTS order_refl)
show "homeomorphism (S - rel_interior S) (T - rel_interior T) f g"
unfolding homeomorphism_def
proof (intro conjI ballI)
show gf: "⋀x. x ∈ S - rel_interior S ⟹ g (f x) = x"
using S rel_interior_subset by blast
show fg: "⋀y. y ∈ T - rel_interior T ⟹ f (g y) = y"
using T mem_rel_interior_ball by blast
show "f ` (S - rel_interior S) = T - rel_interior T"
using S fST fim gim by auto
show "continuous_on (S - rel_interior S) f"
using contf continuous_on_subset rel_interior_subset by blast
show "g ` (T - rel_interior T) = S - rel_interior S"
using T gTS gim fim by auto
show "continuous_on (T - rel_interior T) g"
using contg continuous_on_subset rel_interior_subset by blast
qed
qed

lemma homeomorphic_rel_boundaries:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "S homeomorphic T" "rel_interior S = {} ⟷ rel_interior T = {}"
shows "(S - rel_interior S) homeomorphic (T - rel_interior T)"
proof (cases "rel_interior T = {}")
case True
with assms show ?thesis by auto
next
case False
obtain f g
where S: "∀x∈S. f x ∈ T ∧ g (f x) = x" and T: "∀y∈T. g y ∈ S ∧ f (g y) = y"
and contf: "continuous_on S f" and contg: "continuous_on T g"
using  assms [unfolded homeomorphic_minimal] by auto
have "aff_dim (affine hull S) ≤ aff_dim (affine hull T)"
apply (rule invariance_of_dimension_affine_sets [of _ "rel_interior S" _ f])
apply (simp_all add: openin_rel_interior False assms)
using contf continuous_on_subset rel_interior_subset apply blast
apply (meson S hull_subset image_subsetI rel_interior_subset rev_subsetD)
apply (metis S inj_on_inverseI inj_on_subset rel_interior_subset)
done
moreover have "aff_dim (affine hull T) ≤ aff_dim (affine hull S)"
apply (rule invariance_of_dimension_affine_sets [of _ "rel_interior T" _ g])
apply (simp_all add: openin_rel_interior False assms)
using contg continuous_on_subset rel_interior_subset apply blast
apply (meson T hull_subset image_subsetI rel_interior_subset rev_subsetD)
apply (metis T inj_on_inverseI inj_on_subset rel_interior_subset)
done
ultimately have "aff_dim S = aff_dim T" by force
then show ?thesis
by (rule homeomorphic_rel_boundaries_same_dimension [OF ‹S homeomorphic T›])
qed

proposition uniformly_continuous_homeomorphism_UNIV_trivial:
fixes f :: "'a::euclidean_space ⇒ 'a"
assumes contf: "uniformly_continuous_on S f" and hom: "homeomorphism S UNIV f g"
shows "S = UNIV"
proof (cases "S = {}")
case True
then show ?thesis
by (metis UNIV_I hom empty_iff homeomorphism_def image_eqI)
next
case False
have "inj g"
by (metis UNIV_I hom homeomorphism_apply2 injI)
then have "open (g ` UNIV)"
by (blast intro: invariance_of_domain hom homeomorphism_cont2)
then have "open S"
using hom homeomorphism_image2 by blast
moreover have "complete S"
unfolding complete_def
proof clarify
fix σ
assume σ: "∀n. σ n ∈ S" and "Cauchy σ"
have "Cauchy (f o σ)"
using uniformly_continuous_imp_Cauchy_continuous ‹Cauchy σ› σ contf by blast
then obtain l where "(f ∘ σ) ⇢ l"
by (auto simp: convergent_eq_Cauchy [symmetric])
show "∃l∈S. σ ⇢ l"
proof
show "g l ∈ S"
using hom homeomorphism_image2 by blast
have "(g ∘ (f ∘ σ)) ⇢ g l"
by (meson UNIV_I ‹(f ∘ σ) ⇢ l› continuous_on_sequentially hom homeomorphism_cont2)
then show "σ ⇢ g l"
proof -
have "∀n. σ n = (g ∘ (f ∘ σ)) n"
by (metis (no_types) σ comp_eq_dest_lhs hom homeomorphism_apply1)
then show ?thesis
by (metis (no_types) LIMSEQ_iff ‹(g ∘ (f ∘ σ)) ⇢ g l›)
qed
qed
qed
then have "closed S"
ultimately show ?thesis
using clopen [of S] False  by simp
qed

subsection‹Dimension-based conditions for various homeomorphisms›

lemma homeomorphic_subspaces_eq:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "subspace S" "subspace T"
shows "S homeomorphic T ⟷ dim S = dim T"
proof
assume "S homeomorphic T"
then obtain f g where hom: "homeomorphism S T f g"
using homeomorphic_def by blast
show "dim S = dim T"
proof (rule order_antisym)
show "dim S ≤ dim T"
by (metis assms dual_order.refl inj_onI homeomorphism_cont1 [OF hom] homeomorphism_apply1 [OF hom] homeomorphism_image1 [OF hom] continuous_injective_image_subspace_dim_le)
show "dim T ≤ dim S"
by (metis assms dual_order.refl inj_onI homeomorphism_cont2 [OF hom] homeomorphism_apply2 [OF hom] homeomorphism_image2 [OF hom] continuous_injective_image_subspace_dim_le)
qed
next
assume "dim S = dim T"
then show "S homeomorphic T"
qed

lemma homeomorphic_affine_sets_eq:
fixes S :: "'a::euclidean_space set" and T :: "'b::euclidean_space set"
assumes "affine S" "affine T"
shows "S homeomorphic T ⟷ aff_dim S = aff_dim T"
proof (cases "S = {} ∨ T = {}")
case True
then show ?thesis
using assms homeomorphic_affine_sets by force
next
case False
then obtain a b where "a ∈ S" "b ∈ T"
by blast
then have "subspace ((+) (- a) ` S)" "subspace ((+) (- b) ` T)"
using affine_diffs_subspace assms by blast+
then show ?thesis
by (metis affine_imp_convex assms homeomorphic_affine_sets homeomorphic_convex_sets)
qed

lemma homeomorphic_hyperplanes_eq:
fixes a :: "'a::euclidean_space" and c :: "'b::euclidean_space"
assumes "a ≠ 0" "c ≠ 0"
shows "({x. a ∙ x = b} homeomorphic {x. c ∙ x = d} ⟷ DIM('a) = DIM('b))"
apply (auto simp: homeomorphic_affine_sets_eq affine_hyperplane assms)
by (metis DIM_positive Suc_pred)

lemma homeomorphic_UNIV_UNIV:
shows "(UNIV::'a set) homeomorphic (UNIV::'b set) ⟷
DIM('a::euclidean_space) = DIM('b::euclidean_space)"

lemma simply_connected_sphere_gen:
assumes "convex S" "bounded S" and 3: "3 ≤ aff_dim S"
shows "simply_connected(rel_frontier S)"
proof -
have pa: "path_connected (rel_frontier S)"
using assms by (simp add: path_connected_sphere_gen)
show ?thesis
proof (clarsimp simp add: simply_connected_eq_contractible_circlemap pa)
fix f
assume f: "continuous_on (sphere (0::complex) 1) f" "f ` sphere 0 1 ⊆ rel_frontier S"
have eq: "sphere (0::complex) 1 = rel_frontier(cball 0 1)"
by simp
have "convex (cball (0::complex) 1)"
by (rule convex_cball)
then obtain c where "homotopic_with (λz. True) (sphere (0::complex) 1) (rel_frontier S) f (λx. c)"
apply (rule inessential_spheremap_lowdim_gen [OF _ bounded_cball ‹convex S› ‹bounded S›, where f=f])
using f 3
apply (auto simp: aff_dim_cball)
done
then show "∃a. homotopic_with (λh. True) (sphere 0 1) (rel_frontier S) f (λx. a)"
by blast
qed
qed

subsection‹more invariance of domain›

proposition invariance_of_domain_sphere_affine_set_gen:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes contf: "continuous_on S f" and injf: "inj_on f S" and fim: "f ` S ⊆ T"
and U: "bounded U" "convex U"
and "affine T" and affTU: "aff_dim T < aff_dim U"
and ope: "openin (subtopology euclidean (rel_frontier U)) S"
shows "openin (subtopology euclidean T) (f ` S)"
proof (cases "rel_frontier U = {}")
case True
then show ?thesis
using ope openin_subset by force
next
case False
obtain b c where b: "b ∈ rel_frontier U" and c: "c ∈ rel_frontier U" and "b ≠ c"
using ‹bounded U› rel_frontier_not_sing [of U] subset_singletonD False  by fastforce
obtain V :: "'a set" where "affine V" and affV: "aff_dim V = aff_dim U - 1"
proof (rule choose_affine_subset [OF affine_UNIV])
show "- 1 ≤ aff_dim U - 1"
by (metis aff_dim_empty aff_dim_geq aff_dim_negative_iff affTU diff_0 diff_right_mono not_le)
show "aff_dim U - 1 ≤ aff_dim (UNIV::'a set)"
by (metis aff_dim_UNIV aff_dim_le_DIM le_cases not_le zle_diff1_eq)
qed auto
have SU: "S ⊆ rel_frontier U"
using ope openin_imp_subset by auto
have homb: "rel_frontier U - {b} homeomorphic V"
and homc: "rel_frontier U - {c} homeomorphic V"
using homeomorphic_punctured_sphere_affine_gen [of U _ V]
by (simp_all add: ‹affine V› affV U b c)
then obtain g h j k
where gh: "homeomorphism (rel_frontier U - {b}) V g h"
and jk: "homeomorphism (rel_frontier U - {c}) V j k"
by (auto simp: homeomorphic_def)
with SU have hgsub: "(h ` g ` (S - {b})) ⊆ S" and kjsub: "(k ` j ` (S - {c})) ⊆ S"
have [simp]: "aff_dim T ≤ aff_dim V"
have "openin (subtopology euclidean T) ((f ∘ h) ` g ` (S - {b}))"
proof (rule invariance_of_domain_affine_sets [OF _ ‹affine V›])
show "openin (subtopology euclidean V) (g ` (S - {b}))"
apply (rule homeomorphism_imp_open_map [OF gh])
by (meson Diff_mono Diff_subset SU ope openin_delete openin_subset_trans order_refl)
show "continuous_on (g ` (S - {b})) (f ∘ h)"
apply (rule continuous_on_compose)
apply (meson Diff_mono SU homeomorphism_def homeomorphism_of_subsets gh set_eq_subset)
using contf continuous_on_subset hgsub by blast
show "inj_on (f ∘ h) (g ` (S - {b}))"
using kjsub
by (metis SU b homeomorphism_def inj_onD injf insert_Diff insert_iff gh rev_subsetD)
show "(f ∘ h) ` g ` (S - {b}) ⊆ T"
by (metis fim image_comp image_mono hgsub subset_trans)
qed (auto simp: assms)
moreover
have "openin (subtopology euclidean T) ((f ∘ k) ` j ` (S - {c}))"
proof (rule invariance_of_domain_affine_sets [OF _ ‹affine V›])
show "openin (subtopology euclidean V) (j ` (S - {c}))"
apply (rule homeomorphism_imp_open_map [OF jk])
by (meson Diff_mono Diff_subset SU ope openin_delete openin_subset_trans order_refl)
show "continuous_on (j ` (S - {c})) (f ∘ k)"
apply (rule continuous_on_compose)
apply (meson Diff_mono SU homeomorphism_def homeomorphism_of_subsets jk set_eq_subset)
using contf continuous_on_subset kjsub by blast
show "inj_on (f ∘ k) (j ` (S - {c}))"
using kjsub
by (metis SU c homeomorphism_def inj_onD injf insert_Diff insert_iff jk rev_subsetD)
show "(f ∘ k) ` j ` (S - {c}) ⊆ T"
by (metis fim image_comp image_mono kjsub subset_trans)
qed (auto simp: assms)
ultimately have "openin (subtopology euclidean T) ((f ∘ h) ` g ` (S - {b}) ∪ ((f ∘ k) ` j ` (S - {c})))"
by (rule openin_Un)
moreover have "(f ∘ h) ` g ` (S - {b}) = f ` (S - {b})"
proof -
have "h ` g ` (S - {b}) = (S - {b})"
proof
show "h ` g ` (S - {b}) ⊆ S - {b}"
using homeomorphism_apply1 [OF gh] SU
by (fastforce simp add: image_iff image_subset_iff)
show "S - {b} ⊆ h ` g ` (S - {b})"
apply clarify
by  (metis SU subsetD homeomorphism_apply1 [OF gh] image_iff member_remove remove_def)
qed
then show ?thesis
by (metis image_comp)
qed
moreover have "(f ∘ k) ` j ` (S - {c}) = f ` (S - {c})"
proof -
have "k ` j ` (S - {c}) = (S - {c})"
proof
show "k ` j ` (S - {c}) ⊆ S - {c}"
using homeomorphism_apply1 [OF jk] SU
by (fastforce simp add: image_iff image_subset_iff)
show "S - {c} ⊆ k ` j ` (S - {c})"
apply clarify
by  (metis SU subsetD homeomorphism_apply1 [OF jk] image_iff member_remove remove_def)
qed
then show ?thesis
by (metis image_comp)
qed
moreover have "f ` (S - {b}) ∪ f ` (S - {c}) = f ` (S)"
using ‹b ≠ c› by blast
ultimately show ?thesis
by simp
qed

lemma invariance_of_domain_sphere_affine_set:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes contf: "continuous_on S f" and injf: "inj_on f S" and fim: "f ` S ⊆ T"
and "r ≠ 0" "affine T" and affTU: "aff_dim T < DIM('a)"
and ope: "openin (subtopology euclidean (sphere a r)) S"
shows "openin (subtopology euclidean T) (f ` S)"
proof (cases "sphere a r = {}")
case True
then show ?thesis
using ope openin_subset by force
next
case False
show ?thesis
proof (rule invariance_of_domain_sphere_affine_set_gen [OF contf injf fim bounded_cball convex_cball ‹affine T›])
show "aff_dim T < aff_dim (cball a r)"
by (metis False affTU aff_dim_cball assms(4) linorder_cases sphere_empty)
show "openin (subtopology euclidean (rel_frontier (cball a r))) S"
by (simp add: ‹r ≠ 0› ope)
qed
qed

lemma no_embedding_sphere_lowdim:
fixes f :: "'a::euclidean_space ⇒ 'b::euclidean_space"
assumes contf: "continuous_on (sphere a r) f" and injf: "inj_on f (sphere a r)" and "r > 0"
shows "DIM('a) ≤ DIM('b)"
proof -
have "False" if "DIM('a) > DIM('b)"
proof -
have "compact (f ` sphere a r)"
using compact_continuous_image
then have "¬ open (f ` sphere a r)"
using compact_open
by (metis assms(3) image_is_empty not_less_iff_gr_or_eq sphere_eq_empty)
then show False
using invariance_of_domain_sphere_affine_set [OF contf injf subset_UNIV] ‹r > 0›
by (metis aff_dim_UNIV affine_UNIV less_irrefl of_nat_less_iff open_openin openin_subtopology_self subtopology_UNIV that)
qed
then show ?thesis
using not_less by blast
qed

lemma simply_connected_sphere:
fixes a :: "'a::euclidean_space"
assumes "3 ≤ DIM('a)"
shows "simply_connected(sphere a r)"
proof (cases rule: linorder_cases [of r 0])
case less
then show ?thesis by simp
next
case equal
then show ?thesis  by (auto simp: convex_imp_simply_connected)
next
case greater
then show ?thesis
using simply_connected_sphere_gen [of "cball a r"] assms
qed

lemma simply_connected_sphere_eq:
fixes a :: "'a::euclidean_space"
shows "simply_connected(sphere a r) ⟷ 3 ≤ DIM('a) ∨ r ≤ 0"  (is "?lhs = ?rhs")
proof (cases "r ≤ 0")
case True
have "simply_connected (sphere a r)"
apply (rule convex_imp_simply_connected)
using True less_eq_real_def by auto
with True show ?thesis by auto
next
case False
show ?thesis
proof
assume L: ?lhs
have "False" if "DIM('a) = 1 ∨ DIM('a) = 2"
using that
proof
assume "DIM('a) = 1"
with L show False
using connected_sphere_eq simply_connected_imp_connected
by (metis False Suc_1 not_less_eq_eq order_refl)
next
assume "DIM('a) = 2"
then have "sphere a r homeomorphic sphere (0::complex) 1"
by (metis DIM_complex False homeomorphic_spheres_gen not_less zero_less_one)
then have "simply_connected(sphere (0::complex) 1)"
using L homeomorphic_simply_connected_eq by blast
then obtain a::complex where "homotopic_with (λh. True) (sphere 0 1) (sphere 0 1) id (λx. a)"
by (metis continuous_on_id' id_apply image_id subset_refl)
then show False
using contractible_sphere contractible_def not_one_le_zero by blast
qed
with False show ?rhs
apply simp
by (metis DIM_ge_Suc0 le_antisym not_less_eq_eq numeral_2_eq_2 numeral_3_eq_3)
next
assume ?rhs
with False show ?lhs by (simp add: simply_connected_sphere)
qed
qed

lemma simply_connected_punctured_universe_eq:
fixes a :: "'a::euclidean_space"
shows "simply_connected(- {a}) ⟷ 3 ≤ DIM('a)"
proof -
have [simp]: "a ∈ rel_interior (cball a 1)"
have [simp]: "affine hull cball a 1 - {a} = -{a}"
by (metis Compl_eq_Diff_UNIV aff_dim_cball aff_dim_lt_full not_less_iff_gr_or_eq zero_less_one)
have "simply_connected(- {a}) ⟷ simply_connected(sphere a 1)"
apply (rule sym)
apply (rule homotopy_eqv_simple_connectedness)
using homotopy_eqv_rel_frontier_punctured_affine_hull [of "cball a 1" a] apply auto
done
also have "...  ⟷ 3 ≤ DIM('a)"
finally show ?thesis .
qed

lemma not_simply_connected_circle:
fixes a :: complex
shows "0 < r ⟹ ¬ simply_connected(sphere a r)"

proposition simply_connected_punctured_convex:
fixes a :: "'a::euclidean_space"
assumes "convex S" and 3: "3 ≤ aff_dim S"
shows "simply_connected(S - {a})"
proof (cases "a ∈ rel_interior S")
case True
then obtain e where "a ∈ S" "0 < e" and e: "cball a e ∩ affine hull S ⊆ S"
by (auto simp: rel_interior_cball)
have con: "convex (cball a e ∩ affine hull S)"
have bo: "bounded (cball a e ∩ affine hull S)"
have "affine hull S ∩ interior (cball a e) ≠ {}"
using ‹0 < e› ‹a ∈ S› hull_subset by fastforce
then have "3 ≤ aff_dim (affine hull S ∩ cball a e)"
by (simp add: 3 aff_dim_convex_Int_nonempty_interior [OF convex_affine_hull])
also have "... = aff_dim (cball a e ∩ affine hull S)"
finally have "3 ≤ aff_dim (cball a e ∩ affine hull S)" .
moreover have "rel_frontier (cball a e ∩ affine hull S) homotopy_eqv S - {a}"
proof (rule homotopy_eqv_rel_frontier_punctured_convex)
show "a ∈ rel_interior (cball a e ∩ affine hull S)"
by (meson IntI Int_mono ‹a ∈ S› ‹0 < e› e ‹cball a e ∩ affine hull S ⊆ S› ball_subset_cball centre_in_cball dual_order.strict_implies_order hull_inc hull_mono mem_rel_interior_ball)
have "closed (cball a e ∩ affine hull S)"
by blast
then show "rel_frontier (cball a e ∩ affine hull S) ⊆ S"
using e by blast
show "S ⊆ affine hull (cball a e ∩ affine hull S)"
by (metis (no_types, lifting) IntI ‹a ∈ S› ‹0 < e› affine_hull_convex_Int_nonempty_interior centre_in_ball convex_affine_hull empty_iff hull_subset inf_commute interior_cball subsetCE subsetI)
qed (auto simp: assms con bo)
ultimately show ?thesis
using homotopy_eqv_simple_connectedness simply_connected_sphere_gen [OF con bo]
by blast
next
case False
show ?thesis
apply (rule contractible_imp_simply_connected)
apply (rule contractible_convex_tweak_boundary_points [OF ‹convex S›])
apply (simp add: False rel_interior_subset subset_Diff_insert)
by (meson Diff_subset closure_subset subset_trans)
qed

corollary simply_connected_punctured_universe:
fixes a :: "'a::euclidean_space"
assumes "3 ≤ DIM('a)"
shows "simply_connected(- {a})"
proof -
have [simp]: "affine hull cball a 1 = UNIV"
apply auto
by (metis UNIV_I aff_dim_cball aff_dim_lt_full zero_less_one not_less_iff_gr_or_eq)
have "simply_connected (rel_frontier (cball a 1)) = simply_connected (affine hull cball a 1 - {a})"
apply (rule homotopy_eqv_simple_connectedness)
apply (rule homotopy_eqv_rel_frontier_punctured_affine_hull)
apply (force simp: rel_interior_cball intro: homotopy_eqv_simple_connectedness homotopy_eqv_rel_frontier_punctured_affine_hull)+
done
then show ?thesis
using simply_connected_sphere [of a 1, OF assms] by (auto simp: Compl_eq_Diff_UNIV)
qed

subsection‹The power, squaring and exponential functions as covering maps›

proposition covering_space_power_punctured_plane:
assumes "0 < n"
shows "covering_space (- {0}) (λz::complex. z^n) (- {0})"
proof -
consider "n = 1" | "2 ≤ n" using assms by linarith
then obtain e where "0 < e"
and e: "⋀w z. cmod(w - z) < e * cmod z ⟹ (w^n = z^n ⟷ w = z)"
proof cases
assume "n = 1" then show ?thesis
by (rule_tac e=1 in that) auto
next
assume "2 ≤ n"
have eq_if_pow_eq:
"w = z" if lt: "cmod (w - z) < 2 * sin (pi / real n) * cmod z"
and eq: "w^n = z^n" for w z
proof (cases "z = 0")
case True with eq assms show ?thesis by (auto simp: power_0_left)
next
case False
then have "z ≠ 0" by auto
have "(w/z)^n = 1"
by (metis False divide_self_if eq power_divide power_one)
then obtain j where j: "w / z = exp (2 * of_real pi * 𝗂 * j / n)" and "j < n"
using Suc_leI assms ‹2 ≤ n› complex_roots_unity [THEN eqset_imp_iff, of n "w/z"]
by force
have "cmod (w/z - 1) < 2 * sin (pi / real n)"
using lt assms ‹z ≠ 0› by (simp add: divide_simps norm_divide)
then have "cmod (exp (𝗂 * of_real (2 * pi * j / n)) - 1) < 2 * sin (pi / real n)"
then have "2 * ¦sin((2 * pi * j / n) / 2)¦ < 2 * sin (pi / real n)"
by (simp only: dist_exp_i_1)
then have sin_less: "sin((pi * j / n)) < sin (pi / real n)"
then have "w / z = 1"
proof (cases "j = 0")
case True then show ?thesis by (auto simp: j)
next
case False
then have "sin (pi / real n) ≤ sin((pi * j / n))"
proof (cases "j / n ≤ 1/2")
case True
show ?thesis
apply (rule sin_monotone_2pi_le)
using ‹j ≠ 0 › ‹j < n› True
apply (auto simp: field_simps intro: order_trans [of _ 0])
done
next
case False
then have seq: "sin(pi * j / n) = sin(pi * (n - j) / n)"
using ‹j < n› by (simp add: algebra_simps diff_divide_distrib of_nat_diff)
show ?thesis
apply (simp only: seq)
apply (rule sin_monotone_2pi_le)
using ‹j < n› False
apply (auto simp: field_simps intro: order_trans [of _ 0])
done
qed
with sin_less show ?thesis by force
qed
then show ?thesis by simp
qed
show ?thesis
apply (rule_tac e = "2 * sin(pi / n)" in that)
apply (force simp: ‹2 ≤ n› sin_pi_divide_n_gt_0)
apply (meson eq_if_pow_eq)
done
qed
have zn1: "continuous_on (- {0}) (λz::complex. z^n)"
by (rule continuous_intros)+
have zn2: "(λz::complex. z^n) ` (- {0}) = - {0}"
using assms by (auto simp: image_def elim: exists_complex_root_nonzero [where n = n])
have zn3: "∃T. z^n ∈ T ∧ open T ∧ 0 ∉ T ∧
(∃v. ⋃v = -{0} ∩ (λz. z ^ n) -` T ∧
(∀u∈v. open u ∧ 0 ∉ u) ∧
pairwise disjnt v ∧
(∀u∈v. Ex (homeomorphism u T (λz. z^n))))"
if "z ≠ 0" for z::complex
proof -
define d where "d ≡ min (1/2) (e/4) * norm z"
have "0 < d"
by (simp add: d_def ‹0 < e› ‹z ≠ 0›)
have iff_x_eq_y: "x^n = y^n ⟷ x = y"
if eq: "w^n = z^n" and x: "x ∈ ball w d" and y: "y ∈ ball w d" for w x y
proof -
have [simp]: "norm z = norm w" using that
show ?thesis
proof (cases "w = 0")
case True with ‹z ≠ 0› assms eq
show ?thesis by (auto simp: power_0_left)
next
case False
have "cmod (x - y) < 2*d"
using x y
also have "... ≤ 2 * e / 4 * norm w"
using ‹e > 0› by (simp add: d_def min_mult_distrib_right)
also have "... = e * (cmod w / 2)"
by simp
also have "... ≤ e * cmod y"
apply (rule mult_left_mono)
using ‹e > 0› y
apply (simp_all add: dist_norm d_def min_mult_distrib_right del: divide_const_simps)
apply (metis dist_0_norm dist_complex_def dist_triangle_half_l linorder_not_less order_less_irrefl)
done
finally have "cmod (x - y) < e * cmod y" .
then show ?thesis by (rule e)
qed
qed
then have inj: "inj_on (λw. w^n) (ball z d)"
have cont: "continuous_on (ball z d) (λw. w ^ n)"
by (intro continuous_intros)
have noncon: "¬ (λw::complex. w^n) constant_on UNIV"
by (metis UNIV_I assms constant_on_def power_one zero_neq_one zero_power)
have im_eq: "(λw. w^n) ` ball z' d = (λw. w^n) ` ball z d"
if z': "z'^n = z^n" for z'
proof -
have nz': "norm z' = norm z" using that assms power_eq_imp_eq_norm by blast
have "(w ∈ (λw. w^n) ` ball z' d) = (w ∈ (λw. w^n) ` ball z d)" for w
proof (cases "w=0")
case True with assms show ?thesis
by (simp add: image_def ball_def nz')
next
case False
have "z' ≠ 0" using ‹z ≠ 0› nz' by force
have [simp]: "(z*x / z')^n = x^n" if "x ≠ 0" for x
using z' that by (simp add: field_simps ‹z ≠ 0›)
have [simp]: "cmod (z - z * x / z') = cmod (z' - x)" if "x ≠ 0" for x
proof -
have "cmod (z - z * x / z') = cmod z * cmod (1 - x / z')"
by (metis (no_types) ab_semigroup_mult_class.mult_ac(1) divide_complex_def mult.right_neutral norm_mult right_diff_distrib')
also have "... = cmod z' * cmod (1 - x / z')"
also have "... = cmod (z' - x)"
by (simp add: ‹z' ≠ 0› diff_divide_eq_iff norm_divide)
finally show ?thesis .
qed
have [simp]: "(z'*x / z)^n = x^n" if "x ≠ 0" for x
using z' that by (simp add: field_simps ‹z ≠ 0›)
have [simp]: "cmod (z' - z' * x / z) = cmod (z - x)" if "x ≠ 0" for x
proof -
have "cmod (z * (1 - x * inverse z)) = cmod (z - x)"
by (metis ‹z ≠ 0› diff_divide_distrib divide_complex_def divide_self_if nonzero_eq_divide_eq semiring_normalization_rules(7))
then show ?thesis
by (metis (no_types) mult.assoc divide_complex_def mult.right_neutral norm_mult nz' right_diff_distrib')
qed
show ?thesis
unfolding image_def ball_def
apply safe
apply simp_all
apply (rule_tac x="z/z' * x" in exI)
using assms False apply (simp add: dist_norm)
apply (rule_tac x="z'/z * x" in exI)
using assms False apply (simp add: dist_norm)
done
qed
then show ?thesis by blast
qed

have ex_ball: "∃B. (∃z'. B = ball z' d ∧ z'^n = z^n) ∧ x ∈ B"
if "x ≠ 0" and eq: "x^n = w^n" and dzw: "dist z w < d" for x w
proof -
have "w ≠ 0" by (metis assms power_eq_0_iff that(1) that(2))
have [simp]: "cmod x = cmod w"
using assms power_eq_imp_eq_norm eq by blast
have [simp]: "cmod (x * z / w - x) = cmod (z - w)"
proof -
have "cmod (x * z / w - x) = cmod x * cmod (z / w - 1)"
by (metis (no_types) mult.right_neutral norm_mult right_diff_distrib' times_divide_eq_right)
also have "... = cmod w * cmod (z / w - 1)"
by simp
also have "... = cmod (z - w)"
by (simp add: ‹w ≠ 0› divide_diff_eq_iff nonzero_norm_divide)
finally show ?thesis .
qed
show ?thesis
apply (rule_tac x="ball (z / w * x) d" in exI)
using ‹d > 0› that
apply (simp add: ‹z ≠ 0› ‹w ≠ 0› field_simps)
done
qed

show ?thesis
proof (rule exI, intro conjI)
show "z ^ n ∈ (λw. w ^ n) ` ball z d"
using ‹d > 0› by simp
show "open ((λw. w ^ n) ` ball z d)"
by (rule invariance_of_domain [OF cont open_ball inj])
show "0 ∉ (λw. w ^ n) ` ball z d"
using ‹z ≠ 0› assms by (force simp: d_def)
show "∃v. ⋃v = - {0} ∩ (λz. z ^ n) -` (λw. w ^ n) ` ball z d ∧
(∀u∈v. open u ∧ 0 ∉ u) ∧
disjoint v ∧
(∀u∈v. Ex (homeomorphism u ((λw. w ^ n) ` ball z d) (λz. z ^ n)))"
proof (rule exI, intro ballI conjI)
show "⋃{ball z' d |z'. z'^n = z^n} = - {0} ∩ (λz. z ^ n) -` (λw. w ^ n) ` ball z d" (is "?l = ?r")
proof
show "?l ⊆ ?r"
apply auto
apply (simp add: assms d_def power_eq_imp_eq_norm that)
by (metis im_eq image_eqI mem_ball)
show "?r ⊆ ?l"
by auto (meson ex_ball)
qed
show "⋀u. u ∈ {ball z' d |z'. z' ^ n = z ^ n} ⟹ 0 ∉ u"
by (force simp add: assms d_def power_eq_imp_eq_norm that)

show "disjoint {ball z' d |z'. z' ^ n = z ^ n}"
proof (clarsimp simp add: pairwise_def disjnt_iff)
fix ξ ζ x
assume "ξ^n = z^n" "ζ^n = z^n" "ball ξ d ≠ ball ζ d"
and "dist ξ x < d" "dist ζ x < d"
then have "dist ξ ζ < d+d"
then have "cmod (ξ - ζ) < 2*d"
also have "... ≤ e * cmod z"
using mult_right_mono ‹0 < e› that by (auto simp: d_def)
finally have "cmod (ξ - ζ) < e * cmod z" .
with e have "ξ = ζ"
by (metis ‹ξ^n = z^n› ‹ζ^n = z^n› assms power_eq_imp_eq_norm)
then show "False"
using ‹ball ξ d ≠ ball ζ d› by blast
qed
show "Ex (homeomorphism u ((λw. w ^ n) ` ball z d) (λz. z ^ n))"
if "u ∈ {ball z' d |z'. z' ^ n = z ^ n}" for u
proof (rule invariance_of_domain_homeomorphism [of "u" "λz. z^n"])
show "open u"
using that by auto
show "continuous_on u (λz. z ^ n)"
by (intro continuous_intros)
show "inj_on (λz. z ^ n) u"
using that by (auto simp: iff_x_eq_y inj_on_def)
show "⋀g. homeomorphism u ((λz. z ^ n) ` u) (λz. z ^ n) g ⟹ Ex (homeomorphism u ((λw. w ^ n) ` ball z d) (λz. z ^ n))"
using im_eq that by clarify metis
qed auto
qed auto
qed
qed
show ?thesis
using assms
apply (simp add: covering_space_def zn1 zn2)
apply (subst zn2 [symmetric])
apply (blast intro: zn3)
done
qed

corollary covering_space_square_punctured_plane:
"covering_space (- {0}) (λz::complex. z^2) (- {0})"

proposition covering_space_exp_punctured_plane:
"covering_space UNIV (λz::complex. exp z) (- {0})"
proof (simp add: covering_space_def, intro conjI ballI)
show "continuous_on UNIV (λz::complex. exp z)"
by (rule continuous_on_exp [OF continuous_on_id])
show "range exp = - {0::complex}"
by auto (metis exp_Ln range_eqI)
show "∃T. z ∈ T ∧ openin (subtopology euclidean (- {0})) T ∧
(∃v. ⋃v = exp -` T ∧ (∀u∈v. open u) ∧ disjoint v ∧
(∀u∈v. ∃q. homeomorphism u T exp q))"
if "z ∈ - {0::complex}" for z
proof -
have "z ≠ 0"
using that by auto
have inj_exp: "inj_on exp (ball (Ln z) 1)"
apply (rule inj_on_subset [OF inj_on_exp_pi [of "Ln z"]])
using pi_ge_two by (simp add: ball_subset_ball_iff)
define 𝒱 where "𝒱 ≡ range (λn. (λx. x + of_real (2 * of_int n * pi) * 𝗂) ` (ball(Ln z) 1))"
show ?thesis
proof (intro exI conjI)
show "z ∈ exp ` (ball(Ln z) 1)"
by (metis ‹z ≠ 0› centre_in_ball exp_Ln rev_image_eqI zero_less_one)
have "open (- {0::complex})"
by blast
moreover have "inj_on exp (ball (Ln z) 1)"
apply (rule inj_on_subset [OF inj_on_exp_pi [of "Ln z"]])
using pi_ge_two by (simp add: ball_subset_ball_iff)
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