(* Author: L C Paulson, University of Cambridge
Author: Amine Chaieb, University of Cambridge
Author: Robert Himmelmann, TU Muenchen
Author: Brian Huffman, Portland State University
*)
section \<open>Elementary Metric Spaces\<close>
theory Elementary_Metric_Spaces
imports
Abstract_Topology_2
begin
subsection \<open>Open and closed balls\<close>
definition%important ball :: "'a::metric_space \<Rightarrow> real \<Rightarrow> 'a set"
where "ball x e = {y. dist x y < e}"
definition%important cball :: "'a::metric_space \<Rightarrow> real \<Rightarrow> 'a set"
where "cball x e = {y. dist x y \<le> e}"
definition%important sphere :: "'a::metric_space \<Rightarrow> real \<Rightarrow> 'a set"
where "sphere x e = {y. dist x y = e}"
lemma mem_ball [simp]: "y \<in> ball x e \<longleftrightarrow> dist x y < e"
by (simp add: ball_def)
lemma mem_cball [simp]: "y \<in> cball x e \<longleftrightarrow> dist x y \<le> e"
by (simp add: cball_def)
lemma mem_sphere [simp]: "y \<in> sphere x e \<longleftrightarrow> dist x y = e"
by (simp add: sphere_def)
lemma ball_trivial [simp]: "ball x 0 = {}"
by (simp add: ball_def)
lemma cball_trivial [simp]: "cball x 0 = {x}"
by (simp add: cball_def)
lemma sphere_trivial [simp]: "sphere x 0 = {x}"
by (simp add: sphere_def)
lemma disjoint_ballI: "dist x y \<ge> r+s \<Longrightarrow> ball x r \<inter> ball y s = {}"
using dist_triangle_less_add not_le by fastforce
lemma disjoint_cballI: "dist x y > r + s \<Longrightarrow> cball x r \<inter> cball y s = {}"
by (metis add_mono disjoint_iff_not_equal dist_triangle2 dual_order.trans leD mem_cball)
lemma sphere_empty [simp]: "r < 0 \<Longrightarrow> sphere a r = {}"
for a :: "'a::metric_space"
by auto
lemma centre_in_ball [simp]: "x \<in> ball x e \<longleftrightarrow> 0 < e"
by simp
lemma centre_in_cball [simp]: "x \<in> cball x e \<longleftrightarrow> 0 \<le> e"
by simp
lemma ball_subset_cball [simp, intro]: "ball x e \<subseteq> cball x e"
by (simp add: subset_eq)
lemma mem_ball_imp_mem_cball: "x \<in> ball y e \<Longrightarrow> x \<in> cball y e"
by (auto simp: mem_ball mem_cball)
lemma sphere_cball [simp,intro]: "sphere z r \<subseteq> cball z r"
by force
lemma cball_diff_sphere: "cball a r - sphere a r = ball a r"
by auto
lemma subset_ball[intro]: "d \<le> e \<Longrightarrow> ball x d \<subseteq> ball x e"
by (simp add: subset_eq)
lemma subset_cball[intro]: "d \<le> e \<Longrightarrow> cball x d \<subseteq> cball x e"
by (simp add: subset_eq)
lemma mem_ball_leI: "x \<in> ball y e \<Longrightarrow> e \<le> f \<Longrightarrow> x \<in> ball y f"
by (auto simp: mem_ball mem_cball)
lemma mem_cball_leI: "x \<in> cball y e \<Longrightarrow> e \<le> f \<Longrightarrow> x \<in> cball y f"
by (auto simp: mem_ball mem_cball)
lemma cball_trans: "y \<in> cball z b \<Longrightarrow> x \<in> cball y a \<Longrightarrow> x \<in> cball z (b + a)"
unfolding mem_cball
proof -
have "dist z x \<le> dist z y + dist y x"
by (rule dist_triangle)
also assume "dist z y \<le> b"
also assume "dist y x \<le> a"
finally show "dist z x \<le> b + a" by arith
qed
lemma ball_max_Un: "ball a (max r s) = ball a r \<union> ball a s"
by (simp add: set_eq_iff) arith
lemma ball_min_Int: "ball a (min r s) = ball a r \<inter> ball a s"
by (simp add: set_eq_iff)
lemma cball_max_Un: "cball a (max r s) = cball a r \<union> cball a s"
by (simp add: set_eq_iff) arith
lemma cball_min_Int: "cball a (min r s) = cball a r \<inter> cball a s"
by (simp add: set_eq_iff)
lemma cball_diff_eq_sphere: "cball a r - ball a r = sphere a r"
by (auto simp: cball_def ball_def dist_commute)
lemma open_ball [intro, simp]: "open (ball x e)"
proof -
have "open (dist x -` {..<e})"
by (intro open_vimage open_lessThan continuous_intros)
also have "dist x -` {..<e} = ball x e"
by auto
finally show ?thesis .
qed
lemma open_contains_ball: "open S \<longleftrightarrow> (\<forall>x\<in>S. \<exists>e>0. ball x e \<subseteq> S)"
by (simp add: open_dist subset_eq mem_ball Ball_def dist_commute)
lemma openI [intro?]: "(\<And>x. x\<in>S \<Longrightarrow> \<exists>e>0. ball x e \<subseteq> S) \<Longrightarrow> open S"
by (auto simp: open_contains_ball)
lemma openE[elim?]:
assumes "open S" "x\<in>S"
obtains e where "e>0" "ball x e \<subseteq> S"
using assms unfolding open_contains_ball by auto
lemma open_contains_ball_eq: "open S \<Longrightarrow> x\<in>S \<longleftrightarrow> (\<exists>e>0. ball x e \<subseteq> S)"
by (metis open_contains_ball subset_eq centre_in_ball)
lemma ball_eq_empty[simp]: "ball x e = {} \<longleftrightarrow> e \<le> 0"
unfolding mem_ball set_eq_iff
apply (simp add: not_less)
apply (metis zero_le_dist order_trans dist_self)
done
lemma ball_empty: "e \<le> 0 \<Longrightarrow> ball x e = {}" by simp
lemma closed_cball [iff]: "closed (cball x e)"
proof -
have "closed (dist x -` {..e})"
by (intro closed_vimage closed_atMost continuous_intros)
also have "dist x -` {..e} = cball x e"
by auto
finally show ?thesis .
qed
lemma open_contains_cball: "open S \<longleftrightarrow> (\<forall>x\<in>S. \<exists>e>0. cball x e \<subseteq> S)"
proof -
{
fix x and e::real
assume "x\<in>S" "e>0" "ball x e \<subseteq> S"
then have "\<exists>d>0. cball x d \<subseteq> S" unfolding subset_eq by (rule_tac x="e/2" in exI, auto)
}
moreover
{
fix x and e::real
assume "x\<in>S" "e>0" "cball x e \<subseteq> S"
then have "\<exists>d>0. ball x d \<subseteq> S"
unfolding subset_eq
apply (rule_tac x="e/2" in exI, auto)
done
}
ultimately show ?thesis
unfolding open_contains_ball by auto
qed
lemma open_contains_cball_eq: "open S \<Longrightarrow> (\<forall>x. x \<in> S \<longleftrightarrow> (\<exists>e>0. cball x e \<subseteq> S))"
by (metis open_contains_cball subset_eq order_less_imp_le centre_in_cball)
lemma eventually_nhds_ball: "d > 0 \<Longrightarrow> eventually (\<lambda>x. x \<in> ball z d) (nhds z)"
by (rule eventually_nhds_in_open) simp_all
lemma eventually_at_ball: "d > 0 \<Longrightarrow> eventually (\<lambda>t. t \<in> ball z d \<and> t \<in> A) (at z within A)"
unfolding eventually_at by (intro exI[of _ d]) (simp_all add: dist_commute)
lemma eventually_at_ball': "d > 0 \<Longrightarrow> eventually (\<lambda>t. t \<in> ball z d \<and> t \<noteq> z \<and> t \<in> A) (at z within A)"
unfolding eventually_at by (intro exI[of _ d]) (simp_all add: dist_commute)
lemma at_within_ball: "e > 0 \<Longrightarrow> dist x y < e \<Longrightarrow> at y within ball x e = at y"
by (subst at_within_open) auto
lemma atLeastAtMost_eq_cball:
fixes a b::real
shows "{a .. b} = cball ((a + b)/2) ((b - a)/2)"
by (auto simp: dist_real_def field_simps mem_cball)
lemma greaterThanLessThan_eq_ball:
fixes a b::real
shows "{a <..< b} = ball ((a + b)/2) ((b - a)/2)"
by (auto simp: dist_real_def field_simps mem_ball)
lemma interior_ball [simp]: "interior (ball x e) = ball x e"
by (simp add: interior_open)
lemma cball_eq_empty [simp]: "cball x e = {} \<longleftrightarrow> e < 0"
apply (simp add: set_eq_iff not_le)
apply (metis zero_le_dist dist_self order_less_le_trans)
done
lemma cball_empty [simp]: "e < 0 \<Longrightarrow> cball x e = {}"
by simp
lemma cball_sing:
fixes x :: "'a::metric_space"
shows "e = 0 \<Longrightarrow> cball x e = {x}"
by (auto simp: set_eq_iff)
lemma ball_divide_subset: "d \<ge> 1 \<Longrightarrow> ball x (e/d) \<subseteq> ball x e"
apply (cases "e \<le> 0")
apply (simp add: ball_empty divide_simps)
apply (rule subset_ball)
apply (simp add: divide_simps)
done
lemma ball_divide_subset_numeral: "ball x (e / numeral w) \<subseteq> ball x e"
using ball_divide_subset one_le_numeral by blast
lemma cball_divide_subset: "d \<ge> 1 \<Longrightarrow> cball x (e/d) \<subseteq> cball x e"
apply (cases "e < 0")
apply (simp add: divide_simps)
apply (rule subset_cball)
apply (metis div_by_1 frac_le not_le order_refl zero_less_one)
done
lemma cball_divide_subset_numeral: "cball x (e / numeral w) \<subseteq> cball x e"
using cball_divide_subset one_le_numeral by blast
subsection \<open>Limit Points\<close>
lemma islimpt_approachable:
fixes x :: "'a::metric_space"
shows "x islimpt S \<longleftrightarrow> (\<forall>e>0. \<exists>x'\<in>S. x' \<noteq> x \<and> dist x' x < e)"
unfolding islimpt_iff_eventually eventually_at by fast
lemma islimpt_approachable_le: "x islimpt S \<longleftrightarrow> (\<forall>e>0. \<exists>x'\<in> S. x' \<noteq> x \<and> dist x' x \<le> e)"
for x :: "'a::metric_space"
unfolding islimpt_approachable
using approachable_lt_le [where f="\<lambda>y. dist y x" and P="\<lambda>y. y \<notin> S \<or> y = x",
THEN arg_cong [where f=Not]]
by (simp add: Bex_def conj_commute conj_left_commute)
lemma limpt_of_limpts: "x islimpt {y. y islimpt S} \<Longrightarrow> x islimpt S"
for x :: "'a::metric_space"
apply (clarsimp simp add: islimpt_approachable)
apply (drule_tac x="e/2" in spec)
apply (auto simp: simp del: less_divide_eq_numeral1)
apply (drule_tac x="dist x' x" in spec)
apply (auto simp: zero_less_dist_iff simp del: less_divide_eq_numeral1)
apply (erule rev_bexI)
apply (metis dist_commute dist_triangle_half_r less_trans less_irrefl)
done
lemma closed_limpts: "closed {x::'a::metric_space. x islimpt S}"
using closed_limpt limpt_of_limpts by blast
lemma limpt_of_closure: "x islimpt closure S \<longleftrightarrow> x islimpt S"
for x :: "'a::metric_space"
by (auto simp: closure_def islimpt_Un dest: limpt_of_limpts)
lemma islimpt_eq_infinite_ball: "x islimpt S \<longleftrightarrow> (\<forall>e>0. infinite(S \<inter> ball x e))"
apply (simp add: islimpt_eq_acc_point, safe)
apply (metis Int_commute open_ball centre_in_ball)
by (metis open_contains_ball Int_mono finite_subset inf_commute subset_refl)
lemma islimpt_eq_infinite_cball: "x islimpt S \<longleftrightarrow> (\<forall>e>0. infinite(S \<inter> cball x e))"
apply (simp add: islimpt_eq_infinite_ball, safe)
apply (meson Int_mono ball_subset_cball finite_subset order_refl)
by (metis open_ball centre_in_ball finite_Int inf.absorb_iff2 inf_assoc open_contains_cball_eq)
subsection \<open>Perfect Metric Spaces\<close>
lemma perfect_choose_dist: "0 < r \<Longrightarrow> \<exists>a. a \<noteq> x \<and> dist a x < r"
for x :: "'a::{perfect_space,metric_space}"
using islimpt_UNIV [of x] by (simp add: islimpt_approachable)
lemma cball_eq_sing:
fixes x :: "'a::{metric_space,perfect_space}"
shows "cball x e = {x} \<longleftrightarrow> e = 0"
proof (rule linorder_cases)
assume e: "0 < e"
obtain a where "a \<noteq> x" "dist a x < e"
using perfect_choose_dist [OF e] by auto
then have "a \<noteq> x" "dist x a \<le> e"
by (auto simp: dist_commute)
with e show ?thesis by (auto simp: set_eq_iff)
qed auto
subsection \<open>?\<close>
lemma finite_ball_include:
fixes a :: "'a::metric_space"
assumes "finite S"
shows "\<exists>e>0. S \<subseteq> ball a e"
using assms
proof induction
case (insert x S)
then obtain e0 where "e0>0" and e0:"S \<subseteq> ball a e0" by auto
define e where "e = max e0 (2 * dist a x)"
have "e>0" unfolding e_def using \<open>e0>0\<close> by auto
moreover have "insert x S \<subseteq> ball a e"
using e0 \<open>e>0\<close> unfolding e_def by auto
ultimately show ?case by auto
qed (auto intro: zero_less_one)
lemma finite_set_avoid:
fixes a :: "'a::metric_space"
assumes "finite S"
shows "\<exists>d>0. \<forall>x\<in>S. x \<noteq> a \<longrightarrow> d \<le> dist a x"
using assms
proof induction
case (insert x S)
then obtain d where "d > 0" and d: "\<forall>x\<in>S. x \<noteq> a \<longrightarrow> d \<le> dist a x"
by blast
show ?case
proof (cases "x = a")
case True
with \<open>d > 0 \<close>d show ?thesis by auto
next
case False
let ?d = "min d (dist a x)"
from False \<open>d > 0\<close> have dp: "?d > 0"
by auto
from d have d': "\<forall>x\<in>S. x \<noteq> a \<longrightarrow> ?d \<le> dist a x"
by auto
with dp False show ?thesis
by (metis insert_iff le_less min_less_iff_conj not_less)
qed
qed (auto intro: zero_less_one)
lemma discrete_imp_closed:
fixes S :: "'a::metric_space set"
assumes e: "0 < e"
and d: "\<forall>x \<in> S. \<forall>y \<in> S. dist y x < e \<longrightarrow> y = x"
shows "closed S"
proof -
have False if C: "\<And>e. e>0 \<Longrightarrow> \<exists>x'\<in>S. x' \<noteq> x \<and> dist x' x < e" for x
proof -
from e have e2: "e/2 > 0" by arith
from C[rule_format, OF e2] obtain y where y: "y \<in> S" "y \<noteq> x" "dist y x < e/2"
by blast
let ?m = "min (e/2) (dist x y) "
from e2 y(2) have mp: "?m > 0"
by simp
from C[OF mp] obtain z where z: "z \<in> S" "z \<noteq> x" "dist z x < ?m"
by blast
from z y have "dist z y < e"
by (intro dist_triangle_lt [where z=x]) simp
from d[rule_format, OF y(1) z(1) this] y z show ?thesis
by (auto simp: dist_commute)
qed
then show ?thesis
by (metis islimpt_approachable closed_limpt [where 'a='a])
qed
subsection \<open>Interior\<close>
lemma mem_interior: "x \<in> interior S \<longleftrightarrow> (\<exists>e>0. ball x e \<subseteq> S)"
using open_contains_ball_eq [where S="interior S"]
by (simp add: open_subset_interior)
lemma mem_interior_cball: "x \<in> interior S \<longleftrightarrow> (\<exists>e>0. cball x e \<subseteq> S)"
by (meson ball_subset_cball interior_subset mem_interior open_contains_cball open_interior
subset_trans)
subsection \<open>Frontier\<close>
lemma frontier_straddle:
fixes a :: "'a::metric_space"
shows "a \<in> frontier S \<longleftrightarrow> (\<forall>e>0. (\<exists>x\<in>S. dist a x < e) \<and> (\<exists>x. x \<notin> S \<and> dist a x < e))"
unfolding frontier_def closure_interior
by (auto simp: mem_interior subset_eq ball_def)
subsection \<open>Limits\<close>
proposition Lim: "(f \<longlongrightarrow> l) net \<longleftrightarrow> trivial_limit net \<or> (\<forall>e>0. eventually (\<lambda>x. dist (f x) l < e) net)"
by (auto simp: tendsto_iff trivial_limit_eq)
text \<open>Show that they yield usual definitions in the various cases.\<close>
proposition Lim_within_le: "(f \<longlongrightarrow> l)(at a within S) \<longleftrightarrow>
(\<forall>e>0. \<exists>d>0. \<forall>x\<in>S. 0 < dist x a \<and> dist x a \<le> d \<longrightarrow> dist (f x) l < e)"
by (auto simp: tendsto_iff eventually_at_le)
proposition Lim_within: "(f \<longlongrightarrow> l) (at a within S) \<longleftrightarrow>
(\<forall>e >0. \<exists>d>0. \<forall>x \<in> S. 0 < dist x a \<and> dist x a < d \<longrightarrow> dist (f x) l < e)"
by (auto simp: tendsto_iff eventually_at)
corollary Lim_withinI [intro?]:
assumes "\<And>e. e > 0 \<Longrightarrow> \<exists>d>0. \<forall>x \<in> S. 0 < dist x a \<and> dist x a < d \<longrightarrow> dist (f x) l \<le> e"
shows "(f \<longlongrightarrow> l) (at a within S)"
apply (simp add: Lim_within, clarify)
apply (rule ex_forward [OF assms [OF half_gt_zero]], auto)
done
proposition Lim_at: "(f \<longlongrightarrow> l) (at a) \<longleftrightarrow>
(\<forall>e >0. \<exists>d>0. \<forall>x. 0 < dist x a \<and> dist x a < d \<longrightarrow> dist (f x) l < e)"
by (auto simp: tendsto_iff eventually_at)
lemma Lim_transform_within_set:
fixes a :: "'a::metric_space" and l :: "'b::metric_space"
shows "\<lbrakk>(f \<longlongrightarrow> l) (at a within S); eventually (\<lambda>x. x \<in> S \<longleftrightarrow> x \<in> T) (at a)\<rbrakk>
\<Longrightarrow> (f \<longlongrightarrow> l) (at a within T)"
apply (clarsimp simp: eventually_at Lim_within)
apply (drule_tac x=e in spec, clarify)
apply (rename_tac k)
apply (rule_tac x="min d k" in exI, simp)
done
text \<open>Another limit point characterization.\<close>
lemma limpt_sequential_inj:
fixes x :: "'a::metric_space"
shows "x islimpt S \<longleftrightarrow>
(\<exists>f. (\<forall>n::nat. f n \<in> S - {x}) \<and> inj f \<and> (f \<longlongrightarrow> x) sequentially)"
(is "?lhs = ?rhs")
proof
assume ?lhs
then have "\<forall>e>0. \<exists>x'\<in>S. x' \<noteq> x \<and> dist x' x < e"
by (force simp: islimpt_approachable)
then obtain y where y: "\<And>e. e>0 \<Longrightarrow> y e \<in> S \<and> y e \<noteq> x \<and> dist (y e) x < e"
by metis
define f where "f \<equiv> rec_nat (y 1) (\<lambda>n fn. y (min (inverse(2 ^ (Suc n))) (dist fn x)))"
have [simp]: "f 0 = y 1"
"f(Suc n) = y (min (inverse(2 ^ (Suc n))) (dist (f n) x))" for n
by (simp_all add: f_def)
have f: "f n \<in> S \<and> (f n \<noteq> x) \<and> dist (f n) x < inverse(2 ^ n)" for n
proof (induction n)
case 0 show ?case
by (simp add: y)
next
case (Suc n) then show ?case
apply (auto simp: y)
by (metis half_gt_zero_iff inverse_positive_iff_positive less_divide_eq_numeral1(1) min_less_iff_conj y zero_less_dist_iff zero_less_numeral zero_less_power)
qed
show ?rhs
proof (rule_tac x=f in exI, intro conjI allI)
show "\<And>n. f n \<in> S - {x}"
using f by blast
have "dist (f n) x < dist (f m) x" if "m < n" for m n
using that
proof (induction n)
case 0 then show ?case by simp
next
case (Suc n)
then consider "m < n" | "m = n" using less_Suc_eq by blast
then show ?case
proof cases
assume "m < n"
have "dist (f(Suc n)) x = dist (y (min (inverse(2 ^ (Suc n))) (dist (f n) x))) x"
by simp
also have "\<dots> < dist (f n) x"
by (metis dist_pos_lt f min.strict_order_iff min_less_iff_conj y)
also have "\<dots> < dist (f m) x"
using Suc.IH \<open>m < n\<close> by blast
finally show ?thesis .
next
assume "m = n" then show ?case
by simp (metis dist_pos_lt f half_gt_zero_iff inverse_positive_iff_positive min_less_iff_conj y zero_less_numeral zero_less_power)
qed
qed
then show "inj f"
by (metis less_irrefl linorder_injI)
show "f \<longlonglongrightarrow> x"
apply (rule tendstoI)
apply (rule_tac c="nat (ceiling(1/e))" in eventually_sequentiallyI)
apply (rule less_trans [OF f [THEN conjunct2, THEN conjunct2]])
apply (simp add: field_simps)
by (meson le_less_trans mult_less_cancel_left not_le of_nat_less_two_power)
qed
next
assume ?rhs
then show ?lhs
by (fastforce simp add: islimpt_approachable lim_sequentially)
qed
lemma Lim_dist_ubound:
assumes "\<not>(trivial_limit net)"
and "(f \<longlongrightarrow> l) net"
and "eventually (\<lambda>x. dist a (f x) \<le> e) net"
shows "dist a l \<le> e"
using assms by (fast intro: tendsto_le tendsto_intros)
subsection \<open>Continuity\<close>
text\<open>Derive the epsilon-delta forms, which we often use as "definitions"\<close>
proposition continuous_within_eps_delta:
"continuous (at x within s) f \<longleftrightarrow> (\<forall>e>0. \<exists>d>0. \<forall>x'\<in> s. dist x' x < d --> dist (f x') (f x) < e)"
unfolding continuous_within and Lim_within by fastforce
corollary continuous_at_eps_delta:
"continuous (at x) f \<longleftrightarrow> (\<forall>e > 0. \<exists>d > 0. \<forall>x'. dist x' x < d \<longrightarrow> dist (f x') (f x) < e)"
using continuous_within_eps_delta [of x UNIV f] by simp
lemma continuous_at_right_real_increasing:
fixes f :: "real \<Rightarrow> real"
assumes nondecF: "\<And>x y. x \<le> y \<Longrightarrow> f x \<le> f y"
shows "continuous (at_right a) f \<longleftrightarrow> (\<forall>e>0. \<exists>d>0. f (a + d) - f a < e)"
apply (simp add: greaterThan_def dist_real_def continuous_within Lim_within_le)
apply (intro all_cong ex_cong, safe)
apply (erule_tac x="a + d" in allE, simp)
apply (simp add: nondecF field_simps)
apply (drule nondecF, simp)
done
lemma continuous_at_left_real_increasing:
assumes nondecF: "\<And> x y. x \<le> y \<Longrightarrow> f x \<le> ((f y) :: real)"
shows "(continuous (at_left (a :: real)) f) = (\<forall>e > 0. \<exists>delta > 0. f a - f (a - delta) < e)"
apply (simp add: lessThan_def dist_real_def continuous_within Lim_within_le)
apply (intro all_cong ex_cong, safe)
apply (erule_tac x="a - d" in allE, simp)
apply (simp add: nondecF field_simps)
apply (cut_tac x="a - d" and y=x in nondecF, simp_all)
done
text\<open>Versions in terms of open balls.\<close>
lemma continuous_within_ball:
"continuous (at x within s) f \<longleftrightarrow>
(\<forall>e > 0. \<exists>d > 0. f ` (ball x d \<inter> s) \<subseteq> ball (f x) e)"
(is "?lhs = ?rhs")
proof
assume ?lhs
{
fix e :: real
assume "e > 0"
then obtain d where d: "d>0" "\<forall>xa\<in>s. 0 < dist xa x \<and> dist xa x < d \<longrightarrow> dist (f xa) (f x) < e"
using \<open>?lhs\<close>[unfolded continuous_within Lim_within] by auto
{
fix y
assume "y \<in> f ` (ball x d \<inter> s)"
then have "y \<in> ball (f x) e"
using d(2)
apply (auto simp: dist_commute)
apply (erule_tac x=xa in ballE, auto)
using \<open>e > 0\<close>
apply auto
done
}
then have "\<exists>d>0. f ` (ball x d \<inter> s) \<subseteq> ball (f x) e"
using \<open>d > 0\<close>
unfolding subset_eq ball_def by (auto simp: dist_commute)
}
then show ?rhs by auto
next
assume ?rhs
then show ?lhs
unfolding continuous_within Lim_within ball_def subset_eq
apply (auto simp: dist_commute)
apply (erule_tac x=e in allE, auto)
done
qed
lemma continuous_at_ball:
"continuous (at x) f \<longleftrightarrow> (\<forall>e>0. \<exists>d>0. f ` (ball x d) \<subseteq> ball (f x) e)" (is "?lhs = ?rhs")
proof
assume ?lhs
then show ?rhs
unfolding continuous_at Lim_at subset_eq Ball_def Bex_def image_iff mem_ball
apply auto
apply (erule_tac x=e in allE, auto)
apply (rule_tac x=d in exI, auto)
apply (erule_tac x=xa in allE)
apply (auto simp: dist_commute)
done
next
assume ?rhs
then show ?lhs
unfolding continuous_at Lim_at subset_eq Ball_def Bex_def image_iff mem_ball
apply auto
apply (erule_tac x=e in allE, auto)
apply (rule_tac x=d in exI, auto)
apply (erule_tac x="f xa" in allE)
apply (auto simp: dist_commute)
done
qed
text\<open>Define setwise continuity in terms of limits within the set.\<close>
lemma continuous_on_iff:
"continuous_on s f \<longleftrightarrow>
(\<forall>x\<in>s. \<forall>e>0. \<exists>d>0. \<forall>x'\<in>s. dist x' x < d \<longrightarrow> dist (f x') (f x) < e)"
unfolding continuous_on_def Lim_within
by (metis dist_pos_lt dist_self)
lemma continuous_within_E:
assumes "continuous (at x within s) f" "e>0"
obtains d where "d>0" "\<And>x'. \<lbrakk>x'\<in> s; dist x' x \<le> d\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e"
using assms apply (simp add: continuous_within_eps_delta)
apply (drule spec [of _ e], clarify)
apply (rule_tac d="d/2" in that, auto)
done
lemma continuous_onI [intro?]:
assumes "\<And>x e. \<lbrakk>e > 0; x \<in> s\<rbrakk> \<Longrightarrow> \<exists>d>0. \<forall>x'\<in>s. dist x' x < d \<longrightarrow> dist (f x') (f x) \<le> e"
shows "continuous_on s f"
apply (simp add: continuous_on_iff, clarify)
apply (rule ex_forward [OF assms [OF half_gt_zero]], auto)
done
text\<open>Some simple consequential lemmas.\<close>
lemma continuous_onE:
assumes "continuous_on s f" "x\<in>s" "e>0"
obtains d where "d>0" "\<And>x'. \<lbrakk>x' \<in> s; dist x' x \<le> d\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e"
using assms
apply (simp add: continuous_on_iff)
apply (elim ballE allE)
apply (auto intro: that [where d="d/2" for d])
done
text\<open>The usual transformation theorems.\<close>
lemma continuous_transform_within:
fixes f g :: "'a::metric_space \<Rightarrow> 'b::topological_space"
assumes "continuous (at x within s) f"
and "0 < d"
and "x \<in> s"
and "\<And>x'. \<lbrakk>x' \<in> s; dist x' x < d\<rbrakk> \<Longrightarrow> f x' = g x'"
shows "continuous (at x within s) g"
using assms
unfolding continuous_within
by (force intro: Lim_transform_within)
subsection \<open>Closure and Limit Characterization\<close>
lemma closure_approachable:
fixes S :: "'a::metric_space set"
shows "x \<in> closure S \<longleftrightarrow> (\<forall>e>0. \<exists>y\<in>S. dist y x < e)"
apply (auto simp: closure_def islimpt_approachable)
apply (metis dist_self)
done
lemma closure_approachable_le:
fixes S :: "'a::metric_space set"
shows "x \<in> closure S \<longleftrightarrow> (\<forall>e>0. \<exists>y\<in>S. dist y x \<le> e)"
unfolding closure_approachable
using dense by force
lemma closure_approachableD:
assumes "x \<in> closure S" "e>0"
shows "\<exists>y\<in>S. dist x y < e"
using assms unfolding closure_approachable by (auto simp: dist_commute)
lemma closed_approachable:
fixes S :: "'a::metric_space set"
shows "closed S \<Longrightarrow> (\<forall>e>0. \<exists>y\<in>S. dist y x < e) \<longleftrightarrow> x \<in> S"
by (metis closure_closed closure_approachable)
lemma closure_contains_Inf:
fixes S :: "real set"
assumes "S \<noteq> {}" "bdd_below S"
shows "Inf S \<in> closure S"
proof -
have *: "\<forall>x\<in>S. Inf S \<le> x"
using cInf_lower[of _ S] assms by metis
{
fix e :: real
assume "e > 0"
then have "Inf S < Inf S + e" by simp
with assms obtain x where "x \<in> S" "x < Inf S + e"
by (subst (asm) cInf_less_iff) auto
with * have "\<exists>x\<in>S. dist x (Inf S) < e"
by (intro bexI[of _ x]) (auto simp: dist_real_def)
}
then show ?thesis unfolding closure_approachable by auto
qed
lemma not_trivial_limit_within_ball:
"\<not> trivial_limit (at x within S) \<longleftrightarrow> (\<forall>e>0. S \<inter> ball x e - {x} \<noteq> {})"
(is "?lhs \<longleftrightarrow> ?rhs")
proof
show ?rhs if ?lhs
proof -
{
fix e :: real
assume "e > 0"
then obtain y where "y \<in> S - {x}" and "dist y x < e"
using \<open>?lhs\<close> not_trivial_limit_within[of x S] closure_approachable[of x "S - {x}"]
by auto
then have "y \<in> S \<inter> ball x e - {x}"
unfolding ball_def by (simp add: dist_commute)
then have "S \<inter> ball x e - {x} \<noteq> {}" by blast
}
then show ?thesis by auto
qed
show ?lhs if ?rhs
proof -
{
fix e :: real
assume "e > 0"
then obtain y where "y \<in> S \<inter> ball x e - {x}"
using \<open>?rhs\<close> by blast
then have "y \<in> S - {x}" and "dist y x < e"
unfolding ball_def by (simp_all add: dist_commute)
then have "\<exists>y \<in> S - {x}. dist y x < e"
by auto
}
then show ?thesis
using not_trivial_limit_within[of x S] closure_approachable[of x "S - {x}"]
by auto
qed
qed
subsection \<open>Boundedness\<close>
(* FIXME: This has to be unified with BSEQ!! *)
definition%important (in metric_space) bounded :: "'a set \<Rightarrow> bool"
where "bounded S \<longleftrightarrow> (\<exists>x e. \<forall>y\<in>S. dist x y \<le> e)"
lemma bounded_subset_cball: "bounded S \<longleftrightarrow> (\<exists>e x. S \<subseteq> cball x e \<and> 0 \<le> e)"
unfolding bounded_def subset_eq by auto (meson order_trans zero_le_dist)
lemma bounded_any_center: "bounded S \<longleftrightarrow> (\<exists>e. \<forall>y\<in>S. dist a y \<le> e)"
unfolding bounded_def
by auto (metis add.commute add_le_cancel_right dist_commute dist_triangle_le)
lemma bounded_iff: "bounded S \<longleftrightarrow> (\<exists>a. \<forall>x\<in>S. norm x \<le> a)"
unfolding bounded_any_center [where a=0]
by (simp add: dist_norm)
lemma bdd_above_norm: "bdd_above (norm ` X) \<longleftrightarrow> bounded X"
by (simp add: bounded_iff bdd_above_def)
lemma bounded_norm_comp: "bounded ((\<lambda>x. norm (f x)) ` S) = bounded (f ` S)"
by (simp add: bounded_iff)
lemma boundedI:
assumes "\<And>x. x \<in> S \<Longrightarrow> norm x \<le> B"
shows "bounded S"
using assms bounded_iff by blast
lemma bounded_empty [simp]: "bounded {}"
by (simp add: bounded_def)
lemma bounded_subset: "bounded T \<Longrightarrow> S \<subseteq> T \<Longrightarrow> bounded S"
by (metis bounded_def subset_eq)
lemma bounded_interior[intro]: "bounded S \<Longrightarrow> bounded(interior S)"
by (metis bounded_subset interior_subset)
lemma bounded_closure[intro]:
assumes "bounded S"
shows "bounded (closure S)"
proof -
from assms obtain x and a where a: "\<forall>y\<in>S. dist x y \<le> a"
unfolding bounded_def by auto
{
fix y
assume "y \<in> closure S"
then obtain f where f: "\<forall>n. f n \<in> S" "(f \<longlongrightarrow> y) sequentially"
unfolding closure_sequential by auto
have "\<forall>n. f n \<in> S \<longrightarrow> dist x (f n) \<le> a" using a by simp
then have "eventually (\<lambda>n. dist x (f n) \<le> a) sequentially"
by (simp add: f(1))
have "dist x y \<le> a"
apply (rule Lim_dist_ubound [of sequentially f])
apply (rule trivial_limit_sequentially)
apply (rule f(2))
apply fact
done
}
then show ?thesis
unfolding bounded_def by auto
qed
lemma bounded_closure_image: "bounded (f ` closure S) \<Longrightarrow> bounded (f ` S)"
by (simp add: bounded_subset closure_subset image_mono)
lemma bounded_cball[simp,intro]: "bounded (cball x e)"
apply (simp add: bounded_def)
apply (rule_tac x=x in exI)
apply (rule_tac x=e in exI, auto)
done
lemma bounded_ball[simp,intro]: "bounded (ball x e)"
by (metis ball_subset_cball bounded_cball bounded_subset)
lemma bounded_Un[simp]: "bounded (S \<union> T) \<longleftrightarrow> bounded S \<and> bounded T"
by (auto simp: bounded_def) (metis Un_iff bounded_any_center le_max_iff_disj)
lemma bounded_Union[intro]: "finite F \<Longrightarrow> \<forall>S\<in>F. bounded S \<Longrightarrow> bounded (\<Union>F)"
by (induct rule: finite_induct[of F]) auto
lemma bounded_UN [intro]: "finite A \<Longrightarrow> \<forall>x\<in>A. bounded (B x) \<Longrightarrow> bounded (\<Union>x\<in>A. B x)"
by (induct set: finite) auto
lemma bounded_insert [simp]: "bounded (insert x S) \<longleftrightarrow> bounded S"
proof -
have "\<forall>y\<in>{x}. dist x y \<le> 0"
by simp
then have "bounded {x}"
unfolding bounded_def by fast
then show ?thesis
by (metis insert_is_Un bounded_Un)
qed
lemma bounded_subset_ballI: "S \<subseteq> ball x r \<Longrightarrow> bounded S"
by (meson bounded_ball bounded_subset)
lemma bounded_subset_ballD:
assumes "bounded S" shows "\<exists>r. 0 < r \<and> S \<subseteq> ball x r"
proof -
obtain e::real and y where "S \<subseteq> cball y e" "0 \<le> e"
using assms by (auto simp: bounded_subset_cball)
then show ?thesis
apply (rule_tac x="dist x y + e + 1" in exI)
apply (simp add: add.commute add_pos_nonneg)
apply (erule subset_trans)
apply (clarsimp simp add: cball_def)
by (metis add_le_cancel_right add_strict_increasing dist_commute dist_triangle_le zero_less_one)
qed
lemma finite_imp_bounded [intro]: "finite S \<Longrightarrow> bounded S"
by (induct set: finite) simp_all
lemma bounded_Int[intro]: "bounded S \<or> bounded T \<Longrightarrow> bounded (S \<inter> T)"
by (metis Int_lower1 Int_lower2 bounded_subset)
lemma bounded_diff[intro]: "bounded S \<Longrightarrow> bounded (S - T)"
by (metis Diff_subset bounded_subset)
lemma bounded_dist_comp:
assumes "bounded (f ` S)" "bounded (g ` S)"
shows "bounded ((\<lambda>x. dist (f x) (g x)) ` S)"
proof -
from assms obtain M1 M2 where *: "dist (f x) undefined \<le> M1" "dist undefined (g x) \<le> M2" if "x \<in> S" for x
by (auto simp: bounded_any_center[of _ undefined] dist_commute)
have "dist (f x) (g x) \<le> M1 + M2" if "x \<in> S" for x
using *[OF that]
by (rule order_trans[OF dist_triangle add_mono])
then show ?thesis
by (auto intro!: boundedI)
qed
lemma bounded_Times:
assumes "bounded s" "bounded t"
shows "bounded (s \<times> t)"
proof -
obtain x y a b where "\<forall>z\<in>s. dist x z \<le> a" "\<forall>z\<in>t. dist y z \<le> b"
using assms [unfolded bounded_def] by auto
then have "\<forall>z\<in>s \<times> t. dist (x, y) z \<le> sqrt (a\<^sup>2 + b\<^sup>2)"
by (auto simp: dist_Pair_Pair real_sqrt_le_mono add_mono power_mono)
then show ?thesis unfolding bounded_any_center [where a="(x, y)"] by auto
qed
subsection \<open>Compactness\<close>
lemma compact_imp_bounded:
assumes "compact U"
shows "bounded U"
proof -
have "compact U" "\<forall>x\<in>U. open (ball x 1)" "U \<subseteq> (\<Union>x\<in>U. ball x 1)"
using assms by auto
then obtain D where D: "D \<subseteq> U" "finite D" "U \<subseteq> (\<Union>x\<in>D. ball x 1)"
by (metis compactE_image)
from \<open>finite D\<close> have "bounded (\<Union>x\<in>D. ball x 1)"
by (simp add: bounded_UN)
then show "bounded U" using \<open>U \<subseteq> (\<Union>x\<in>D. ball x 1)\<close>
by (rule bounded_subset)
qed
lemma closure_Int_ball_not_empty:
assumes "S \<subseteq> closure T" "x \<in> S" "r > 0"
shows "T \<inter> ball x r \<noteq> {}"
using assms centre_in_ball closure_iff_nhds_not_empty by blast
lemma compact_sup_maxdistance:
fixes s :: "'a::metric_space set"
assumes "compact s"
and "s \<noteq> {}"
shows "\<exists>x\<in>s. \<exists>y\<in>s. \<forall>u\<in>s. \<forall>v\<in>s. dist u v \<le> dist x y"
proof -
have "compact (s \<times> s)"
using \<open>compact s\<close> by (intro compact_Times)
moreover have "s \<times> s \<noteq> {}"
using \<open>s \<noteq> {}\<close> by auto
moreover have "continuous_on (s \<times> s) (\<lambda>x. dist (fst x) (snd x))"
by (intro continuous_at_imp_continuous_on ballI continuous_intros)
ultimately show ?thesis
using continuous_attains_sup[of "s \<times> s" "\<lambda>x. dist (fst x) (snd x)"] by auto
qed
subsubsection\<open>Totally bounded\<close>
lemma cauchy_def: "Cauchy s \<longleftrightarrow> (\<forall>e>0. \<exists>N. \<forall>m n. m \<ge> N \<and> n \<ge> N \<longrightarrow> dist (s m) (s n) < e)"
unfolding Cauchy_def by metis
proposition seq_compact_imp_totally_bounded:
assumes "seq_compact s"
shows "\<forall>e>0. \<exists>k. finite k \<and> k \<subseteq> s \<and> s \<subseteq> (\<Union>x\<in>k. ball x e)"
proof -
{ fix e::real assume "e > 0" assume *: "\<And>k. finite k \<Longrightarrow> k \<subseteq> s \<Longrightarrow> \<not> s \<subseteq> (\<Union>x\<in>k. ball x e)"
let ?Q = "\<lambda>x n r. r \<in> s \<and> (\<forall>m < (n::nat). \<not> (dist (x m) r < e))"
have "\<exists>x. \<forall>n::nat. ?Q x n (x n)"
proof (rule dependent_wellorder_choice)
fix n x assume "\<And>y. y < n \<Longrightarrow> ?Q x y (x y)"
then have "\<not> s \<subseteq> (\<Union>x\<in>x ` {0..<n}. ball x e)"
using *[of "x ` {0 ..< n}"] by (auto simp: subset_eq)
then obtain z where z:"z\<in>s" "z \<notin> (\<Union>x\<in>x ` {0..<n}. ball x e)"
unfolding subset_eq by auto
show "\<exists>r. ?Q x n r"
using z by auto
qed simp
then obtain x where "\<forall>n::nat. x n \<in> s" and x:"\<And>n m. m < n \<Longrightarrow> \<not> (dist (x m) (x n) < e)"
by blast
then obtain l r where "l \<in> s" and r:"strict_mono r" and "((x \<circ> r) \<longlongrightarrow> l) sequentially"
using assms by (metis seq_compact_def)
from this(3) have "Cauchy (x \<circ> r)"
using LIMSEQ_imp_Cauchy by auto
then obtain N::nat where "\<And>m n. N \<le> m \<Longrightarrow> N \<le> n \<Longrightarrow> dist ((x \<circ> r) m) ((x \<circ> r) n) < e"
unfolding cauchy_def using \<open>e > 0\<close> by blast
then have False
using x[of "r N" "r (N+1)"] r by (auto simp: strict_mono_def) }
then show ?thesis
by metis
qed
subsubsection\<open>Heine-Borel theorem\<close>
proposition seq_compact_imp_Heine_Borel:
fixes s :: "'a :: metric_space set"
assumes "seq_compact s"
shows "compact s"
proof -
from seq_compact_imp_totally_bounded[OF \<open>seq_compact s\<close>]
obtain f where f: "\<forall>e>0. finite (f e) \<and> f e \<subseteq> s \<and> s \<subseteq> (\<Union>x\<in>f e. ball x e)"
unfolding choice_iff' ..
define K where "K = (\<lambda>(x, r). ball x r) ` ((\<Union>e \<in> \<rat> \<inter> {0 <..}. f e) \<times> \<rat>)"
have "countably_compact s"
using \<open>seq_compact s\<close> by (rule seq_compact_imp_countably_compact)
then show "compact s"
proof (rule countably_compact_imp_compact)
show "countable K"
unfolding K_def using f
by (auto intro: countable_finite countable_subset countable_rat
intro!: countable_image countable_SIGMA countable_UN)
show "\<forall>b\<in>K. open b" by (auto simp: K_def)
next
fix T x
assume T: "open T" "x \<in> T" and x: "x \<in> s"
from openE[OF T] obtain e where "0 < e" "ball x e \<subseteq> T"
by auto
then have "0 < e / 2" "ball x (e / 2) \<subseteq> T"
by auto
from Rats_dense_in_real[OF \<open>0 < e / 2\<close>] obtain r where "r \<in> \<rat>" "0 < r" "r < e / 2"
by auto
from f[rule_format, of r] \<open>0 < r\<close> \<open>x \<in> s\<close> obtain k where "k \<in> f r" "x \<in> ball k r"
by auto
from \<open>r \<in> \<rat>\<close> \<open>0 < r\<close> \<open>k \<in> f r\<close> have "ball k r \<in> K"
by (auto simp: K_def)
then show "\<exists>b\<in>K. x \<in> b \<and> b \<inter> s \<subseteq> T"
proof (rule bexI[rotated], safe)
fix y
assume "y \<in> ball k r"
with \<open>r < e / 2\<close> \<open>x \<in> ball k r\<close> have "dist x y < e"
by (intro dist_triangle_half_r [of k _ e]) (auto simp: dist_commute)
with \<open>ball x e \<subseteq> T\<close> show "y \<in> T"
by auto
next
show "x \<in> ball k r" by fact
qed
qed
qed
proposition compact_eq_seq_compact_metric:
"compact (s :: 'a::metric_space set) \<longleftrightarrow> seq_compact s"
using compact_imp_seq_compact seq_compact_imp_Heine_Borel by blast
proposition compact_def: \<comment> \<open>this is the definition of compactness in HOL Light\<close>
"compact (S :: 'a::metric_space set) \<longleftrightarrow>
(\<forall>f. (\<forall>n. f n \<in> S) \<longrightarrow> (\<exists>l\<in>S. \<exists>r::nat\<Rightarrow>nat. strict_mono r \<and> (f \<circ> r) \<longlonglongrightarrow> l))"
unfolding compact_eq_seq_compact_metric seq_compact_def by auto
subsubsection \<open>Complete the chain of compactness variants\<close>
proposition compact_eq_Bolzano_Weierstrass:
fixes s :: "'a::metric_space set"
shows "compact s \<longleftrightarrow> (\<forall>t. infinite t \<and> t \<subseteq> s --> (\<exists>x \<in> s. x islimpt t))"
(is "?lhs = ?rhs")
proof
assume ?lhs
then show ?rhs
using Heine_Borel_imp_Bolzano_Weierstrass[of s] by auto
next
assume ?rhs
then show ?lhs
unfolding compact_eq_seq_compact_metric by (rule Bolzano_Weierstrass_imp_seq_compact)
qed
proposition Bolzano_Weierstrass_imp_bounded:
"\<forall>t. infinite t \<and> t \<subseteq> s \<longrightarrow> (\<exists>x \<in> s. x islimpt t) \<Longrightarrow> bounded s"
using compact_imp_bounded unfolding compact_eq_Bolzano_Weierstrass .
subsection \<open>Banach fixed point theorem\<close>
theorem banach_fix:\<comment> \<open>TODO: rename to \<open>Banach_fix\<close>\<close>
assumes s: "complete s" "s \<noteq> {}"
and c: "0 \<le> c" "c < 1"
and f: "f ` s \<subseteq> s"
and lipschitz: "\<forall>x\<in>s. \<forall>y\<in>s. dist (f x) (f y) \<le> c * dist x y"
shows "\<exists>!x\<in>s. f x = x"
proof -
from c have "1 - c > 0" by simp
from s(2) obtain z0 where z0: "z0 \<in> s" by blast
define z where "z n = (f ^^ n) z0" for n
with f z0 have z_in_s: "z n \<in> s" for n :: nat
by (induct n) auto
define d where "d = dist (z 0) (z 1)"
have fzn: "f (z n) = z (Suc n)" for n
by (simp add: z_def)
have cf_z: "dist (z n) (z (Suc n)) \<le> (c ^ n) * d" for n :: nat
proof (induct n)
case 0
then show ?case
by (simp add: d_def)
next
case (Suc m)
with \<open>0 \<le> c\<close> have "c * dist (z m) (z (Suc m)) \<le> c ^ Suc m * d"
using mult_left_mono[of "dist (z m) (z (Suc m))" "c ^ m * d" c] by simp
then show ?case
using lipschitz[THEN bspec[where x="z m"], OF z_in_s, THEN bspec[where x="z (Suc m)"], OF z_in_s]
by (simp add: fzn mult_le_cancel_left)
qed
have cf_z2: "(1 - c) * dist (z m) (z (m + n)) \<le> (c ^ m) * d * (1 - c ^ n)" for n m :: nat
proof (induct n)
case 0
show ?case by simp
next
case (Suc k)
from c have "(1 - c) * dist (z m) (z (m + Suc k)) \<le>
(1 - c) * (dist (z m) (z (m + k)) + dist (z (m + k)) (z (Suc (m + k))))"
by (simp add: dist_triangle)
also from c cf_z[of "m + k"] have "\<dots> \<le> (1 - c) * (dist (z m) (z (m + k)) + c ^ (m + k) * d)"
by simp
also from Suc have "\<dots> \<le> c ^ m * d * (1 - c ^ k) + (1 - c) * c ^ (m + k) * d"
by (simp add: field_simps)
also have "\<dots> = (c ^ m) * (d * (1 - c ^ k) + (1 - c) * c ^ k * d)"
by (simp add: power_add field_simps)
also from c have "\<dots> \<le> (c ^ m) * d * (1 - c ^ Suc k)"
by (simp add: field_simps)
finally show ?case by simp
qed
have "\<exists>N. \<forall>m n. N \<le> m \<and> N \<le> n \<longrightarrow> dist (z m) (z n) < e" if "e > 0" for e
proof (cases "d = 0")
case True
from \<open>1 - c > 0\<close> have "(1 - c) * x \<le> 0 \<longleftrightarrow> x \<le> 0" for x
by (metis mult_zero_left mult.commute real_mult_le_cancel_iff1)
with c cf_z2[of 0] True have "z n = z0" for n
by (simp add: z_def)
with \<open>e > 0\<close> show ?thesis by simp
next
case False
with zero_le_dist[of "z 0" "z 1"] have "d > 0"
by (metis d_def less_le)
with \<open>1 - c > 0\<close> \<open>e > 0\<close> have "0 < e * (1 - c) / d"
by simp
with c obtain N where N: "c ^ N < e * (1 - c) / d"
using real_arch_pow_inv[of "e * (1 - c) / d" c] by auto
have *: "dist (z m) (z n) < e" if "m > n" and as: "m \<ge> N" "n \<ge> N" for m n :: nat
proof -
from c \<open>n \<ge> N\<close> have *: "c ^ n \<le> c ^ N"
using power_decreasing[OF \<open>n\<ge>N\<close>, of c] by simp
from c \<open>m > n\<close> have "1 - c ^ (m - n) > 0"
using power_strict_mono[of c 1 "m - n"] by simp
with \<open>d > 0\<close> \<open>0 < 1 - c\<close> have **: "d * (1 - c ^ (m - n)) / (1 - c) > 0"
by simp
from cf_z2[of n "m - n"] \<open>m > n\<close>
have "dist (z m) (z n) \<le> c ^ n * d * (1 - c ^ (m - n)) / (1 - c)"
by (simp add: pos_le_divide_eq[OF \<open>1 - c > 0\<close>] mult.commute dist_commute)
also have "\<dots> \<le> c ^ N * d * (1 - c ^ (m - n)) / (1 - c)"
using mult_right_mono[OF * order_less_imp_le[OF **]]
by (simp add: mult.assoc)
also have "\<dots> < (e * (1 - c) / d) * d * (1 - c ^ (m - n)) / (1 - c)"
using mult_strict_right_mono[OF N **] by (auto simp: mult.assoc)
also from c \<open>d > 0\<close> \<open>1 - c > 0\<close> have "\<dots> = e * (1 - c ^ (m - n))"
by simp
also from c \<open>1 - c ^ (m - n) > 0\<close> \<open>e > 0\<close> have "\<dots> \<le> e"
using mult_right_le_one_le[of e "1 - c ^ (m - n)"] by auto
finally show ?thesis by simp
qed
have "dist (z n) (z m) < e" if "N \<le> m" "N \<le> n" for m n :: nat
proof (cases "n = m")
case True
with \<open>e > 0\<close> show ?thesis by simp
next
case False
with *[of n m] *[of m n] and that show ?thesis
by (auto simp: dist_commute nat_neq_iff)
qed
then show ?thesis by auto
qed
then have "Cauchy z"
by (simp add: cauchy_def)
then obtain x where "x\<in>s" and x:"(z \<longlongrightarrow> x) sequentially"
using s(1)[unfolded compact_def complete_def, THEN spec[where x=z]] and z_in_s by auto
define e where "e = dist (f x) x"
have "e = 0"
proof (rule ccontr)
assume "e \<noteq> 0"
then have "e > 0"
unfolding e_def using zero_le_dist[of "f x" x]
by (metis dist_eq_0_iff dist_nz e_def)
then obtain N where N:"\<forall>n\<ge>N. dist (z n) x < e / 2"
using x[unfolded lim_sequentially, THEN spec[where x="e/2"]] by auto
then have N':"dist (z N) x < e / 2" by auto
have *: "c * dist (z N) x \<le> dist (z N) x"
unfolding mult_le_cancel_right2
using zero_le_dist[of "z N" x] and c
by (metis dist_eq_0_iff dist_nz order_less_asym less_le)
have "dist (f (z N)) (f x) \<le> c * dist (z N) x"
using lipschitz[THEN bspec[where x="z N"], THEN bspec[where x=x]]
using z_in_s[of N] \<open>x\<in>s\<close>
using c
by auto
also have "\<dots> < e / 2"
using N' and c using * by auto
finally show False
unfolding fzn
using N[THEN spec[where x="Suc N"]] and dist_triangle_half_r[of "z (Suc N)" "f x" e x]
unfolding e_def
by auto
qed
then have "f x = x" by (auto simp: e_def)
moreover have "y = x" if "f y = y" "y \<in> s" for y
proof -
from \<open>x \<in> s\<close> \<open>f x = x\<close> that have "dist x y \<le> c * dist x y"
using lipschitz[THEN bspec[where x=x], THEN bspec[where x=y]] by simp
with c and zero_le_dist[of x y] have "dist x y = 0"
by (simp add: mult_le_cancel_right1)
then show ?thesis by simp
qed
ultimately show ?thesis
using \<open>x\<in>s\<close> by blast
qed
subsection \<open>Edelstein fixed point theorem\<close>
theorem edelstein_fix:\<comment> \<open>TODO: rename to \<open>Edelstein_fix\<close>\<close>
fixes s :: "'a::metric_space set"
assumes s: "compact s" "s \<noteq> {}"
and gs: "(g ` s) \<subseteq> s"
and dist: "\<forall>x\<in>s. \<forall>y\<in>s. x \<noteq> y \<longrightarrow> dist (g x) (g y) < dist x y"
shows "\<exists>!x\<in>s. g x = x"
proof -
let ?D = "(\<lambda>x. (x, x)) ` s"
have D: "compact ?D" "?D \<noteq> {}"
by (rule compact_continuous_image)
(auto intro!: s continuous_Pair continuous_ident simp: continuous_on_eq_continuous_within)
have "\<And>x y e. x \<in> s \<Longrightarrow> y \<in> s \<Longrightarrow> 0 < e \<Longrightarrow> dist y x < e \<Longrightarrow> dist (g y) (g x) < e"
using dist by fastforce
then have "continuous_on s g"
by (auto simp: continuous_on_iff)
then have cont: "continuous_on ?D (\<lambda>x. dist ((g \<circ> fst) x) (snd x))"
unfolding continuous_on_eq_continuous_within
by (intro continuous_dist ballI continuous_within_compose)
(auto intro!: continuous_fst continuous_snd continuous_ident simp: image_image)
obtain a where "a \<in> s" and le: "\<And>x. x \<in> s \<Longrightarrow> dist (g a) a \<le> dist (g x) x"
using continuous_attains_inf[OF D cont] by auto
have "g a = a"
proof (rule ccontr)
assume "g a \<noteq> a"
with \<open>a \<in> s\<close> gs have "dist (g (g a)) (g a) < dist (g a) a"
by (intro dist[rule_format]) auto
moreover have "dist (g a) a \<le> dist (g (g a)) (g a)"
using \<open>a \<in> s\<close> gs by (intro le) auto
ultimately show False by auto
qed
moreover have "\<And>x. x \<in> s \<Longrightarrow> g x = x \<Longrightarrow> x = a"
using dist[THEN bspec[where x=a]] \<open>g a = a\<close> and \<open>a\<in>s\<close> by auto
ultimately show "\<exists>!x\<in>s. g x = x"
using \<open>a \<in> s\<close> by blast
qed
subsection \<open>The diameter of a set\<close>
definition%important diameter :: "'a::metric_space set \<Rightarrow> real" where
"diameter S = (if S = {} then 0 else SUP (x,y)\<in>S\<times>S. dist x y)"
lemma diameter_empty [simp]: "diameter{} = 0"
by (auto simp: diameter_def)
lemma diameter_singleton [simp]: "diameter{x} = 0"
by (auto simp: diameter_def)
lemma diameter_le:
assumes "S \<noteq> {} \<or> 0 \<le> d"
and no: "\<And>x y. \<lbrakk>x \<in> S; y \<in> S\<rbrakk> \<Longrightarrow> norm(x - y) \<le> d"
shows "diameter S \<le> d"
using assms
by (auto simp: dist_norm diameter_def intro: cSUP_least)
lemma diameter_bounded_bound:
fixes s :: "'a :: metric_space set"
assumes s: "bounded s" "x \<in> s" "y \<in> s"
shows "dist x y \<le> diameter s"
proof -
from s obtain z d where z: "\<And>x. x \<in> s \<Longrightarrow> dist z x \<le> d"
unfolding bounded_def by auto
have "bdd_above (case_prod dist ` (s\<times>s))"
proof (intro bdd_aboveI, safe)
fix a b
assume "a \<in> s" "b \<in> s"
with z[of a] z[of b] dist_triangle[of a b z]
show "dist a b \<le> 2 * d"
by (simp add: dist_commute)
qed
moreover have "(x,y) \<in> s\<times>s" using s by auto
ultimately have "dist x y \<le> (SUP (x,y)\<in>s\<times>s. dist x y)"
by (rule cSUP_upper2) simp
with \<open>x \<in> s\<close> show ?thesis
by (auto simp: diameter_def)
qed
lemma diameter_lower_bounded:
fixes s :: "'a :: metric_space set"
assumes s: "bounded s"
and d: "0 < d" "d < diameter s"
shows "\<exists>x\<in>s. \<exists>y\<in>s. d < dist x y"
proof (rule ccontr)
assume contr: "\<not> ?thesis"
moreover have "s \<noteq> {}"
using d by (auto simp: diameter_def)
ultimately have "diameter s \<le> d"
by (auto simp: not_less diameter_def intro!: cSUP_least)
with \<open>d < diameter s\<close> show False by auto
qed
lemma diameter_bounded:
assumes "bounded s"
shows "\<forall>x\<in>s. \<forall>y\<in>s. dist x y \<le> diameter s"
and "\<forall>d>0. d < diameter s \<longrightarrow> (\<exists>x\<in>s. \<exists>y\<in>s. dist x y > d)"
using diameter_bounded_bound[of s] diameter_lower_bounded[of s] assms
by auto
lemma bounded_two_points:
"bounded S \<longleftrightarrow> (\<exists>e. \<forall>x\<in>S. \<forall>y\<in>S. dist x y \<le> e)"
apply (rule iffI)
subgoal using diameter_bounded(1) by auto
subgoal using bounded_any_center[of S] by meson
done
lemma diameter_compact_attained:
assumes "compact s"
and "s \<noteq> {}"
shows "\<exists>x\<in>s. \<exists>y\<in>s. dist x y = diameter s"
proof -
have b: "bounded s" using assms(1)
by (rule compact_imp_bounded)
then obtain x y where xys: "x\<in>s" "y\<in>s"
and xy: "\<forall>u\<in>s. \<forall>v\<in>s. dist u v \<le> dist x y"
using compact_sup_maxdistance[OF assms] by auto
then have "diameter s \<le> dist x y"
unfolding diameter_def
apply clarsimp
apply (rule cSUP_least, fast+)
done
then show ?thesis
by (metis b diameter_bounded_bound order_antisym xys)
qed
lemma diameter_ge_0:
assumes "bounded S" shows "0 \<le> diameter S"
by (metis all_not_in_conv assms diameter_bounded_bound diameter_empty dist_self order_refl)
lemma diameter_subset:
assumes "S \<subseteq> T" "bounded T"
shows "diameter S \<le> diameter T"
proof (cases "S = {} \<or> T = {}")
case True
with assms show ?thesis
by (force simp: diameter_ge_0)
next
case False
then have "bdd_above ((\<lambda>x. case x of (x, xa) \<Rightarrow> dist x xa) ` (T \<times> T))"
using \<open>bounded T\<close> diameter_bounded_bound by (force simp: bdd_above_def)
with False \<open>S \<subseteq> T\<close> show ?thesis
apply (simp add: diameter_def)
apply (rule cSUP_subset_mono, auto)
done
qed
lemma diameter_closure:
assumes "bounded S"
shows "diameter(closure S) = diameter S"
proof (rule order_antisym)
have "False" if "diameter S < diameter (closure S)"
proof -
define d where "d = diameter(closure S) - diameter(S)"
have "d > 0"
using that by (simp add: d_def)
then have "diameter(closure(S)) - d / 2 < diameter(closure(S))"
by simp
have dd: "diameter (closure S) - d / 2 = (diameter(closure(S)) + diameter(S)) / 2"
by (simp add: d_def divide_simps)
have bocl: "bounded (closure S)"
using assms by blast
moreover have "0 \<le> diameter S"
using assms diameter_ge_0 by blast
ultimately obtain x y where "x \<in> closure S" "y \<in> closure S" and xy: "diameter(closure(S)) - d / 2 < dist x y"
using diameter_bounded(2) [OF bocl, rule_format, of "diameter(closure(S)) - d / 2"] \<open>d > 0\<close> d_def by auto
then obtain x' y' where x'y': "x' \<in> S" "dist x' x < d/4" "y' \<in> S" "dist y' y < d/4"
using closure_approachable
by (metis \<open>0 < d\<close> zero_less_divide_iff zero_less_numeral)
then have "dist x' y' \<le> diameter S"
using assms diameter_bounded_bound by blast
with x'y' have "dist x y \<le> d / 4 + diameter S + d / 4"
by (meson add_mono_thms_linordered_semiring(1) dist_triangle dist_triangle3 less_eq_real_def order_trans)
then show ?thesis
using xy d_def by linarith
qed
then show "diameter (closure S) \<le> diameter S"
by fastforce
next
show "diameter S \<le> diameter (closure S)"
by (simp add: assms bounded_closure closure_subset diameter_subset)
qed
proposition Lebesgue_number_lemma:
assumes "compact S" "\<C> \<noteq> {}" "S \<subseteq> \<Union>\<C>" and ope: "\<And>B. B \<in> \<C> \<Longrightarrow> open B"
obtains \<delta> where "0 < \<delta>" "\<And>T. \<lbrakk>T \<subseteq> S; diameter T < \<delta>\<rbrakk> \<Longrightarrow> \<exists>B \<in> \<C>. T \<subseteq> B"
proof (cases "S = {}")
case True
then show ?thesis
by (metis \<open>\<C> \<noteq> {}\<close> zero_less_one empty_subsetI equals0I subset_trans that)
next
case False
{ fix x assume "x \<in> S"
then obtain C where C: "x \<in> C" "C \<in> \<C>"
using \<open>S \<subseteq> \<Union>\<C>\<close> by blast
then obtain r where r: "r>0" "ball x (2*r) \<subseteq> C"
by (metis mult.commute mult_2_right not_le ope openE field_sum_of_halves zero_le_numeral zero_less_mult_iff)
then have "\<exists>r C. r > 0 \<and> ball x (2*r) \<subseteq> C \<and> C \<in> \<C>"
using C by blast
}
then obtain r where r: "\<And>x. x \<in> S \<Longrightarrow> r x > 0 \<and> (\<exists>C \<in> \<C>. ball x (2*r x) \<subseteq> C)"
by metis
then have "S \<subseteq> (\<Union>x \<in> S. ball x (r x))"
by auto
then obtain \<T> where "finite \<T>" "S \<subseteq> \<Union>\<T>" and \<T>: "\<T> \<subseteq> (\<lambda>x. ball x (r x)) ` S"
by (rule compactE [OF \<open>compact S\<close>]) auto
then obtain S0 where "S0 \<subseteq> S" "finite S0" and S0: "\<T> = (\<lambda>x. ball x (r x)) ` S0"
by (meson finite_subset_image)
then have "S0 \<noteq> {}"
using False \<open>S \<subseteq> \<Union>\<T>\<close> by auto
define \<delta> where "\<delta> = Inf (r ` S0)"
have "\<delta> > 0"
using \<open>finite S0\<close> \<open>S0 \<subseteq> S\<close> \<open>S0 \<noteq> {}\<close> r by (auto simp: \<delta>_def finite_less_Inf_iff)
show ?thesis
proof
show "0 < \<delta>"
by (simp add: \<open>0 < \<delta>\<close>)
show "\<exists>B \<in> \<C>. T \<subseteq> B" if "T \<subseteq> S" and dia: "diameter T < \<delta>" for T
proof (cases "T = {}")
case True
then show ?thesis
using \<open>\<C> \<noteq> {}\<close> by blast
next
case False
then obtain y where "y \<in> T" by blast
then have "y \<in> S"
using \<open>T \<subseteq> S\<close> by auto
then obtain x where "x \<in> S0" and x: "y \<in> ball x (r x)"
using \<open>S \<subseteq> \<Union>\<T>\<close> S0 that by blast
have "ball y \<delta> \<subseteq> ball y (r x)"
by (metis \<delta>_def \<open>S0 \<noteq> {}\<close> \<open>finite S0\<close> \<open>x \<in> S0\<close> empty_is_image finite_imageI finite_less_Inf_iff imageI less_irrefl not_le subset_ball)
also have "... \<subseteq> ball x (2*r x)"
by clarsimp (metis dist_commute dist_triangle_less_add mem_ball mult_2 x)
finally obtain C where "C \<in> \<C>" "ball y \<delta> \<subseteq> C"
by (meson r \<open>S0 \<subseteq> S\<close> \<open>x \<in> S0\<close> dual_order.trans subsetCE)
have "bounded T"
using \<open>compact S\<close> bounded_subset compact_imp_bounded \<open>T \<subseteq> S\<close> by blast
then have "T \<subseteq> ball y \<delta>"
using \<open>y \<in> T\<close> dia diameter_bounded_bound by fastforce
then show ?thesis
apply (rule_tac x=C in bexI)
using \<open>ball y \<delta> \<subseteq> C\<close> \<open>C \<in> \<C>\<close> by auto
qed
qed
qed
subsection \<open>Metric spaces with the Heine-Borel property\<close>
text \<open>
A metric space (or topological vector space) is said to have the
Heine-Borel property if every closed and bounded subset is compact.
\<close>
class heine_borel = metric_space +
assumes bounded_imp_convergent_subsequence:
"bounded (range f) \<Longrightarrow> \<exists>l r. strict_mono (r::nat\<Rightarrow>nat) \<and> ((f \<circ> r) \<longlongrightarrow> l) sequentially"
proposition bounded_closed_imp_seq_compact:
fixes s::"'a::heine_borel set"
assumes "bounded s"
and "closed s"
shows "seq_compact s"
proof (unfold seq_compact_def, clarify)
fix f :: "nat \<Rightarrow> 'a"
assume f: "\<forall>n. f n \<in> s"
with \<open>bounded s\<close> have "bounded (range f)"
by (auto intro: bounded_subset)
obtain l r where r: "strict_mono (r :: nat \<Rightarrow> nat)" and l: "((f \<circ> r) \<longlongrightarrow> l) sequentially"
using bounded_imp_convergent_subsequence [OF \<open>bounded (range f)\<close>] by auto
from f have fr: "\<forall>n. (f \<circ> r) n \<in> s"
by simp
have "l \<in> s" using \<open>closed s\<close> fr l
by (rule closed_sequentially)
show "\<exists>l\<in>s. \<exists>r. strict_mono r \<and> ((f \<circ> r) \<longlongrightarrow> l) sequentially"
using \<open>l \<in> s\<close> r l by blast
qed
lemma compact_eq_bounded_closed:
fixes s :: "'a::heine_borel set"
shows "compact s \<longleftrightarrow> bounded s \<and> closed s"
(is "?lhs = ?rhs")
proof
assume ?lhs
then show ?rhs
using compact_imp_closed compact_imp_bounded
by blast
next
assume ?rhs
then show ?lhs
using bounded_closed_imp_seq_compact[of s]
unfolding compact_eq_seq_compact_metric
by auto
qed
lemma compact_Inter:
fixes \<F> :: "'a :: heine_borel set set"
assumes com: "\<And>S. S \<in> \<F> \<Longrightarrow> compact S" and "\<F> \<noteq> {}"
shows "compact(\<Inter> \<F>)"
using assms
by (meson Inf_lower all_not_in_conv bounded_subset closed_Inter compact_eq_bounded_closed)
lemma compact_closure [simp]:
fixes S :: "'a::heine_borel set"
shows "compact(closure S) \<longleftrightarrow> bounded S"
by (meson bounded_closure bounded_subset closed_closure closure_subset compact_eq_bounded_closed)
instance%important real :: heine_borel
proof%unimportant
fix f :: "nat \<Rightarrow> real"
assume f: "bounded (range f)"
obtain r :: "nat \<Rightarrow> nat" where r: "strict_mono r" "monoseq (f \<circ> r)"
unfolding comp_def by (metis seq_monosub)
then have "Bseq (f \<circ> r)"
unfolding Bseq_eq_bounded using f
by (metis BseqI' bounded_iff comp_apply rangeI)
with r show "\<exists>l r. strict_mono r \<and> (f \<circ> r) \<longlonglongrightarrow> l"
using Bseq_monoseq_convergent[of "f \<circ> r"] by (auto simp: convergent_def)
qed
lemma compact_lemma_general:
fixes f :: "nat \<Rightarrow> 'a"
fixes proj::"'a \<Rightarrow> 'b \<Rightarrow> 'c::heine_borel" (infixl "proj" 60)
fixes unproj:: "('b \<Rightarrow> 'c) \<Rightarrow> 'a"
assumes finite_basis: "finite basis"
assumes bounded_proj: "\<And>k. k \<in> basis \<Longrightarrow> bounded ((\<lambda>x. x proj k) ` range f)"
assumes proj_unproj: "\<And>e k. k \<in> basis \<Longrightarrow> (unproj e) proj k = e k"
assumes unproj_proj: "\<And>x. unproj (\<lambda>k. x proj k) = x"
shows "\<forall>d\<subseteq>basis. \<exists>l::'a. \<exists> r::nat\<Rightarrow>nat.
strict_mono r \<and> (\<forall>e>0. eventually (\<lambda>n. \<forall>i\<in>d. dist (f (r n) proj i) (l proj i) < e) sequentially)"
proof safe
fix d :: "'b set"
assume d: "d \<subseteq> basis"
with finite_basis have "finite d"
by (blast intro: finite_subset)
from this d show "\<exists>l::'a. \<exists>r::nat\<Rightarrow>nat. strict_mono r \<and>
(\<forall>e>0. eventually (\<lambda>n. \<forall>i\<in>d. dist (f (r n) proj i) (l proj i) < e) sequentially)"
proof (induct d)
case empty
then show ?case
unfolding strict_mono_def by auto
next
case (insert k d)
have k[intro]: "k \<in> basis"
using insert by auto
have s': "bounded ((\<lambda>x. x proj k) ` range f)"
using k
by (rule bounded_proj)
obtain l1::"'a" and r1 where r1: "strict_mono r1"
and lr1: "\<forall>e > 0. eventually (\<lambda>n. \<forall>i\<in>d. dist (f (r1 n) proj i) (l1 proj i) < e) sequentially"
using insert(3) using insert(4) by auto
have f': "\<forall>n. f (r1 n) proj k \<in> (\<lambda>x. x proj k) ` range f"
by simp
have "bounded (range (\<lambda>i. f (r1 i) proj k))"
by (metis (lifting) bounded_subset f' image_subsetI s')
then obtain l2 r2 where r2:"strict_mono r2" and lr2:"((\<lambda>i. f (r1 (r2 i)) proj k) \<longlongrightarrow> l2) sequentially"
using bounded_imp_convergent_subsequence[of "\<lambda>i. f (r1 i) proj k"]
by (auto simp: o_def)
define r where "r = r1 \<circ> r2"
have r:"strict_mono r"
using r1 and r2 unfolding r_def o_def strict_mono_def by auto
moreover
define l where "l = unproj (\<lambda>i. if i = k then l2 else l1 proj i)"
{
fix e::real
assume "e > 0"
from lr1 \<open>e > 0\<close> have N1: "eventually (\<lambda>n. \<forall>i\<in>d. dist (f (r1 n) proj i) (l1 proj i) < e) sequentially"
by blast
from lr2 \<open>e > 0\<close> have N2:"eventually (\<lambda>n. dist (f (r1 (r2 n)) proj k) l2 < e) sequentially"
by (rule tendstoD)
from r2 N1 have N1': "eventually (\<lambda>n. \<forall>i\<in>d. dist (f (r1 (r2 n)) proj i) (l1 proj i) < e) sequentially"
by (rule eventually_subseq)
have "eventually (\<lambda>n. \<forall>i\<in>(insert k d). dist (f (r n) proj i) (l proj i) < e) sequentially"
using N1' N2
by eventually_elim (insert insert.prems, auto simp: l_def r_def o_def proj_unproj)
}
ultimately show ?case by auto
qed
qed
lemma bounded_fst: "bounded s \<Longrightarrow> bounded (fst ` s)"
unfolding bounded_def
by (metis (erased, hide_lams) dist_fst_le image_iff order_trans)
lemma bounded_snd: "bounded s \<Longrightarrow> bounded (snd ` s)"
unfolding bounded_def
by (metis (no_types, hide_lams) dist_snd_le image_iff order.trans)
instance%important prod :: (heine_borel, heine_borel) heine_borel
proof%unimportant
fix f :: "nat \<Rightarrow> 'a \<times> 'b"
assume f: "bounded (range f)"
then have "bounded (fst ` range f)"
by (rule bounded_fst)
then have s1: "bounded (range (fst \<circ> f))"
by (simp add: image_comp)
obtain l1 r1 where r1: "strict_mono r1" and l1: "(\<lambda>n. fst (f (r1 n))) \<longlonglongrightarrow> l1"
using bounded_imp_convergent_subsequence [OF s1] unfolding o_def by fast
from f have s2: "bounded (range (snd \<circ> f \<circ> r1))"
by (auto simp: image_comp intro: bounded_snd bounded_subset)
obtain l2 r2 where r2: "strict_mono r2" and l2: "((\<lambda>n. snd (f (r1 (r2 n)))) \<longlongrightarrow> l2) sequentially"
using bounded_imp_convergent_subsequence [OF s2]
unfolding o_def by fast
have l1': "((\<lambda>n. fst (f (r1 (r2 n)))) \<longlongrightarrow> l1) sequentially"
using LIMSEQ_subseq_LIMSEQ [OF l1 r2] unfolding o_def .
have l: "((f \<circ> (r1 \<circ> r2)) \<longlongrightarrow> (l1, l2)) sequentially"
using tendsto_Pair [OF l1' l2] unfolding o_def by simp
have r: "strict_mono (r1 \<circ> r2)"
using r1 r2 unfolding strict_mono_def by simp
show "\<exists>l r. strict_mono r \<and> ((f \<circ> r) \<longlongrightarrow> l) sequentially"
using l r by fast
qed
subsection \<open>Completeness\<close>
proposition (in metric_space) completeI:
assumes "\<And>f. \<forall>n. f n \<in> s \<Longrightarrow> Cauchy f \<Longrightarrow> \<exists>l\<in>s. f \<longlonglongrightarrow> l"
shows "complete s"
using assms unfolding complete_def by fast
proposition (in metric_space) completeE:
assumes "complete s" and "\<forall>n. f n \<in> s" and "Cauchy f"
obtains l where "l \<in> s" and "f \<longlonglongrightarrow> l"
using assms unfolding complete_def by fast
(* TODO: generalize to uniform spaces *)
lemma compact_imp_complete:
fixes s :: "'a::metric_space set"
assumes "compact s"
shows "complete s"
proof -
{
fix f
assume as: "(\<forall>n::nat. f n \<in> s)" "Cauchy f"
from as(1) obtain l r where lr: "l\<in>s" "strict_mono r" "(f \<circ> r) \<longlonglongrightarrow> l"
using assms unfolding compact_def by blast
note lr' = seq_suble [OF lr(2)]
{
fix e :: real
assume "e > 0"
from as(2) obtain N where N:"\<forall>m n. N \<le> m \<and> N \<le> n \<longrightarrow> dist (f m) (f n) < e/2"
unfolding cauchy_def
using \<open>e > 0\<close>
apply (erule_tac x="e/2" in allE, auto)
done
from lr(3)[unfolded lim_sequentially, THEN spec[where x="e/2"]]
obtain M where M:"\<forall>n\<ge>M. dist ((f \<circ> r) n) l < e/2"
using \<open>e > 0\<close> by auto
{
fix n :: nat
assume n: "n \<ge> max N M"
have "dist ((f \<circ> r) n) l < e/2"
using n M by auto
moreover have "r n \<ge> N"
using lr'[of n] n by auto
then have "dist (f n) ((f \<circ> r) n) < e / 2"
using N and n by auto
ultimately have "dist (f n) l < e"
using dist_triangle_half_r[of "f (r n)" "f n" e l]
by (auto simp: dist_commute)
}
then have "\<exists>N. \<forall>n\<ge>N. dist (f n) l < e" by blast
}
then have "\<exists>l\<in>s. (f \<longlongrightarrow> l) sequentially" using \<open>l\<in>s\<close>
unfolding lim_sequentially by auto
}
then show ?thesis unfolding complete_def by auto
qed
proposition compact_eq_totally_bounded:
"compact s \<longleftrightarrow> complete s \<and> (\<forall>e>0. \<exists>k. finite k \<and> s \<subseteq> (\<Union>x\<in>k. ball x e))"
(is "_ \<longleftrightarrow> ?rhs")
proof
assume assms: "?rhs"
then obtain k where k: "\<And>e. 0 < e \<Longrightarrow> finite (k e)" "\<And>e. 0 < e \<Longrightarrow> s \<subseteq> (\<Union>x\<in>k e. ball x e)"
by (auto simp: choice_iff')
show "compact s"
proof cases
assume "s = {}"
then show "compact s" by (simp add: compact_def)
next
assume "s \<noteq> {}"
show ?thesis
unfolding compact_def
proof safe
fix f :: "nat \<Rightarrow> 'a"
assume f: "\<forall>n. f n \<in> s"
define e where "e n = 1 / (2 * Suc n)" for n
then have [simp]: "\<And>n. 0 < e n" by auto
define B where "B n U = (SOME b. infinite {n. f n \<in> b} \<and> (\<exists>x. b \<subseteq> ball x (e n) \<inter> U))" for n U
{
fix n U
assume "infinite {n. f n \<in> U}"
then have "\<exists>b\<in>k (e n). infinite {i\<in>{n. f n \<in> U}. f i \<in> ball b (e n)}"
using k f by (intro pigeonhole_infinite_rel) (auto simp: subset_eq)
then obtain a where
"a \<in> k (e n)"
"infinite {i \<in> {n. f n \<in> U}. f i \<in> ball a (e n)}" ..
then have "\<exists>b. infinite {i. f i \<in> b} \<and> (\<exists>x. b \<subseteq> ball x (e n) \<inter> U)"
by (intro exI[of _ "ball a (e n) \<inter> U"] exI[of _ a]) (auto simp: ac_simps)
from someI_ex[OF this]
have "infinite {i. f i \<in> B n U}" "\<exists>x. B n U \<subseteq> ball x (e n) \<inter> U"
unfolding B_def by auto
}
note B = this
define F where "F = rec_nat (B 0 UNIV) B"
{
fix n
have "infinite {i. f i \<in> F n}"
by (induct n) (auto simp: F_def B)
}
then have F: "\<And>n. \<exists>x. F (Suc n) \<subseteq> ball x (e n) \<inter> F n"
using B by (simp add: F_def)
then have F_dec: "\<And>m n. m \<le> n \<Longrightarrow> F n \<subseteq> F m"
using decseq_SucI[of F] by (auto simp: decseq_def)
obtain sel where sel: "\<And>k i. i < sel k i" "\<And>k i. f (sel k i) \<in> F k"
proof (atomize_elim, unfold all_conj_distrib[symmetric], intro choice allI)
fix k i
have "infinite ({n. f n \<in> F k} - {.. i})"
using \<open>infinite {n. f n \<in> F k}\<close> by auto
from infinite_imp_nonempty[OF this]
show "\<exists>x>i. f x \<in> F k"
by (simp add: set_eq_iff not_le conj_commute)
qed
define t where "t = rec_nat (sel 0 0) (\<lambda>n i. sel (Suc n) i)"
have "strict_mono t"
unfolding strict_mono_Suc_iff by (simp add: t_def sel)
moreover have "\<forall>i. (f \<circ> t) i \<in> s"
using f by auto
moreover
{
fix n
have "(f \<circ> t) n \<in> F n"
by (cases n) (simp_all add: t_def sel)
}
note t = this
have "Cauchy (f \<circ> t)"
proof (safe intro!: metric_CauchyI exI elim!: nat_approx_posE)
fix r :: real and N n m
assume "1 / Suc N < r" "Suc N \<le> n" "Suc N \<le> m"
then have "(f \<circ> t) n \<in> F (Suc N)" "(f \<circ> t) m \<in> F (Suc N)" "2 * e N < r"
using F_dec t by (auto simp: e_def field_simps of_nat_Suc)
with F[of N] obtain x where "dist x ((f \<circ> t) n) < e N" "dist x ((f \<circ> t) m) < e N"
by (auto simp: subset_eq)
with dist_triangle[of "(f \<circ> t) m" "(f \<circ> t) n" x] \<open>2 * e N < r\<close>
show "dist ((f \<circ> t) m) ((f \<circ> t) n) < r"
by (simp add: dist_commute)
qed
ultimately show "\<exists>l\<in>s. \<exists>r. strict_mono r \<and> (f \<circ> r) \<longlonglongrightarrow> l"
using assms unfolding complete_def by blast
qed
qed
qed (metis compact_imp_complete compact_imp_seq_compact seq_compact_imp_totally_bounded)
lemma cauchy_imp_bounded:
assumes "Cauchy s"
shows "bounded (range s)"
proof -
from assms obtain N :: nat where "\<forall>m n. N \<le> m \<and> N \<le> n \<longrightarrow> dist (s m) (s n) < 1"
unfolding cauchy_def by force
then have N:"\<forall>n. N \<le> n \<longrightarrow> dist (s N) (s n) < 1" by auto
moreover
have "bounded (s ` {0..N})"
using finite_imp_bounded[of "s ` {1..N}"] by auto
then obtain a where a:"\<forall>x\<in>s ` {0..N}. dist (s N) x \<le> a"
unfolding bounded_any_center [where a="s N"] by auto
ultimately show "?thesis"
unfolding bounded_any_center [where a="s N"]
apply (rule_tac x="max a 1" in exI, auto)
apply (erule_tac x=y in allE)
apply (erule_tac x=y in ballE, auto)
done
qed
instance heine_borel < complete_space
proof
fix f :: "nat \<Rightarrow> 'a" assume "Cauchy f"
then have "bounded (range f)"
by (rule cauchy_imp_bounded)
then have "compact (closure (range f))"
unfolding compact_eq_bounded_closed by auto
then have "complete (closure (range f))"
by (rule compact_imp_complete)
moreover have "\<forall>n. f n \<in> closure (range f)"
using closure_subset [of "range f"] by auto
ultimately have "\<exists>l\<in>closure (range f). (f \<longlongrightarrow> l) sequentially"
using \<open>Cauchy f\<close> unfolding complete_def by auto
then show "convergent f"
unfolding convergent_def by auto
qed
lemma complete_UNIV: "complete (UNIV :: ('a::complete_space) set)"
proof (rule completeI)
fix f :: "nat \<Rightarrow> 'a" assume "Cauchy f"
then have "convergent f" by (rule Cauchy_convergent)
then show "\<exists>l\<in>UNIV. f \<longlonglongrightarrow> l" unfolding convergent_def by simp
qed
lemma complete_imp_closed:
fixes S :: "'a::metric_space set"
assumes "complete S"
shows "closed S"
proof (unfold closed_sequential_limits, clarify)
fix f x assume "\<forall>n. f n \<in> S" and "f \<longlonglongrightarrow> x"
from \<open>f \<longlonglongrightarrow> x\<close> have "Cauchy f"
by (rule LIMSEQ_imp_Cauchy)
with \<open>complete S\<close> and \<open>\<forall>n. f n \<in> S\<close> obtain l where "l \<in> S" and "f \<longlonglongrightarrow> l"
by (rule completeE)
from \<open>f \<longlonglongrightarrow> x\<close> and \<open>f \<longlonglongrightarrow> l\<close> have "x = l"
by (rule LIMSEQ_unique)
with \<open>l \<in> S\<close> show "x \<in> S"
by simp
qed
lemma complete_Int_closed:
fixes S :: "'a::metric_space set"
assumes "complete S" and "closed t"
shows "complete (S \<inter> t)"
proof (rule completeI)
fix f assume "\<forall>n. f n \<in> S \<inter> t" and "Cauchy f"
then have "\<forall>n. f n \<in> S" and "\<forall>n. f n \<in> t"
by simp_all
from \<open>complete S\<close> obtain l where "l \<in> S" and "f \<longlonglongrightarrow> l"
using \<open>\<forall>n. f n \<in> S\<close> and \<open>Cauchy f\<close> by (rule completeE)
from \<open>closed t\<close> and \<open>\<forall>n. f n \<in> t\<close> and \<open>f \<longlonglongrightarrow> l\<close> have "l \<in> t"
by (rule closed_sequentially)
with \<open>l \<in> S\<close> and \<open>f \<longlonglongrightarrow> l\<close> show "\<exists>l\<in>S \<inter> t. f \<longlonglongrightarrow> l"
by fast
qed
lemma complete_closed_subset:
fixes S :: "'a::metric_space set"
assumes "closed S" and "S \<subseteq> t" and "complete t"
shows "complete S"
using assms complete_Int_closed [of t S] by (simp add: Int_absorb1)
lemma complete_eq_closed:
fixes S :: "('a::complete_space) set"
shows "complete S \<longleftrightarrow> closed S"
proof
assume "closed S" then show "complete S"
using subset_UNIV complete_UNIV by (rule complete_closed_subset)
next
assume "complete S" then show "closed S"
by (rule complete_imp_closed)
qed
lemma convergent_eq_Cauchy:
fixes S :: "nat \<Rightarrow> 'a::complete_space"
shows "(\<exists>l. (S \<longlongrightarrow> l) sequentially) \<longleftrightarrow> Cauchy S"
unfolding Cauchy_convergent_iff convergent_def ..
lemma convergent_imp_bounded:
fixes S :: "nat \<Rightarrow> 'a::metric_space"
shows "(S \<longlongrightarrow> l) sequentially \<Longrightarrow> bounded (range S)"
by (intro cauchy_imp_bounded LIMSEQ_imp_Cauchy)
lemma frontier_subset_compact:
fixes S :: "'a::heine_borel set"
shows "compact S \<Longrightarrow> frontier S \<subseteq> S"
using frontier_subset_closed compact_eq_bounded_closed
by blast
lemma continuous_closed_imp_Cauchy_continuous:
fixes S :: "('a::complete_space) set"
shows "\<lbrakk>continuous_on S f; closed S; Cauchy \<sigma>; \<And>n. (\<sigma> n) \<in> S\<rbrakk> \<Longrightarrow> Cauchy(f \<circ> \<sigma>)"
apply (simp add: complete_eq_closed [symmetric] continuous_on_sequentially)
by (meson LIMSEQ_imp_Cauchy complete_def)
lemma banach_fix_type:
fixes f::"'a::complete_space\<Rightarrow>'a"
assumes c:"0 \<le> c" "c < 1"
and lipschitz:"\<forall>x. \<forall>y. dist (f x) (f y) \<le> c * dist x y"
shows "\<exists>!x. (f x = x)"
using assms banach_fix[OF complete_UNIV UNIV_not_empty assms(1,2) subset_UNIV, of f]
by auto
subsection%unimportant\<open> Finite intersection property\<close>
text\<open>Also developed in HOL's toplogical spaces theory, but the Heine-Borel type class isn't available there.\<close>
lemma closed_imp_fip:
fixes S :: "'a::heine_borel set"
assumes "closed S"
and T: "T \<in> \<F>" "bounded T"
and clof: "\<And>T. T \<in> \<F> \<Longrightarrow> closed T"
and none: "\<And>\<F>'. \<lbrakk>finite \<F>'; \<F>' \<subseteq> \<F>\<rbrakk> \<Longrightarrow> S \<inter> \<Inter>\<F>' \<noteq> {}"
shows "S \<inter> \<Inter>\<F> \<noteq> {}"
proof -
have "compact (S \<inter> T)"
using \<open>closed S\<close> clof compact_eq_bounded_closed T by blast
then have "(S \<inter> T) \<inter> \<Inter>\<F> \<noteq> {}"
apply (rule compact_imp_fip)
apply (simp add: clof)
by (metis Int_assoc complete_lattice_class.Inf_insert finite_insert insert_subset none \<open>T \<in> \<F>\<close>)
then show ?thesis by blast
qed
lemma closed_imp_fip_compact:
fixes S :: "'a::heine_borel set"
shows
"\<lbrakk>closed S; \<And>T. T \<in> \<F> \<Longrightarrow> compact T;
\<And>\<F>'. \<lbrakk>finite \<F>'; \<F>' \<subseteq> \<F>\<rbrakk> \<Longrightarrow> S \<inter> \<Inter>\<F>' \<noteq> {}\<rbrakk>
\<Longrightarrow> S \<inter> \<Inter>\<F> \<noteq> {}"
by (metis Inf_greatest closed_imp_fip compact_eq_bounded_closed empty_subsetI finite.emptyI inf.orderE)
lemma closed_fip_Heine_Borel:
fixes \<F> :: "'a::heine_borel set set"
assumes "closed S" "T \<in> \<F>" "bounded T"
and "\<And>T. T \<in> \<F> \<Longrightarrow> closed T"
and "\<And>\<F>'. \<lbrakk>finite \<F>'; \<F>' \<subseteq> \<F>\<rbrakk> \<Longrightarrow> \<Inter>\<F>' \<noteq> {}"
shows "\<Inter>\<F> \<noteq> {}"
proof -
have "UNIV \<inter> \<Inter>\<F> \<noteq> {}"
using assms closed_imp_fip [OF closed_UNIV] by auto
then show ?thesis by simp
qed
lemma compact_fip_Heine_Borel:
fixes \<F> :: "'a::heine_borel set set"
assumes clof: "\<And>T. T \<in> \<F> \<Longrightarrow> compact T"
and none: "\<And>\<F>'. \<lbrakk>finite \<F>'; \<F>' \<subseteq> \<F>\<rbrakk> \<Longrightarrow> \<Inter>\<F>' \<noteq> {}"
shows "\<Inter>\<F> \<noteq> {}"
by (metis InterI all_not_in_conv clof closed_fip_Heine_Borel compact_eq_bounded_closed none)
lemma compact_sequence_with_limit:
fixes f :: "nat \<Rightarrow> 'a::heine_borel"
shows "(f \<longlongrightarrow> l) sequentially \<Longrightarrow> compact (insert l (range f))"
apply (simp add: compact_eq_bounded_closed, auto)
apply (simp add: convergent_imp_bounded)
by (simp add: closed_limpt islimpt_insert sequence_unique_limpt)
subsection \<open>Properties of Balls and Spheres\<close>
lemma compact_cball[simp]:
fixes x :: "'a::heine_borel"
shows "compact (cball x e)"
using compact_eq_bounded_closed bounded_cball closed_cball
by blast
lemma compact_frontier_bounded[intro]:
fixes S :: "'a::heine_borel set"
shows "bounded S \<Longrightarrow> compact (frontier S)"
unfolding frontier_def
using compact_eq_bounded_closed
by blast
lemma compact_frontier[intro]:
fixes S :: "'a::heine_borel set"
shows "compact S \<Longrightarrow> compact (frontier S)"
using compact_eq_bounded_closed compact_frontier_bounded
by blast
subsection \<open>Distance from a Set\<close>
lemma distance_attains_sup:
assumes "compact s" "s \<noteq> {}"
shows "\<exists>x\<in>s. \<forall>y\<in>s. dist a y \<le> dist a x"
proof (rule continuous_attains_sup [OF assms])
{
fix x
assume "x\<in>s"
have "(dist a \<longlongrightarrow> dist a x) (at x within s)"
by (intro tendsto_dist tendsto_const tendsto_ident_at)
}
then show "continuous_on s (dist a)"
unfolding continuous_on ..
qed
text \<open>For \emph{minimal} distance, we only need closure, not compactness.\<close>
lemma distance_attains_inf:
fixes a :: "'a::heine_borel"
assumes "closed s" and "s \<noteq> {}"
obtains x where "x\<in>s" "\<And>y. y \<in> s \<Longrightarrow> dist a x \<le> dist a y"
proof -
from assms obtain b where "b \<in> s" by auto
let ?B = "s \<inter> cball a (dist b a)"
have "?B \<noteq> {}" using \<open>b \<in> s\<close>
by (auto simp: dist_commute)
moreover have "continuous_on ?B (dist a)"
by (auto intro!: continuous_at_imp_continuous_on continuous_dist continuous_ident continuous_const)
moreover have "compact ?B"
by (intro closed_Int_compact \<open>closed s\<close> compact_cball)
ultimately obtain x where "x \<in> ?B" "\<forall>y\<in>?B. dist a x \<le> dist a y"
by (metis continuous_attains_inf)
with that show ?thesis by fastforce
qed
subsection \<open>Infimum Distance\<close>
definition%important "infdist x A = (if A = {} then 0 else INF a\<in>A. dist x a)"
lemma bdd_below_image_dist[intro, simp]: "bdd_below (dist x ` A)"
by (auto intro!: zero_le_dist)
lemma infdist_notempty: "A \<noteq> {} \<Longrightarrow> infdist x A = (INF a\<in>A. dist x a)"
by (simp add: infdist_def)
lemma infdist_nonneg: "0 \<le> infdist x A"
by (auto simp: infdist_def intro: cINF_greatest)
lemma infdist_le: "a \<in> A \<Longrightarrow> infdist x A \<le> dist x a"
by (auto intro: cINF_lower simp add: infdist_def)
lemma infdist_le2: "a \<in> A \<Longrightarrow> dist x a \<le> d \<Longrightarrow> infdist x A \<le> d"
by (auto intro!: cINF_lower2 simp add: infdist_def)
lemma infdist_zero[simp]: "a \<in> A \<Longrightarrow> infdist a A = 0"
by (auto intro!: antisym infdist_nonneg infdist_le2)
lemma infdist_triangle: "infdist x A \<le> infdist y A + dist x y"
proof (cases "A = {}")
case True
then show ?thesis by (simp add: infdist_def)
next
case False
then obtain a where "a \<in> A" by auto
have "infdist x A \<le> Inf {dist x y + dist y a |a. a \<in> A}"
proof (rule cInf_greatest)
from \<open>A \<noteq> {}\<close> show "{dist x y + dist y a |a. a \<in> A} \<noteq> {}"
by simp
fix d
assume "d \<in> {dist x y + dist y a |a. a \<in> A}"
then obtain a where d: "d = dist x y + dist y a" "a \<in> A"
by auto
show "infdist x A \<le> d"
unfolding infdist_notempty[OF \<open>A \<noteq> {}\<close>]
proof (rule cINF_lower2)
show "a \<in> A" by fact
show "dist x a \<le> d"
unfolding d by (rule dist_triangle)
qed simp
qed
also have "\<dots> = dist x y + infdist y A"
proof (rule cInf_eq, safe)
fix a
assume "a \<in> A"
then show "dist x y + infdist y A \<le> dist x y + dist y a"
by (auto intro: infdist_le)
next
fix i
assume inf: "\<And>d. d \<in> {dist x y + dist y a |a. a \<in> A} \<Longrightarrow> i \<le> d"
then have "i - dist x y \<le> infdist y A"
unfolding infdist_notempty[OF \<open>A \<noteq> {}\<close>] using \<open>a \<in> A\<close>
by (intro cINF_greatest) (auto simp: field_simps)
then show "i \<le> dist x y + infdist y A"
by simp
qed
finally show ?thesis by simp
qed
lemma in_closure_iff_infdist_zero:
assumes "A \<noteq> {}"
shows "x \<in> closure A \<longleftrightarrow> infdist x A = 0"
proof
assume "x \<in> closure A"
show "infdist x A = 0"
proof (rule ccontr)
assume "infdist x A \<noteq> 0"
with infdist_nonneg[of x A] have "infdist x A > 0"
by auto
then have "ball x (infdist x A) \<inter> closure A = {}"
apply auto
apply (metis \<open>x \<in> closure A\<close> closure_approachable dist_commute infdist_le not_less)
done
then have "x \<notin> closure A"
by (metis \<open>0 < infdist x A\<close> centre_in_ball disjoint_iff_not_equal)
then show False using \<open>x \<in> closure A\<close> by simp
qed
next
assume x: "infdist x A = 0"
then obtain a where "a \<in> A"
by atomize_elim (metis all_not_in_conv assms)
show "x \<in> closure A"
unfolding closure_approachable
apply safe
proof (rule ccontr)
fix e :: real
assume "e > 0"
assume "\<not> (\<exists>y\<in>A. dist y x < e)"
then have "infdist x A \<ge> e" using \<open>a \<in> A\<close>
unfolding infdist_def
by (force simp: dist_commute intro: cINF_greatest)
with x \<open>e > 0\<close> show False by auto
qed
qed
lemma in_closed_iff_infdist_zero:
assumes "closed A" "A \<noteq> {}"
shows "x \<in> A \<longleftrightarrow> infdist x A = 0"
proof -
have "x \<in> closure A \<longleftrightarrow> infdist x A = 0"
by (rule in_closure_iff_infdist_zero) fact
with assms show ?thesis by simp
qed
lemma infdist_pos_not_in_closed:
assumes "closed S" "S \<noteq> {}" "x \<notin> S"
shows "infdist x S > 0"
using in_closed_iff_infdist_zero[OF assms(1) assms(2), of x] assms(3) infdist_nonneg le_less by fastforce
lemma
infdist_attains_inf:
fixes X::"'a::heine_borel set"
assumes "closed X"
assumes "X \<noteq> {}"
obtains x where "x \<in> X" "infdist y X = dist y x"
proof -
have "bdd_below (dist y ` X)"
by auto
from distance_attains_inf[OF assms, of y]
obtain x where INF: "x \<in> X" "\<And>z. z \<in> X \<Longrightarrow> dist y x \<le> dist y z" by auto
have "infdist y X = dist y x"
by (auto simp: infdist_def assms
intro!: antisym cINF_lower[OF _ \<open>x \<in> X\<close>] cINF_greatest[OF assms(2) INF(2)])
with \<open>x \<in> X\<close> show ?thesis ..
qed
text \<open>Every metric space is a T4 space:\<close>
instance metric_space \<subseteq> t4_space
proof
fix S T::"'a set" assume H: "closed S" "closed T" "S \<inter> T = {}"
consider "S = {}" | "T = {}" | "S \<noteq> {} \<and> T \<noteq> {}" by auto
then show "\<exists>U V. open U \<and> open V \<and> S \<subseteq> U \<and> T \<subseteq> V \<and> U \<inter> V = {}"
proof (cases)
case 1
show ?thesis
apply (rule exI[of _ "{}"], rule exI[of _ UNIV]) using 1 by auto
next
case 2
show ?thesis
apply (rule exI[of _ UNIV], rule exI[of _ "{}"]) using 2 by auto
next
case 3
define U where "U = (\<Union>x\<in>S. ball x ((infdist x T)/2))"
have A: "open U" unfolding U_def by auto
have "infdist x T > 0" if "x \<in> S" for x
using H that 3 by (auto intro!: infdist_pos_not_in_closed)
then have B: "S \<subseteq> U" unfolding U_def by auto
define V where "V = (\<Union>x\<in>T. ball x ((infdist x S)/2))"
have C: "open V" unfolding V_def by auto
have "infdist x S > 0" if "x \<in> T" for x
using H that 3 by (auto intro!: infdist_pos_not_in_closed)
then have D: "T \<subseteq> V" unfolding V_def by auto
have "(ball x ((infdist x T)/2)) \<inter> (ball y ((infdist y S)/2)) = {}" if "x \<in> S" "y \<in> T" for x y
proof (auto)
fix z assume H: "dist x z * 2 < infdist x T" "dist y z * 2 < infdist y S"
have "2 * dist x y \<le> 2 * dist x z + 2 * dist y z"
using dist_triangle[of x y z] by (auto simp add: dist_commute)
also have "... < infdist x T + infdist y S"
using H by auto
finally have "dist x y < infdist x T \<or> dist x y < infdist y S"
by auto
then show False
using infdist_le[OF \<open>x \<in> S\<close>, of y] infdist_le[OF \<open>y \<in> T\<close>, of x] by (auto simp add: dist_commute)
qed
then have E: "U \<inter> V = {}"
unfolding U_def V_def by auto
show ?thesis
apply (rule exI[of _ U], rule exI[of _ V]) using A B C D E by auto
qed
qed
lemma tendsto_infdist [tendsto_intros]:
assumes f: "(f \<longlongrightarrow> l) F"
shows "((\<lambda>x. infdist (f x) A) \<longlongrightarrow> infdist l A) F"
proof (rule tendstoI)
fix e ::real
assume "e > 0"
from tendstoD[OF f this]
show "eventually (\<lambda>x. dist (infdist (f x) A) (infdist l A) < e) F"
proof (eventually_elim)
fix x
from infdist_triangle[of l A "f x"] infdist_triangle[of "f x" A l]
have "dist (infdist (f x) A) (infdist l A) \<le> dist (f x) l"
by (simp add: dist_commute dist_real_def)
also assume "dist (f x) l < e"
finally show "dist (infdist (f x) A) (infdist l A) < e" .
qed
qed
lemma continuous_infdist[continuous_intros]:
assumes "continuous F f"
shows "continuous F (\<lambda>x. infdist (f x) A)"
using assms unfolding continuous_def by (rule tendsto_infdist)
lemma compact_infdist_le:
fixes A::"'a::heine_borel set"
assumes "A \<noteq> {}"
assumes "compact A"
assumes "e > 0"
shows "compact {x. infdist x A \<le> e}"
proof -
from continuous_closed_vimage[of "{0..e}" "\<lambda>x. infdist x A"]
continuous_infdist[OF continuous_ident, of _ UNIV A]
have "closed {x. infdist x A \<le> e}" by (auto simp: vimage_def infdist_nonneg)
moreover
from assms obtain x0 b where b: "\<And>x. x \<in> A \<Longrightarrow> dist x0 x \<le> b" "closed A"
by (auto simp: compact_eq_bounded_closed bounded_def)
{
fix y
assume le: "infdist y A \<le> e"
from infdist_attains_inf[OF \<open>closed A\<close> \<open>A \<noteq> {}\<close>, of y]
obtain z where z: "z \<in> A" "infdist y A = dist y z" by blast
have "dist x0 y \<le> dist y z + dist x0 z"
by (metis dist_commute dist_triangle)
also have "dist y z \<le> e" using le z by simp
also have "dist x0 z \<le> b" using b z by simp
finally have "dist x0 y \<le> b + e" by arith
} then
have "bounded {x. infdist x A \<le> e}"
by (auto simp: bounded_any_center[where a=x0] intro!: exI[where x="b + e"])
ultimately show "compact {x. infdist x A \<le> e}"
by (simp add: compact_eq_bounded_closed)
qed
subsection \<open>Separation between Points and Sets\<close>
proposition separate_point_closed:
fixes s :: "'a::heine_borel set"
assumes "closed s" and "a \<notin> s"
shows "\<exists>d>0. \<forall>x\<in>s. d \<le> dist a x"
proof (cases "s = {}")
case True
then show ?thesis by(auto intro!: exI[where x=1])
next
case False
from assms obtain x where "x\<in>s" "\<forall>y\<in>s. dist a x \<le> dist a y"
using \<open>s \<noteq> {}\<close> by (blast intro: distance_attains_inf [of s a])
with \<open>x\<in>s\<close> show ?thesis using dist_pos_lt[of a x] and\<open>a \<notin> s\<close>
by blast
qed
proposition separate_compact_closed:
fixes s t :: "'a::heine_borel set"
assumes "compact s"
and t: "closed t" "s \<inter> t = {}"
shows "\<exists>d>0. \<forall>x\<in>s. \<forall>y\<in>t. d \<le> dist x y"
proof cases
assume "s \<noteq> {} \<and> t \<noteq> {}"
then have "s \<noteq> {}" "t \<noteq> {}" by auto
let ?inf = "\<lambda>x. infdist x t"
have "continuous_on s ?inf"
by (auto intro!: continuous_at_imp_continuous_on continuous_infdist continuous_ident)
then obtain x where x: "x \<in> s" "\<forall>y\<in>s. ?inf x \<le> ?inf y"
using continuous_attains_inf[OF \<open>compact s\<close> \<open>s \<noteq> {}\<close>] by auto
then have "0 < ?inf x"
using t \<open>t \<noteq> {}\<close> in_closed_iff_infdist_zero by (auto simp: less_le infdist_nonneg)
moreover have "\<forall>x'\<in>s. \<forall>y\<in>t. ?inf x \<le> dist x' y"
using x by (auto intro: order_trans infdist_le)
ultimately show ?thesis by auto
qed (auto intro!: exI[of _ 1])
proposition separate_closed_compact:
fixes s t :: "'a::heine_borel set"
assumes "closed s"
and "compact t"
and "s \<inter> t = {}"
shows "\<exists>d>0. \<forall>x\<in>s. \<forall>y\<in>t. d \<le> dist x y"
proof -
have *: "t \<inter> s = {}"
using assms(3) by auto
show ?thesis
using separate_compact_closed[OF assms(2,1) *] by (force simp: dist_commute)
qed
proposition compact_in_open_separated:
fixes A::"'a::heine_borel set"
assumes "A \<noteq> {}"
assumes "compact A"
assumes "open B"
assumes "A \<subseteq> B"
obtains e where "e > 0" "{x. infdist x A \<le> e} \<subseteq> B"
proof atomize_elim
have "closed (- B)" "compact A" "- B \<inter> A = {}"
using assms by (auto simp: open_Diff compact_eq_bounded_closed)
from separate_closed_compact[OF this]
obtain d'::real where d': "d'>0" "\<And>x y. x \<notin> B \<Longrightarrow> y \<in> A \<Longrightarrow> d' \<le> dist x y"
by auto
define d where "d = d' / 2"
hence "d>0" "d < d'" using d' by auto
with d' have d: "\<And>x y. x \<notin> B \<Longrightarrow> y \<in> A \<Longrightarrow> d < dist x y"
by force
show "\<exists>e>0. {x. infdist x A \<le> e} \<subseteq> B"
proof (rule ccontr)
assume "\<nexists>e. 0 < e \<and> {x. infdist x A \<le> e} \<subseteq> B"
with \<open>d > 0\<close> obtain x where x: "infdist x A \<le> d" "x \<notin> B"
by auto
from assms have "closed A" "A \<noteq> {}" by (auto simp: compact_eq_bounded_closed)
from infdist_attains_inf[OF this]
obtain y where y: "y \<in> A" "infdist x A = dist x y"
by auto
have "dist x y \<le> d" using x y by simp
also have "\<dots> < dist x y" using y d x by auto
finally show False by simp
qed
qed
subsection \<open>Uniform Continuity\<close>
lemma uniformly_continuous_onE:
assumes "uniformly_continuous_on s f" "0 < e"
obtains d where "d>0" "\<And>x x'. \<lbrakk>x\<in>s; x'\<in>s; dist x' x < d\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e"
using assms
by (auto simp: uniformly_continuous_on_def)
lemma uniformly_continuous_on_sequentially:
"uniformly_continuous_on s f \<longleftrightarrow> (\<forall>x y. (\<forall>n. x n \<in> s) \<and> (\<forall>n. y n \<in> s) \<and>
(\<lambda>n. dist (x n) (y n)) \<longlonglongrightarrow> 0 \<longrightarrow> (\<lambda>n. dist (f(x n)) (f(y n))) \<longlonglongrightarrow> 0)" (is "?lhs = ?rhs")
proof
assume ?lhs
{
fix x y
assume x: "\<forall>n. x n \<in> s"
and y: "\<forall>n. y n \<in> s"
and xy: "((\<lambda>n. dist (x n) (y n)) \<longlongrightarrow> 0) sequentially"
{
fix e :: real
assume "e > 0"
then obtain d where "d > 0" and d: "\<forall>x\<in>s. \<forall>x'\<in>s. dist x' x < d \<longrightarrow> dist (f x') (f x) < e"
using \<open>?lhs\<close>[unfolded uniformly_continuous_on_def, THEN spec[where x=e]] by auto
obtain N where N: "\<forall>n\<ge>N. dist (x n) (y n) < d"
using xy[unfolded lim_sequentially dist_norm] and \<open>d>0\<close> by auto
{
fix n
assume "n\<ge>N"
then have "dist (f (x n)) (f (y n)) < e"
using N[THEN spec[where x=n]]
using d[THEN bspec[where x="x n"], THEN bspec[where x="y n"]]
using x and y
by (simp add: dist_commute)
}
then have "\<exists>N. \<forall>n\<ge>N. dist (f (x n)) (f (y n)) < e"
by auto
}
then have "((\<lambda>n. dist (f(x n)) (f(y n))) \<longlongrightarrow> 0) sequentially"
unfolding lim_sequentially and dist_real_def by auto
}
then show ?rhs by auto
next
assume ?rhs
{
assume "\<not> ?lhs"
then obtain e where "e > 0" "\<forall>d>0. \<exists>x\<in>s. \<exists>x'\<in>s. dist x' x < d \<and> \<not> dist (f x') (f x) < e"
unfolding uniformly_continuous_on_def by auto
then obtain fa where fa:
"\<forall>x. 0 < x \<longrightarrow> fst (fa x) \<in> s \<and> snd (fa x) \<in> s \<and> dist (fst (fa x)) (snd (fa x)) < x \<and> \<not> dist (f (fst (fa x))) (f (snd (fa x))) < e"
using choice[of "\<lambda>d x. d>0 \<longrightarrow> fst x \<in> s \<and> snd x \<in> s \<and> dist (snd x) (fst x) < d \<and> \<not> dist (f (snd x)) (f (fst x)) < e"]
unfolding Bex_def
by (auto simp: dist_commute)
define x where "x n = fst (fa (inverse (real n + 1)))" for n
define y where "y n = snd (fa (inverse (real n + 1)))" for n
have xyn: "\<forall>n. x n \<in> s \<and> y n \<in> s"
and xy0: "\<forall>n. dist (x n) (y n) < inverse (real n + 1)"
and fxy:"\<forall>n. \<not> dist (f (x n)) (f (y n)) < e"
unfolding x_def and y_def using fa
by auto
{
fix e :: real
assume "e > 0"
then obtain N :: nat where "N \<noteq> 0" and N: "0 < inverse (real N) \<and> inverse (real N) < e"
unfolding real_arch_inverse[of e] by auto
{
fix n :: nat
assume "n \<ge> N"
then have "inverse (real n + 1) < inverse (real N)"
using of_nat_0_le_iff and \<open>N\<noteq>0\<close> by auto
also have "\<dots> < e" using N by auto
finally have "inverse (real n + 1) < e" by auto
then have "dist (x n) (y n) < e"
using xy0[THEN spec[where x=n]] by auto
}
then have "\<exists>N. \<forall>n\<ge>N. dist (x n) (y n) < e" by auto
}
then have "\<forall>e>0. \<exists>N. \<forall>n\<ge>N. dist (f (x n)) (f (y n)) < e"
using \<open>?rhs\<close>[THEN spec[where x=x], THEN spec[where x=y]] and xyn
unfolding lim_sequentially dist_real_def by auto
then have False using fxy and \<open>e>0\<close> by auto
}
then show ?lhs
unfolding uniformly_continuous_on_def by blast
qed
subsection \<open>Continuity on a Compact Domain Implies Uniform Continuity\<close>
text\<open>From the proof of the Heine-Borel theorem: Lemma 2 in section 3.7, page 69 of
J. C. Burkill and H. Burkill. A Second Course in Mathematical Analysis (CUP, 2002)\<close>
lemma Heine_Borel_lemma:
assumes "compact S" and Ssub: "S \<subseteq> \<Union>\<G>" and opn: "\<And>G. G \<in> \<G> \<Longrightarrow> open G"
obtains e where "0 < e" "\<And>x. x \<in> S \<Longrightarrow> \<exists>G \<in> \<G>. ball x e \<subseteq> G"
proof -
have False if neg: "\<And>e. 0 < e \<Longrightarrow> \<exists>x \<in> S. \<forall>G \<in> \<G>. \<not> ball x e \<subseteq> G"
proof -
have "\<exists>x \<in> S. \<forall>G \<in> \<G>. \<not> ball x (1 / Suc n) \<subseteq> G" for n
using neg by simp
then obtain f where "\<And>n. f n \<in> S" and fG: "\<And>G n. G \<in> \<G> \<Longrightarrow> \<not> ball (f n) (1 / Suc n) \<subseteq> G"
by metis
then obtain l r where "l \<in> S" "strict_mono r" and to_l: "(f \<circ> r) \<longlonglongrightarrow> l"
using \<open>compact S\<close> compact_def that by metis
then obtain G where "l \<in> G" "G \<in> \<G>"
using Ssub by auto
then obtain e where "0 < e" and e: "\<And>z. dist z l < e \<Longrightarrow> z \<in> G"
using opn open_dist by blast
obtain N1 where N1: "\<And>n. n \<ge> N1 \<Longrightarrow> dist (f (r n)) l < e/2"
using to_l apply (simp add: lim_sequentially)
using \<open>0 < e\<close> half_gt_zero that by blast
obtain N2 where N2: "of_nat N2 > 2/e"
using reals_Archimedean2 by blast
obtain x where "x \<in> ball (f (r (max N1 N2))) (1 / real (Suc (r (max N1 N2))))" and "x \<notin> G"
using fG [OF \<open>G \<in> \<G>\<close>, of "r (max N1 N2)"] by blast
then have "dist (f (r (max N1 N2))) x < 1 / real (Suc (r (max N1 N2)))"
by simp
also have "... \<le> 1 / real (Suc (max N1 N2))"
apply (simp add: divide_simps del: max.bounded_iff)
using \<open>strict_mono r\<close> seq_suble by blast
also have "... \<le> 1 / real (Suc N2)"
by (simp add: field_simps)
also have "... < e/2"
using N2 \<open>0 < e\<close> by (simp add: field_simps)
finally have "dist (f (r (max N1 N2))) x < e / 2" .
moreover have "dist (f (r (max N1 N2))) l < e/2"
using N1 max.cobounded1 by blast
ultimately have "dist x l < e"
using dist_triangle_half_r by blast
then show ?thesis
using e \<open>x \<notin> G\<close> by blast
qed
then show ?thesis
by (meson that)
qed
lemma compact_uniformly_equicontinuous:
assumes "compact S"
and cont: "\<And>x e. \<lbrakk>x \<in> S; 0 < e\<rbrakk>
\<Longrightarrow> \<exists>d. 0 < d \<and>
(\<forall>f \<in> \<F>. \<forall>x' \<in> S. dist x' x < d \<longrightarrow> dist (f x') (f x) < e)"
and "0 < e"
obtains d where "0 < d"
"\<And>f x x'. \<lbrakk>f \<in> \<F>; x \<in> S; x' \<in> S; dist x' x < d\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e"
proof -
obtain d where d_pos: "\<And>x e. \<lbrakk>x \<in> S; 0 < e\<rbrakk> \<Longrightarrow> 0 < d x e"
and d_dist : "\<And>x x' e f. \<lbrakk>dist x' x < d x e; x \<in> S; x' \<in> S; 0 < e; f \<in> \<F>\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e"
using cont by metis
let ?\<G> = "((\<lambda>x. ball x (d x (e / 2))) ` S)"
have Ssub: "S \<subseteq> \<Union> ?\<G>"
by clarsimp (metis d_pos \<open>0 < e\<close> dist_self half_gt_zero_iff)
then obtain k where "0 < k" and k: "\<And>x. x \<in> S \<Longrightarrow> \<exists>G \<in> ?\<G>. ball x k \<subseteq> G"
by (rule Heine_Borel_lemma [OF \<open>compact S\<close>]) auto
moreover have "dist (f v) (f u) < e" if "f \<in> \<F>" "u \<in> S" "v \<in> S" "dist v u < k" for f u v
proof -
obtain G where "G \<in> ?\<G>" "u \<in> G" "v \<in> G"
using k that
by (metis \<open>dist v u < k\<close> \<open>u \<in> S\<close> \<open>0 < k\<close> centre_in_ball subsetD dist_commute mem_ball)
then obtain w where w: "dist w u < d w (e / 2)" "dist w v < d w (e / 2)" "w \<in> S"
by auto
with that d_dist have "dist (f w) (f v) < e/2"
by (metis \<open>0 < e\<close> dist_commute half_gt_zero)
moreover
have "dist (f w) (f u) < e/2"
using that d_dist w by (metis \<open>0 < e\<close> dist_commute divide_pos_pos zero_less_numeral)
ultimately show ?thesis
using dist_triangle_half_r by blast
qed
ultimately show ?thesis using that by blast
qed
corollary compact_uniformly_continuous:
fixes f :: "'a :: metric_space \<Rightarrow> 'b :: metric_space"
assumes f: "continuous_on S f" and S: "compact S"
shows "uniformly_continuous_on S f"
using f
unfolding continuous_on_iff uniformly_continuous_on_def
by (force intro: compact_uniformly_equicontinuous [OF S, of "{f}"])
subsection%unimportant\<open> Theorems relating continuity and uniform continuity to closures\<close>
lemma continuous_on_closure:
"continuous_on (closure S) f \<longleftrightarrow>
(\<forall>x e. x \<in> closure S \<and> 0 < e
\<longrightarrow> (\<exists>d. 0 < d \<and> (\<forall>y. y \<in> S \<and> dist y x < d \<longrightarrow> dist (f y) (f x) < e)))"
(is "?lhs = ?rhs")
proof
assume ?lhs then show ?rhs
unfolding continuous_on_iff by (metis Un_iff closure_def)
next
assume R [rule_format]: ?rhs
show ?lhs
proof
fix x and e::real
assume "0 < e" and x: "x \<in> closure S"
obtain \<delta>::real where "\<delta> > 0"
and \<delta>: "\<And>y. \<lbrakk>y \<in> S; dist y x < \<delta>\<rbrakk> \<Longrightarrow> dist (f y) (f x) < e/2"
using R [of x "e/2"] \<open>0 < e\<close> x by auto
have "dist (f y) (f x) \<le> e" if y: "y \<in> closure S" and dyx: "dist y x < \<delta>/2" for y
proof -
obtain \<delta>'::real where "\<delta>' > 0"
and \<delta>': "\<And>z. \<lbrakk>z \<in> S; dist z y < \<delta>'\<rbrakk> \<Longrightarrow> dist (f z) (f y) < e/2"
using R [of y "e/2"] \<open>0 < e\<close> y by auto
obtain z where "z \<in> S" and z: "dist z y < min \<delta>' \<delta> / 2"
using closure_approachable y
by (metis \<open>0 < \<delta>'\<close> \<open>0 < \<delta>\<close> divide_pos_pos min_less_iff_conj zero_less_numeral)
have "dist (f z) (f y) < e/2"
apply (rule \<delta>' [OF \<open>z \<in> S\<close>])
using z \<open>0 < \<delta>'\<close> by linarith
moreover have "dist (f z) (f x) < e/2"
apply (rule \<delta> [OF \<open>z \<in> S\<close>])
using z \<open>0 < \<delta>\<close> dist_commute[of y z] dist_triangle_half_r [of y] dyx by auto
ultimately show ?thesis
by (metis dist_commute dist_triangle_half_l less_imp_le)
qed
then show "\<exists>d>0. \<forall>x'\<in>closure S. dist x' x < d \<longrightarrow> dist (f x') (f x) \<le> e"
by (rule_tac x="\<delta>/2" in exI) (simp add: \<open>\<delta> > 0\<close>)
qed
qed
lemma continuous_on_closure_sequentially:
fixes f :: "'a::metric_space \<Rightarrow> 'b :: metric_space"
shows
"continuous_on (closure S) f \<longleftrightarrow>
(\<forall>x a. a \<in> closure S \<and> (\<forall>n. x n \<in> S) \<and> x \<longlonglongrightarrow> a \<longrightarrow> (f \<circ> x) \<longlonglongrightarrow> f a)"
(is "?lhs = ?rhs")
proof -
have "continuous_on (closure S) f \<longleftrightarrow>
(\<forall>x \<in> closure S. continuous (at x within S) f)"
by (force simp: continuous_on_closure continuous_within_eps_delta)
also have "... = ?rhs"
by (force simp: continuous_within_sequentially)
finally show ?thesis .
qed
lemma uniformly_continuous_on_closure:
fixes f :: "'a::metric_space \<Rightarrow> 'b::metric_space"
assumes ucont: "uniformly_continuous_on S f"
and cont: "continuous_on (closure S) f"
shows "uniformly_continuous_on (closure S) f"
unfolding uniformly_continuous_on_def
proof (intro allI impI)
fix e::real
assume "0 < e"
then obtain d::real
where "d>0"
and d: "\<And>x x'. \<lbrakk>x\<in>S; x'\<in>S; dist x' x < d\<rbrakk> \<Longrightarrow> dist (f x') (f x) < e/3"
using ucont [unfolded uniformly_continuous_on_def, rule_format, of "e/3"] by auto
show "\<exists>d>0. \<forall>x\<in>closure S. \<forall>x'\<in>closure S. dist x' x < d \<longrightarrow> dist (f x') (f x) < e"
proof (rule exI [where x="d/3"], clarsimp simp: \<open>d > 0\<close>)
fix x y
assume x: "x \<in> closure S" and y: "y \<in> closure S" and dyx: "dist y x * 3 < d"
obtain d1::real where "d1 > 0"
and d1: "\<And>w. \<lbrakk>w \<in> closure S; dist w x < d1\<rbrakk> \<Longrightarrow> dist (f w) (f x) < e/3"
using cont [unfolded continuous_on_iff, rule_format, of "x" "e/3"] \<open>0 < e\<close> x by auto
obtain x' where "x' \<in> S" and x': "dist x' x < min d1 (d / 3)"
using closure_approachable [of x S]
by (metis \<open>0 < d1\<close> \<open>0 < d\<close> divide_pos_pos min_less_iff_conj x zero_less_numeral)
obtain d2::real where "d2 > 0"
and d2: "\<forall>w \<in> closure S. dist w y < d2 \<longrightarrow> dist (f w) (f y) < e/3"
using cont [unfolded continuous_on_iff, rule_format, of "y" "e/3"] \<open>0 < e\<close> y by auto
obtain y' where "y' \<in> S" and y': "dist y' y < min d2 (d / 3)"
using closure_approachable [of y S]
by (metis \<open>0 < d2\<close> \<open>0 < d\<close> divide_pos_pos min_less_iff_conj y zero_less_numeral)
have "dist x' x < d/3" using x' by auto
moreover have "dist x y < d/3"
by (metis dist_commute dyx less_divide_eq_numeral1(1))
moreover have "dist y y' < d/3"
by (metis (no_types) dist_commute min_less_iff_conj y')
ultimately have "dist x' y' < d/3 + d/3 + d/3"
by (meson dist_commute_lessI dist_triangle_lt add_strict_mono)
then have "dist x' y' < d" by simp
then have "dist (f x') (f y') < e/3"
by (rule d [OF \<open>y' \<in> S\<close> \<open>x' \<in> S\<close>])
moreover have "dist (f x') (f x) < e/3" using \<open>x' \<in> S\<close> closure_subset x' d1
by (simp add: closure_def)
moreover have "dist (f y') (f y) < e/3" using \<open>y' \<in> S\<close> closure_subset y' d2
by (simp add: closure_def)
ultimately have "dist (f y) (f x) < e/3 + e/3 + e/3"
by (meson dist_commute_lessI dist_triangle_lt add_strict_mono)
then show "dist (f y) (f x) < e" by simp
qed
qed
lemma uniformly_continuous_on_extension_at_closure:
fixes f::"'a::metric_space \<Rightarrow> 'b::complete_space"
assumes uc: "uniformly_continuous_on X f"
assumes "x \<in> closure X"
obtains l where "(f \<longlongrightarrow> l) (at x within X)"
proof -
from assms obtain xs where xs: "xs \<longlonglongrightarrow> x" "\<And>n. xs n \<in> X"
by (auto simp: closure_sequential)
from uniformly_continuous_on_Cauchy[OF uc LIMSEQ_imp_Cauchy, OF xs]
obtain l where l: "(\<lambda>n. f (xs n)) \<longlonglongrightarrow> l"
by atomize_elim (simp only: convergent_eq_Cauchy)
have "(f \<longlongrightarrow> l) (at x within X)"
proof (safe intro!: Lim_within_LIMSEQ)
fix xs'
assume "\<forall>n. xs' n \<noteq> x \<and> xs' n \<in> X"
and xs': "xs' \<longlonglongrightarrow> x"
then have "xs' n \<noteq> x" "xs' n \<in> X" for n by auto
from uniformly_continuous_on_Cauchy[OF uc LIMSEQ_imp_Cauchy, OF \<open>xs' \<longlonglongrightarrow> x\<close> \<open>xs' _ \<in> X\<close>]
obtain l' where l': "(\<lambda>n. f (xs' n)) \<longlonglongrightarrow> l'"
by atomize_elim (simp only: convergent_eq_Cauchy)
show "(\<lambda>n. f (xs' n)) \<longlonglongrightarrow> l"
proof (rule tendstoI)
fix e::real assume "e > 0"
define e' where "e' \<equiv> e / 2"
have "e' > 0" using \<open>e > 0\<close> by (simp add: e'_def)
have "\<forall>\<^sub>F n in sequentially. dist (f (xs n)) l < e'"
by (simp add: \<open>0 < e'\<close> l tendstoD)
moreover
from uc[unfolded uniformly_continuous_on_def, rule_format, OF \<open>e' > 0\<close>]
obtain d where d: "d > 0" "\<And>x x'. x \<in> X \<Longrightarrow> x' \<in> X \<Longrightarrow> dist x x' < d \<Longrightarrow> dist (f x) (f x') < e'"
by auto
have "\<forall>\<^sub>F n in sequentially. dist (xs n) (xs' n) < d"
by (auto intro!: \<open>0 < d\<close> order_tendstoD tendsto_eq_intros xs xs')
ultimately
show "\<forall>\<^sub>F n in sequentially. dist (f (xs' n)) l < e"
proof eventually_elim
case (elim n)
have "dist (f (xs' n)) l \<le> dist (f (xs n)) (f (xs' n)) + dist (f (xs n)) l"
by (metis dist_triangle dist_commute)
also have "dist (f (xs n)) (f (xs' n)) < e'"
by (auto intro!: d xs \<open>xs' _ \<in> _\<close> elim)
also note \<open>dist (f (xs n)) l < e'\<close>
also have "e' + e' = e" by (simp add: e'_def)
finally show ?case by simp
qed
qed
qed
thus ?thesis ..
qed
lemma uniformly_continuous_on_extension_on_closure:
fixes f::"'a::metric_space \<Rightarrow> 'b::complete_space"
assumes uc: "uniformly_continuous_on X f"
obtains g where "uniformly_continuous_on (closure X) g" "\<And>x. x \<in> X \<Longrightarrow> f x = g x"
"\<And>Y h x. X \<subseteq> Y \<Longrightarrow> Y \<subseteq> closure X \<Longrightarrow> continuous_on Y h \<Longrightarrow> (\<And>x. x \<in> X \<Longrightarrow> f x = h x) \<Longrightarrow> x \<in> Y \<Longrightarrow> h x = g x"
proof -
from uc have cont_f: "continuous_on X f"
by (simp add: uniformly_continuous_imp_continuous)
obtain y where y: "(f \<longlongrightarrow> y x) (at x within X)" if "x \<in> closure X" for x
apply atomize_elim
apply (rule choice)
using uniformly_continuous_on_extension_at_closure[OF assms]
by metis
let ?g = "\<lambda>x. if x \<in> X then f x else y x"
have "uniformly_continuous_on (closure X) ?g"
unfolding uniformly_continuous_on_def
proof safe
fix e::real assume "e > 0"
define e' where "e' \<equiv> e / 3"
have "e' > 0" using \<open>e > 0\<close> by (simp add: e'_def)
from uc[unfolded uniformly_continuous_on_def, rule_format, OF \<open>0 < e'\<close>]
obtain d where "d > 0" and d: "\<And>x x'. x \<in> X \<Longrightarrow> x' \<in> X \<Longrightarrow> dist x' x < d \<Longrightarrow> dist (f x') (f x) < e'"
by auto
define d' where "d' = d / 3"
have "d' > 0" using \<open>d > 0\<close> by (simp add: d'_def)
show "\<exists>d>0. \<forall>x\<in>closure X. \<forall>x'\<in>closure X. dist x' x < d \<longrightarrow> dist (?g x') (?g x) < e"
proof (safe intro!: exI[where x=d'] \<open>d' > 0\<close>)
fix x x' assume x: "x \<in> closure X" and x': "x' \<in> closure X" and dist: "dist x' x < d'"
then obtain xs xs' where xs: "xs \<longlonglongrightarrow> x" "\<And>n. xs n \<in> X"
and xs': "xs' \<longlonglongrightarrow> x'" "\<And>n. xs' n \<in> X"
by (auto simp: closure_sequential)
have "\<forall>\<^sub>F n in sequentially. dist (xs' n) x' < d'"
and "\<forall>\<^sub>F n in sequentially. dist (xs n) x < d'"
by (auto intro!: \<open>0 < d'\<close> order_tendstoD tendsto_eq_intros xs xs')
moreover
have "(\<lambda>x. f (xs x)) \<longlonglongrightarrow> y x" if "x \<in> closure X" "x \<notin> X" "xs \<longlonglongrightarrow> x" "\<And>n. xs n \<in> X" for xs x
using that not_eventuallyD
by (force intro!: filterlim_compose[OF y[OF \<open>x \<in> closure X\<close>]] simp: filterlim_at)
then have "(\<lambda>x. f (xs' x)) \<longlonglongrightarrow> ?g x'" "(\<lambda>x. f (xs x)) \<longlonglongrightarrow> ?g x"
using x x'
by (auto intro!: continuous_on_tendsto_compose[OF cont_f] simp: xs' xs)
then have "\<forall>\<^sub>F n in sequentially. dist (f (xs' n)) (?g x') < e'"
"\<forall>\<^sub>F n in sequentially. dist (f (xs n)) (?g x) < e'"
by (auto intro!: \<open>0 < e'\<close> order_tendstoD tendsto_eq_intros)
ultimately
have "\<forall>\<^sub>F n in sequentially. dist (?g x') (?g x) < e"
proof eventually_elim
case (elim n)
have "dist (?g x') (?g x) \<le>
dist (f (xs' n)) (?g x') + dist (f (xs' n)) (f (xs n)) + dist (f (xs n)) (?g x)"
by (metis add.commute add_le_cancel_left dist_commute dist_triangle dist_triangle_le)
also
{
have "dist (xs' n) (xs n) \<le> dist (xs' n) x' + dist x' x + dist (xs n) x"
by (metis add.commute add_le_cancel_left dist_triangle dist_triangle_le)
also note \<open>dist (xs' n) x' < d'\<close>
also note \<open>dist x' x < d'\<close>
also note \<open>dist (xs n) x < d'\<close>
finally have "dist (xs' n) (xs n) < d" by (simp add: d'_def)
}
with \<open>xs _ \<in> X\<close> \<open>xs' _ \<in> X\<close> have "dist (f (xs' n)) (f (xs n)) < e'"
by (rule d)
also note \<open>dist (f (xs' n)) (?g x') < e'\<close>
also note \<open>dist (f (xs n)) (?g x) < e'\<close>
finally show ?case by (simp add: e'_def)
qed
then show "dist (?g x') (?g x) < e" by simp
qed
qed
moreover have "f x = ?g x" if "x \<in> X" for x using that by simp
moreover
{
fix Y h x
assume Y: "x \<in> Y" "X \<subseteq> Y" "Y \<subseteq> closure X" and cont_h: "continuous_on Y h"
and extension: "(\<And>x. x \<in> X \<Longrightarrow> f x = h x)"
{
assume "x \<notin> X"
have "x \<in> closure X" using Y by auto
then obtain xs where xs: "xs \<longlonglongrightarrow> x" "\<And>n. xs n \<in> X"
by (auto simp: closure_sequential)
from continuous_on_tendsto_compose[OF cont_h xs(1)] xs(2) Y
have hx: "(\<lambda>x. f (xs x)) \<longlonglongrightarrow> h x"
by (auto simp: set_mp extension)
then have "(\<lambda>x. f (xs x)) \<longlonglongrightarrow> y x"
using \<open>x \<notin> X\<close> not_eventuallyD xs(2)
by (force intro!: filterlim_compose[OF y[OF \<open>x \<in> closure X\<close>]] simp: filterlim_at xs)
with hx have "h x = y x" by (rule LIMSEQ_unique)
} then
have "h x = ?g x"
using extension by auto
}
ultimately show ?thesis ..
qed
lemma bounded_uniformly_continuous_image:
fixes f :: "'a :: heine_borel \<Rightarrow> 'b :: heine_borel"
assumes "uniformly_continuous_on S f" "bounded S"
shows "bounded(f ` S)"
by (metis (no_types, lifting) assms bounded_closure_image compact_closure compact_continuous_image compact_eq_bounded_closed image_cong uniformly_continuous_imp_continuous uniformly_continuous_on_extension_on_closure)
subsection \<open>With Abstract Topology (TODO: move and remove dependency?)\<close>
lemma openin_contains_ball:
"openin (subtopology euclidean t) s \<longleftrightarrow>
s \<subseteq> t \<and> (\<forall>x \<in> s. \<exists>e. 0 < e \<and> ball x e \<inter> t \<subseteq> s)"
(is "?lhs = ?rhs")
proof
assume ?lhs
then show ?rhs
apply (simp add: openin_open)
apply (metis Int_commute Int_mono inf.cobounded2 open_contains_ball order_refl subsetCE)
done
next
assume ?rhs
then show ?lhs
apply (simp add: openin_euclidean_subtopology_iff)
by (metis (no_types) Int_iff dist_commute inf.absorb_iff2 mem_ball)
qed
lemma openin_contains_cball:
"openin (subtopology euclidean t) s \<longleftrightarrow>
s \<subseteq> t \<and>
(\<forall>x \<in> s. \<exists>e. 0 < e \<and> cball x e \<inter> t \<subseteq> s)"
apply (simp add: openin_contains_ball)
apply (rule iffI)
apply (auto dest!: bspec)
apply (rule_tac x="e/2" in exI, force+)
done
subsection \<open>Closed Nest\<close>
text \<open>Bounded closed nest property (proof does not use Heine-Borel)\<close>
lemma bounded_closed_nest:
fixes S :: "nat \<Rightarrow> ('a::heine_borel) set"
assumes "\<And>n. closed (S n)"
and "\<And>n. S n \<noteq> {}"
and "\<And>m n. m \<le> n \<Longrightarrow> S n \<subseteq> S m"
and "bounded (S 0)"
obtains a where "\<And>n. a \<in> S n"
proof -
from assms(2) obtain x where x: "\<forall>n. x n \<in> S n"
using choice[of "\<lambda>n x. x \<in> S n"] by auto
from assms(4,1) have "seq_compact (S 0)"
by (simp add: bounded_closed_imp_seq_compact)
then obtain l r where lr: "l \<in> S 0" "strict_mono r" "(x \<circ> r) \<longlonglongrightarrow> l"
using x and assms(3) unfolding seq_compact_def by blast
have "\<forall>n. l \<in> S n"
proof
fix n :: nat
have "closed (S n)"
using assms(1) by simp
moreover have "\<forall>i. (x \<circ> r) i \<in> S i"
using x and assms(3) and lr(2) [THEN seq_suble] by auto
then have "\<forall>i. (x \<circ> r) (i + n) \<in> S n"
using assms(3) by (fast intro!: le_add2)
moreover have "(\<lambda>i. (x \<circ> r) (i + n)) \<longlonglongrightarrow> l"
using lr(3) by (rule LIMSEQ_ignore_initial_segment)
ultimately show "l \<in> S n"
by (rule closed_sequentially)
qed
then show ?thesis
using that by blast
qed
text \<open>Decreasing case does not even need compactness, just completeness.\<close>
lemma decreasing_closed_nest:
fixes S :: "nat \<Rightarrow> ('a::complete_space) set"
assumes "\<And>n. closed (S n)"
"\<And>n. S n \<noteq> {}"
"\<And>m n. m \<le> n \<Longrightarrow> S n \<subseteq> S m"
"\<And>e. e>0 \<Longrightarrow> \<exists>n. \<forall>x\<in>S n. \<forall>y\<in>S n. dist x y < e"
obtains a where "\<And>n. a \<in> S n"
proof -
have "\<forall>n. \<exists>x. x \<in> S n"
using assms(2) by auto
then have "\<exists>t. \<forall>n. t n \<in> S n"
using choice[of "\<lambda>n x. x \<in> S n"] by auto
then obtain t where t: "\<forall>n. t n \<in> S n" by auto
{
fix e :: real
assume "e > 0"
then obtain N where N: "\<forall>x\<in>S N. \<forall>y\<in>S N. dist x y < e"
using assms(4) by blast
{
fix m n :: nat
assume "N \<le> m \<and> N \<le> n"
then have "t m \<in> S N" "t n \<in> S N"
using assms(3) t unfolding subset_eq t by blast+
then have "dist (t m) (t n) < e"
using N by auto
}
then have "\<exists>N. \<forall>m n. N \<le> m \<and> N \<le> n \<longrightarrow> dist (t m) (t n) < e"
by auto
}
then have "Cauchy t"
unfolding cauchy_def by auto
then obtain l where l:"(t \<longlongrightarrow> l) sequentially"
using complete_UNIV unfolding complete_def by auto
{ fix n :: nat
{ fix e :: real
assume "e > 0"
then obtain N :: nat where N: "\<forall>n\<ge>N. dist (t n) l < e"
using l[unfolded lim_sequentially] by auto
have "t (max n N) \<in> S n"
by (meson assms(3) contra_subsetD max.cobounded1 t)
then have "\<exists>y\<in>S n. dist y l < e"
using N max.cobounded2 by blast
}
then have "l \<in> S n"
using closed_approachable[of "S n" l] assms(1) by auto
}
then show ?thesis
using that by blast
qed
text \<open>Strengthen it to the intersection actually being a singleton.\<close>
lemma decreasing_closed_nest_sing:
fixes S :: "nat \<Rightarrow> 'a::complete_space set"
assumes "\<And>n. closed(S n)"
"\<And>n. S n \<noteq> {}"
"\<And>m n. m \<le> n \<Longrightarrow> S n \<subseteq> S m"
"\<And>e. e>0 \<Longrightarrow> \<exists>n. \<forall>x \<in> (S n). \<forall> y\<in>(S n). dist x y < e"
shows "\<exists>a. \<Inter>(range S) = {a}"
proof -
obtain a where a: "\<forall>n. a \<in> S n"
using decreasing_closed_nest[of S] using assms by auto
{ fix b
assume b: "b \<in> \<Inter>(range S)"
{ fix e :: real
assume "e > 0"
then have "dist a b < e"
using assms(4) and b and a by blast
}
then have "dist a b = 0"
by (metis dist_eq_0_iff dist_nz less_le)
}
with a have "\<Inter>(range S) = {a}"
unfolding image_def by auto
then show ?thesis ..
qed
subsection%unimportant \<open>Making a continuous function avoid some value in a neighbourhood\<close>
lemma continuous_within_avoid:
fixes f :: "'a::metric_space \<Rightarrow> 'b::t1_space"
assumes "continuous (at x within s) f"
and "f x \<noteq> a"
shows "\<exists>e>0. \<forall>y \<in> s. dist x y < e --> f y \<noteq> a"
proof -
obtain U where "open U" and "f x \<in> U" and "a \<notin> U"
using t1_space [OF \<open>f x \<noteq> a\<close>] by fast
have "(f \<longlongrightarrow> f x) (at x within s)"
using assms(1) by (simp add: continuous_within)
then have "eventually (\<lambda>y. f y \<in> U) (at x within s)"
using \<open>open U\<close> and \<open>f x \<in> U\<close>
unfolding tendsto_def by fast
then have "eventually (\<lambda>y. f y \<noteq> a) (at x within s)"
using \<open>a \<notin> U\<close> by (fast elim: eventually_mono)
then show ?thesis
using \<open>f x \<noteq> a\<close> by (auto simp: dist_commute eventually_at)
qed
lemma continuous_at_avoid:
fixes f :: "'a::metric_space \<Rightarrow> 'b::t1_space"
assumes "continuous (at x) f"
and "f x \<noteq> a"
shows "\<exists>e>0. \<forall>y. dist x y < e \<longrightarrow> f y \<noteq> a"
using assms continuous_within_avoid[of x UNIV f a] by simp
lemma continuous_on_avoid:
fixes f :: "'a::metric_space \<Rightarrow> 'b::t1_space"
assumes "continuous_on s f"
and "x \<in> s"
and "f x \<noteq> a"
shows "\<exists>e>0. \<forall>y \<in> s. dist x y < e \<longrightarrow> f y \<noteq> a"
using assms(1)[unfolded continuous_on_eq_continuous_within, THEN bspec[where x=x],
OF assms(2)] continuous_within_avoid[of x s f a]
using assms(3)
by auto
lemma continuous_on_open_avoid:
fixes f :: "'a::metric_space \<Rightarrow> 'b::t1_space"
assumes "continuous_on s f"
and "open s"
and "x \<in> s"
and "f x \<noteq> a"
shows "\<exists>e>0. \<forall>y. dist x y < e \<longrightarrow> f y \<noteq> a"
using assms(1)[unfolded continuous_on_eq_continuous_at[OF assms(2)], THEN bspec[where x=x], OF assms(3)]
using continuous_at_avoid[of x f a] assms(4)
by auto
subsection \<open>Consequences for Real Numbers\<close>
lemma closed_contains_Inf:
fixes S :: "real set"
shows "S \<noteq> {} \<Longrightarrow> bdd_below S \<Longrightarrow> closed S \<Longrightarrow> Inf S \<in> S"
by (metis closure_contains_Inf closure_closed)
lemma closed_subset_contains_Inf:
fixes A C :: "real set"
shows "closed C \<Longrightarrow> A \<subseteq> C \<Longrightarrow> A \<noteq> {} \<Longrightarrow> bdd_below A \<Longrightarrow> Inf A \<in> C"
by (metis closure_contains_Inf closure_minimal subset_eq)
lemma atLeastAtMost_subset_contains_Inf:
fixes A :: "real set" and a b :: real
shows "A \<noteq> {} \<Longrightarrow> a \<le> b \<Longrightarrow> A \<subseteq> {a..b} \<Longrightarrow> Inf A \<in> {a..b}"
by (rule closed_subset_contains_Inf)
(auto intro: closed_real_atLeastAtMost intro!: bdd_belowI[of A a])
lemma bounded_real: "bounded (S::real set) \<longleftrightarrow> (\<exists>a. \<forall>x\<in>S. \<bar>x\<bar> \<le> a)"
by (simp add: bounded_iff)
lemma bounded_imp_bdd_above: "bounded S \<Longrightarrow> bdd_above (S :: real set)"
by (auto simp: bounded_def bdd_above_def dist_real_def)
(metis abs_le_D1 abs_minus_commute diff_le_eq)
lemma bounded_imp_bdd_below: "bounded S \<Longrightarrow> bdd_below (S :: real set)"
by (auto simp: bounded_def bdd_below_def dist_real_def)
(metis abs_le_D1 add.commute diff_le_eq)
lemma bounded_has_Sup:
fixes S :: "real set"
assumes "bounded S"
and "S \<noteq> {}"
shows "\<forall>x\<in>S. x \<le> Sup S"
and "\<forall>b. (\<forall>x\<in>S. x \<le> b) \<longrightarrow> Sup S \<le> b"
proof
show "\<forall>b. (\<forall>x\<in>S. x \<le> b) \<longrightarrow> Sup S \<le> b"
using assms by (metis cSup_least)
qed (metis cSup_upper assms(1) bounded_imp_bdd_above)
lemma Sup_insert:
fixes S :: "real set"
shows "bounded S \<Longrightarrow> Sup (insert x S) = (if S = {} then x else max x (Sup S))"
by (auto simp: bounded_imp_bdd_above sup_max cSup_insert_If)
lemma bounded_has_Inf:
fixes S :: "real set"
assumes "bounded S"
and "S \<noteq> {}"
shows "\<forall>x\<in>S. x \<ge> Inf S"
and "\<forall>b. (\<forall>x\<in>S. x \<ge> b) \<longrightarrow> Inf S \<ge> b"
proof
show "\<forall>b. (\<forall>x\<in>S. x \<ge> b) \<longrightarrow> Inf S \<ge> b"
using assms by (metis cInf_greatest)
qed (metis cInf_lower assms(1) bounded_imp_bdd_below)
lemma Inf_insert:
fixes S :: "real set"
shows "bounded S \<Longrightarrow> Inf (insert x S) = (if S = {} then x else min x (Inf S))"
by (auto simp: bounded_imp_bdd_below inf_min cInf_insert_If)
lemma open_real:
fixes s :: "real set"
shows "open s \<longleftrightarrow> (\<forall>x \<in> s. \<exists>e>0. \<forall>x'. \<bar>x' - x\<bar> < e --> x' \<in> s)"
unfolding open_dist dist_norm by simp
lemma islimpt_approachable_real:
fixes s :: "real set"
shows "x islimpt s \<longleftrightarrow> (\<forall>e>0. \<exists>x'\<in> s. x' \<noteq> x \<and> \<bar>x' - x\<bar> < e)"
unfolding islimpt_approachable dist_norm by simp
lemma closed_real:
fixes s :: "real set"
shows "closed s \<longleftrightarrow> (\<forall>x. (\<forall>e>0. \<exists>x' \<in> s. x' \<noteq> x \<and> \<bar>x' - x\<bar> < e) \<longrightarrow> x \<in> s)"
unfolding closed_limpt islimpt_approachable dist_norm by simp
lemma continuous_at_real_range:
fixes f :: "'a::real_normed_vector \<Rightarrow> real"
shows "continuous (at x) f \<longleftrightarrow> (\<forall>e>0. \<exists>d>0. \<forall>x'. norm(x' - x) < d --> \<bar>f x' - f x\<bar> < e)"
unfolding continuous_at
unfolding Lim_at
unfolding dist_norm
apply auto
apply (erule_tac x=e in allE, auto)
apply (rule_tac x=d in exI, auto)
apply (erule_tac x=x' in allE, auto)
apply (erule_tac x=e in allE, auto)
done
lemma continuous_on_real_range:
fixes f :: "'a::real_normed_vector \<Rightarrow> real"
shows "continuous_on s f \<longleftrightarrow>
(\<forall>x \<in> s. \<forall>e>0. \<exists>d>0. (\<forall>x' \<in> s. norm(x' - x) < d \<longrightarrow> \<bar>f x' - f x\<bar> < e))"
unfolding continuous_on_iff dist_norm by simp
lemma continuous_on_closed_Collect_le:
fixes f g :: "'a::topological_space \<Rightarrow> real"
assumes f: "continuous_on s f" and g: "continuous_on s g" and s: "closed s"
shows "closed {x \<in> s. f x \<le> g x}"
proof -
have "closed ((\<lambda>x. g x - f x) -` {0..} \<inter> s)"
using closed_real_atLeast continuous_on_diff [OF g f]
by (simp add: continuous_on_closed_vimage [OF s])
also have "((\<lambda>x. g x - f x) -` {0..} \<inter> s) = {x\<in>s. f x \<le> g x}"
by auto
finally show ?thesis .
qed
lemma continuous_le_on_closure:
fixes a::real
assumes f: "continuous_on (closure s) f"
and x: "x \<in> closure(s)"
and xlo: "\<And>x. x \<in> s ==> f(x) \<le> a"
shows "f(x) \<le> a"
using image_closure_subset [OF f, where T=" {x. x \<le> a}" ] assms
continuous_on_closed_Collect_le[of "UNIV" "\<lambda>x. x" "\<lambda>x. a"]
by auto
lemma continuous_ge_on_closure:
fixes a::real
assumes f: "continuous_on (closure s) f"
and x: "x \<in> closure(s)"
and xlo: "\<And>x. x \<in> s ==> f(x) \<ge> a"
shows "f(x) \<ge> a"
using image_closure_subset [OF f, where T=" {x. a \<le> x}"] assms
continuous_on_closed_Collect_le[of "UNIV" "\<lambda>x. a" "\<lambda>x. x"]
by auto
subsection\<open>The infimum of the distance between two sets\<close>
definition%important setdist :: "'a::metric_space set \<Rightarrow> 'a set \<Rightarrow> real" where
"setdist s t \<equiv>
(if s = {} \<or> t = {} then 0
else Inf {dist x y| x y. x \<in> s \<and> y \<in> t})"
lemma setdist_empty1 [simp]: "setdist {} t = 0"
by (simp add: setdist_def)
lemma setdist_empty2 [simp]: "setdist t {} = 0"
by (simp add: setdist_def)
lemma setdist_pos_le [simp]: "0 \<le> setdist s t"
by (auto simp: setdist_def ex_in_conv [symmetric] intro: cInf_greatest)
lemma le_setdistI:
assumes "s \<noteq> {}" "t \<noteq> {}" "\<And>x y. \<lbrakk>x \<in> s; y \<in> t\<rbrakk> \<Longrightarrow> d \<le> dist x y"
shows "d \<le> setdist s t"
using assms
by (auto simp: setdist_def Set.ex_in_conv [symmetric] intro: cInf_greatest)
lemma setdist_le_dist: "\<lbrakk>x \<in> s; y \<in> t\<rbrakk> \<Longrightarrow> setdist s t \<le> dist x y"
unfolding setdist_def
by (auto intro!: bdd_belowI [where m=0] cInf_lower)
lemma le_setdist_iff:
"d \<le> setdist s t \<longleftrightarrow>
(\<forall>x \<in> s. \<forall>y \<in> t. d \<le> dist x y) \<and> (s = {} \<or> t = {} \<longrightarrow> d \<le> 0)"
apply (cases "s = {} \<or> t = {}")
apply (force simp add: setdist_def)
apply (intro iffI conjI)
using setdist_le_dist apply fastforce
apply (auto simp: intro: le_setdistI)
done
lemma setdist_ltE:
assumes "setdist s t < b" "s \<noteq> {}" "t \<noteq> {}"
obtains x y where "x \<in> s" "y \<in> t" "dist x y < b"
using assms
by (auto simp: not_le [symmetric] le_setdist_iff)
lemma setdist_refl: "setdist s s = 0"
apply (cases "s = {}")
apply (force simp add: setdist_def)
apply (rule antisym [OF _ setdist_pos_le])
apply (metis all_not_in_conv dist_self setdist_le_dist)
done
lemma setdist_sym: "setdist s t = setdist t s"
by (force simp: setdist_def dist_commute intro!: arg_cong [where f=Inf])
lemma setdist_triangle: "setdist s t \<le> setdist s {a} + setdist {a} t"
proof (cases "s = {} \<or> t = {}")
case True then show ?thesis
using setdist_pos_le by fastforce
next
case False
have "\<And>x. x \<in> s \<Longrightarrow> setdist s t - dist x a \<le> setdist {a} t"
apply (rule le_setdistI, blast)
using False apply (fastforce intro: le_setdistI)
apply (simp add: algebra_simps)
apply (metis dist_commute dist_triangle3 order_trans [OF setdist_le_dist])
done
then have "setdist s t - setdist {a} t \<le> setdist s {a}"
using False by (fastforce intro: le_setdistI)
then show ?thesis
by (simp add: algebra_simps)
qed
lemma setdist_singletons [simp]: "setdist {x} {y} = dist x y"
by (simp add: setdist_def)
lemma setdist_Lipschitz: "\<bar>setdist {x} s - setdist {y} s\<bar> \<le> dist x y"
apply (subst setdist_singletons [symmetric])
by (metis abs_diff_le_iff diff_le_eq setdist_triangle setdist_sym)
lemma continuous_at_setdist [continuous_intros]: "continuous (at x) (\<lambda>y. (setdist {y} s))"
by (force simp: continuous_at_eps_delta dist_real_def intro: le_less_trans [OF setdist_Lipschitz])
lemma continuous_on_setdist [continuous_intros]: "continuous_on t (\<lambda>y. (setdist {y} s))"
by (metis continuous_at_setdist continuous_at_imp_continuous_on)
lemma uniformly_continuous_on_setdist: "uniformly_continuous_on t (\<lambda>y. (setdist {y} s))"
by (force simp: uniformly_continuous_on_def dist_real_def intro: le_less_trans [OF setdist_Lipschitz])
lemma setdist_subset_right: "\<lbrakk>t \<noteq> {}; t \<subseteq> u\<rbrakk> \<Longrightarrow> setdist s u \<le> setdist s t"
apply (cases "s = {} \<or> u = {}", force)
apply (auto simp: setdist_def intro!: bdd_belowI [where m=0] cInf_superset_mono)
done
lemma setdist_subset_left: "\<lbrakk>s \<noteq> {}; s \<subseteq> t\<rbrakk> \<Longrightarrow> setdist t u \<le> setdist s u"
by (metis setdist_subset_right setdist_sym)
lemma setdist_closure_1 [simp]: "setdist (closure s) t = setdist s t"
proof (cases "s = {} \<or> t = {}")
case True then show ?thesis by force
next
case False
{ fix y
assume "y \<in> t"
have "continuous_on (closure s) (\<lambda>a. dist a y)"
by (auto simp: continuous_intros dist_norm)
then have *: "\<And>x. x \<in> closure s \<Longrightarrow> setdist s t \<le> dist x y"
apply (rule continuous_ge_on_closure)
apply assumption
apply (blast intro: setdist_le_dist \<open>y \<in> t\<close> )
done
} note * = this
show ?thesis
apply (rule antisym)
using False closure_subset apply (blast intro: setdist_subset_left)
using False *
apply (force simp add: closure_eq_empty intro!: le_setdistI)
done
qed
lemma setdist_closure_2 [simp]: "setdist t (closure s) = setdist t s"
by (metis setdist_closure_1 setdist_sym)
lemma setdist_eq_0I: "\<lbrakk>x \<in> S; x \<in> T\<rbrakk> \<Longrightarrow> setdist S T = 0"
by (metis antisym dist_self setdist_le_dist setdist_pos_le)
lemma setdist_unique:
"\<lbrakk>a \<in> S; b \<in> T; \<And>x y. x \<in> S \<and> y \<in> T ==> dist a b \<le> dist x y\<rbrakk>
\<Longrightarrow> setdist S T = dist a b"
by (force simp add: setdist_le_dist le_setdist_iff intro: antisym)
lemma setdist_le_sing: "x \<in> S ==> setdist S T \<le> setdist {x} T"
using setdist_subset_left by auto
end