src/HOL/Analysis/Connected.thy

tuned;

(*  Author:     L C Paulson, University of Cambridge
    Material split off from Topology_Euclidean_Space
*)

section \<open>Connected Components\<close>

theory Connected
  imports
    Abstract_Topology_2
begin

subsection\<^marker>\<open>tag unimportant\<close> \<open>Connectedness\<close>

lemma connected_local:
 "connected S \<longleftrightarrow>
  \<not> (\<exists>e1 e2.
      openin (top_of_set S) e1 \<and>
      openin (top_of_set S) e2 \<and>
      S \<subseteq> e1 \<union> e2 \<and>
      e1 \<inter> e2 = {} \<and>
      e1 \<noteq> {} \<and>
      e2 \<noteq> {})"
  using connected_openin by blast

lemma exists_diff:
  fixes P :: "'a set \<Rightarrow> bool"
  shows "(\<exists>S. P (- S)) \<longleftrightarrow> (\<exists>S. P S)"
  by (metis boolean_algebra_class.boolean_algebra.double_compl)

lemma connected_clopen: "connected S \<longleftrightarrow>
  (\<forall>T. openin (top_of_set S) T \<and>
     closedin (top_of_set S) T \<longrightarrow> T = {} \<or> T = S)" (is "?lhs \<longleftrightarrow> ?rhs")
proof -
  have "\<not> connected S \<longleftrightarrow>
    (\<exists>e1 e2. open e1 \<and> open (- e2) \<and> S \<subseteq> e1 \<union> (- e2) \<and> e1 \<inter> (- e2) \<inter> S = {} \<and> e1 \<inter> S \<noteq> {} \<and> (- e2) \<inter> S \<noteq> {})"
    unfolding connected_def openin_open closedin_closed
    by (metis double_complement)
  then have th0: "connected S \<longleftrightarrow>
    \<not> (\<exists>e2 e1. closed e2 \<and> open e1 \<and> S \<subseteq> e1 \<union> (- e2) \<and> e1 \<inter> (- e2) \<inter> S = {} \<and> e1 \<inter> S \<noteq> {} \<and> (- e2) \<inter> S \<noteq> {})"
    (is " _ \<longleftrightarrow> \<not> (\<exists>e2 e1. ?P e2 e1)")
    by (simp add: closed_def) metis
  have th1: "?rhs \<longleftrightarrow> \<not> (\<exists>t' t. closed t'\<and>t = S\<inter>t' \<and> t\<noteq>{} \<and> t\<noteq>S \<and> (\<exists>t'. open t' \<and> t = S \<inter> t'))"
    (is "_ \<longleftrightarrow> \<not> (\<exists>t' t. ?Q t' t)")
    unfolding connected_def openin_open closedin_closed by auto
  have "(\<exists>e1. ?P e2 e1) \<longleftrightarrow> (\<exists>t. ?Q e2 t)" for e2
  proof -
    have "?P e2 e1 \<longleftrightarrow> (\<exists>t. closed e2 \<and> t = S\<inter>e2 \<and> open e1 \<and> t = S\<inter>e1 \<and> t\<noteq>{} \<and> t \<noteq> S)" for e1
      by auto
    then show ?thesis
      by metis
  qed
  then have "\<forall>e2. (\<exists>e1. ?P e2 e1) \<longleftrightarrow> (\<exists>t. ?Q e2 t)"
    by blast
  then show ?thesis
    by (simp add: th0 th1)
qed

subsection \<open>Connected components, considered as a connectedness relation or a set\<close>

definition\<^marker>\<open>tag important\<close> "connected_component S x y \<equiv> \<exists>T. connected T \<and> T \<subseteq> S \<and> x \<in> T \<and> y \<in> T"

abbreviation "connected_component_set S x \<equiv> Collect (connected_component S x)"

lemma connected_componentI:
  "connected T \<Longrightarrow> T \<subseteq> S \<Longrightarrow> x \<in> T \<Longrightarrow> y \<in> T \<Longrightarrow> connected_component S x y"
  by (auto simp: connected_component_def)

lemma connected_component_in: "connected_component S x y \<Longrightarrow> x \<in> S \<and> y \<in> S"
  by (auto simp: connected_component_def)

lemma connected_component_refl: "x \<in> S \<Longrightarrow> connected_component S x x"
  using connected_component_def connected_sing by blast
 
lemma connected_component_refl_eq [simp]: "connected_component S x x \<longleftrightarrow> x \<in> S"
  using connected_component_in connected_component_refl by blast

lemma connected_component_sym: "connected_component S x y \<Longrightarrow> connected_component S y x"
  by (auto simp: connected_component_def)

lemma connected_component_trans:
  "connected_component S x y \<Longrightarrow> connected_component S y z \<Longrightarrow> connected_component S x z"
  unfolding connected_component_def
  by (metis Int_iff Un_iff Un_subset_iff equals0D connected_Un)

lemma connected_component_of_subset:
  "connected_component S x y \<Longrightarrow> S \<subseteq> T \<Longrightarrow> connected_component T x y"
  by (auto simp: connected_component_def)

lemma connected_component_Union: "connected_component_set S x = \<Union>{T. connected T \<and> x \<in> T \<and> T \<subseteq> S}"
  by (auto simp: connected_component_def)

lemma connected_connected_component [iff]: "connected (connected_component_set S x)"
  by (auto simp: connected_component_Union intro: connected_Union)

lemma connected_iff_eq_connected_component_set:
  "connected S \<longleftrightarrow> (\<forall>x \<in> S. connected_component_set S x = S)"
  by (metis connected_component_def connected_component_in connected_connected_component mem_Collect_eq subsetI subset_antisym)

lemma connected_component_subset: "connected_component_set S x \<subseteq> S"
  using connected_component_in by blast

lemma connected_component_eq_self: "connected S \<Longrightarrow> x \<in> S \<Longrightarrow> connected_component_set S x = S"
  by (simp add: connected_iff_eq_connected_component_set)

lemma connected_iff_connected_component:
  "connected S \<longleftrightarrow> (\<forall>x \<in> S. \<forall>y \<in> S. connected_component S x y)"
  using connected_component_in by (auto simp: connected_iff_eq_connected_component_set)

lemma connected_component_maximal:
  "x \<in> T \<Longrightarrow> connected T \<Longrightarrow> T \<subseteq> S \<Longrightarrow> T \<subseteq> (connected_component_set S x)"
  using connected_component_eq_self connected_component_of_subset by blast

lemma connected_component_mono:
  "S \<subseteq> T \<Longrightarrow> connected_component_set S x \<subseteq> connected_component_set T x"
  by (simp add: Collect_mono connected_component_of_subset)

lemma connected_component_eq_empty [simp]: "connected_component_set S x = {} \<longleftrightarrow> x \<notin> S"
  using connected_component_refl by (fastforce simp: connected_component_in)

lemma connected_component_set_empty [simp]: "connected_component_set {} x = {}"
  using connected_component_eq_empty by blast

lemma connected_component_eq:
  "y \<in> connected_component_set S x \<Longrightarrow> (connected_component_set S y = connected_component_set S x)"
  by (metis (no_types, lifting)
      Collect_cong connected_component_sym connected_component_trans mem_Collect_eq)

lemma closed_connected_component:
  assumes S: "closed S"
  shows "closed (connected_component_set S x)"
proof (cases "x \<in> S")
  case False
  then show ?thesis
    by (metis connected_component_eq_empty closed_empty)
next
  case True
  show ?thesis
    unfolding closure_eq [symmetric]
  proof
    show "closure (connected_component_set S x) \<subseteq> connected_component_set S x"
    proof (rule connected_component_maximal)
      show "x \<in> closure (connected_component_set S x)"
        by (simp add: closure_def True)
      show "connected (closure (connected_component_set S x))"
        by (simp add: connected_imp_connected_closure)
      show "closure (connected_component_set S x) \<subseteq> S"
        by (simp add: S closure_minimal connected_component_subset)
    qed  
  qed (simp add: closure_subset)
qed

lemma connected_component_disjoint:
  "connected_component_set S a \<inter> connected_component_set S b = {} \<longleftrightarrow>
    a \<notin> connected_component_set S b"
  by (smt (verit) connected_component_eq connected_component_eq_empty connected_component_refl_eq
      disjoint_iff_not_equal mem_Collect_eq)

lemma connected_component_nonoverlap:
  "connected_component_set S a \<inter> connected_component_set S b = {} \<longleftrightarrow>
    a \<notin> S \<or> b \<notin> S \<or> connected_component_set S a \<noteq> connected_component_set S b"
  by (metis connected_component_disjoint connected_component_eq connected_component_eq_empty inf.idem)

lemma connected_component_overlap:
  "connected_component_set S a \<inter> connected_component_set S b \<noteq> {} \<longleftrightarrow>
    a \<in> S \<and> b \<in> S \<and> connected_component_set S a = connected_component_set S b"
  by (auto simp: connected_component_nonoverlap)

lemma connected_component_sym_eq: "connected_component S x y \<longleftrightarrow> connected_component S y x"
  using connected_component_sym by blast

lemma connected_component_eq_eq:
  "connected_component_set S x = connected_component_set S y \<longleftrightarrow>
    x \<notin> S \<and> y \<notin> S \<or> x \<in> S \<and> y \<in> S \<and> connected_component S x y"
  by (metis connected_component_eq connected_component_eq_empty connected_component_refl mem_Collect_eq)


lemma connected_iff_connected_component_eq:
  "connected S \<longleftrightarrow> (\<forall>x \<in> S. \<forall>y \<in> S. connected_component_set S x = connected_component_set S y)"
  by (simp add: connected_component_eq_eq connected_iff_connected_component)

lemma connected_component_idemp:
  "connected_component_set (connected_component_set S x) x = connected_component_set S x"
  by (metis Int_absorb connected_component_disjoint connected_component_eq_empty connected_component_eq_self connected_connected_component)

lemma connected_component_unique:
  "\<lbrakk>x \<in> c; c \<subseteq> S; connected c;
    \<And>c'. \<lbrakk>x \<in> c'; c' \<subseteq> S; connected c'\<rbrakk> \<Longrightarrow> c' \<subseteq> c\<rbrakk>
        \<Longrightarrow> connected_component_set S x = c"
  by (meson connected_component_maximal connected_component_subset connected_connected_component subsetD subset_antisym)

lemma joinable_connected_component_eq:
  "\<lbrakk>connected T; T \<subseteq> S;
    connected_component_set S x \<inter> T \<noteq> {};
    connected_component_set S y \<inter> T \<noteq> {}\<rbrakk>
    \<Longrightarrow> connected_component_set S x = connected_component_set S y"
  by (metis (full_types) subsetD connected_component_eq connected_component_maximal disjoint_iff_not_equal)

lemma Union_connected_component: "\<Union>(connected_component_set S ` S) = S"
proof
  show "\<Union>(connected_component_set S ` S) \<subseteq> S"
    by (simp add: SUP_least connected_component_subset)
qed (use connected_component_refl_eq in force)

lemma complement_connected_component_unions:
    "S - connected_component_set S x =
     \<Union>(connected_component_set S ` S - {connected_component_set S x})"
    (is "?lhs = ?rhs")
proof
  show "?lhs \<subseteq> ?rhs"
    by (metis Diff_subset Diff_subset_conv Sup_insert Union_connected_component insert_Diff_single)
  show "?rhs \<subseteq> ?lhs"
    by clarsimp (metis connected_component_eq_eq connected_component_in)
qed

lemma connected_component_intermediate_subset:
        "\<lbrakk>connected_component_set U a \<subseteq> T; T \<subseteq> U\<rbrakk>
        \<Longrightarrow> connected_component_set T a = connected_component_set U a"
  by (metis connected_component_idemp connected_component_mono subset_antisym)

lemma connected_component_homeomorphismI:
  assumes "homeomorphism A B f g" "connected_component A x y"
  shows   "connected_component B (f x) (f y)"
proof -
  from assms obtain T where T: "connected T" "T \<subseteq> A" "x \<in> T" "y \<in> T"
    unfolding connected_component_def by blast
  have "connected (f ` T)" "f ` T \<subseteq> B" "f x \<in> f ` T" "f y \<in> f ` T"
    using assms T continuous_on_subset[of A f T]
    by (auto intro!: connected_continuous_image simp: homeomorphism_def)
  thus ?thesis
    unfolding connected_component_def by blast
qed

lemma connected_component_homeomorphism_iff:
  assumes "homeomorphism A B f g"
  shows   "connected_component A x y \<longleftrightarrow> x \<in> A \<and> y \<in> A \<and> connected_component B (f x) (f y)"
  by (metis assms connected_component_homeomorphismI connected_component_in homeomorphism_apply1 homeomorphism_sym)

lemma connected_component_set_homeomorphism:
  assumes "homeomorphism A B f g" "x \<in> A"
  shows   "connected_component_set B (f x) = f ` connected_component_set A x" (is "?lhs = ?rhs")
proof -
  have "y \<in> ?lhs \<longleftrightarrow> y \<in> ?rhs" for y
    by (smt (verit, best) assms connected_component_homeomorphism_iff homeomorphism_def image_iff mem_Collect_eq)
  thus ?thesis
    by blast
qed

subsection \<open>The set of connected components of a set\<close>

definition\<^marker>\<open>tag important\<close> components:: "'a::topological_space set \<Rightarrow> 'a set set"
  where "components S \<equiv> connected_component_set S ` S"

lemma components_iff: "S \<in> components U \<longleftrightarrow> (\<exists>x. x \<in> U \<and> S = connected_component_set U x)"
  by (auto simp: components_def)

lemma componentsI: "x \<in> U \<Longrightarrow> connected_component_set U x \<in> components U"
  by (auto simp: components_def)

lemma componentsE:
  assumes "S \<in> components U"
  obtains x where "x \<in> U" "S = connected_component_set U x"
  using assms by (auto simp: components_def)

lemma Union_components [simp]: "\<Union>(components U) = U"
  by (simp add: Union_connected_component components_def)

lemma pairwise_disjoint_components: "pairwise (\<lambda>X Y. X \<inter> Y = {}) (components U)"
  unfolding pairwise_def
  by (metis (full_types) components_iff connected_component_nonoverlap)

lemma in_components_nonempty: "C \<in> components S \<Longrightarrow> C \<noteq> {}"
    by (metis components_iff connected_component_eq_empty)

lemma in_components_subset: "C \<in> components S \<Longrightarrow> C \<subseteq> S"
  using Union_components by blast

lemma in_components_connected: "C \<in> components S \<Longrightarrow> connected C"
  by (metis components_iff connected_connected_component)

lemma in_components_maximal:
  "C \<in> components S \<longleftrightarrow>
    C \<noteq> {} \<and> C \<subseteq> S \<and> connected C \<and> (\<forall>d. d \<noteq> {} \<and> C \<subseteq> d \<and> d \<subseteq> S \<and> connected d \<longrightarrow> d = C)"
(is "?lhs \<longleftrightarrow> ?rhs")
proof
  assume L: ?lhs
  then have "C \<subseteq> S" "connected C"
    by (simp_all add: in_components_subset in_components_connected)
  then show ?rhs
    by (metis (full_types) L components_iff connected_component_maximal connected_component_refl empty_iff mem_Collect_eq subsetD subset_antisym)
next
  show "?rhs \<Longrightarrow> ?lhs"
    by (metis bot.extremum_uniqueI components_iff connected_component_eq_empty connected_component_maximal connected_component_subset connected_connected_component subset_emptyI)
qed


lemma joinable_components_eq:
  "connected T \<and> T \<subseteq> S \<and> c1 \<in> components S \<and> c2 \<in> components S \<and> c1 \<inter> T \<noteq> {} \<and> c2 \<inter> T \<noteq> {} \<Longrightarrow> c1 = c2"
  by (metis (full_types) components_iff joinable_connected_component_eq)

lemma closed_components: "\<lbrakk>closed S; C \<in> components S\<rbrakk> \<Longrightarrow> closed C"
  by (metis closed_connected_component components_iff)

lemma components_nonoverlap:
    "\<lbrakk>C \<in> components S; C' \<in> components S\<rbrakk> \<Longrightarrow> (C \<inter> C' = {}) \<longleftrightarrow> (C \<noteq> C')"
  by (metis (full_types) components_iff connected_component_nonoverlap)

lemma components_eq: "\<lbrakk>C \<in> components S; C' \<in> components S\<rbrakk> \<Longrightarrow> (C = C' \<longleftrightarrow> C \<inter> C' \<noteq> {})"
  by (metis components_nonoverlap)

lemma components_eq_empty [simp]: "components S = {} \<longleftrightarrow> S = {}"
  by (simp add: components_def)

lemma components_empty [simp]: "components {} = {}"
  by simp

lemma connected_eq_connected_components_eq: "connected S \<longleftrightarrow> (\<forall>C \<in> components S. \<forall>C' \<in> components S. C = C')"
  by (metis (no_types, opaque_lifting) components_iff connected_component_eq_eq connected_iff_connected_component)

lemma components_eq_sing_iff: "components S = {S} \<longleftrightarrow> connected S \<and> S \<noteq> {}" (is "?lhs \<longleftrightarrow> ?rhs")
proof
  show "?rhs \<Longrightarrow> ?lhs"
    by (metis components_iff connected_component_eq_self equals0I insert_iff mk_disjoint_insert)
qed (use in_components_connected in fastforce)

lemma components_eq_sing_exists: "(\<exists>a. components S = {a}) \<longleftrightarrow> connected S \<and> S \<noteq> {}"
  by (metis Union_components ccpo_Sup_singleton components_eq_sing_iff)

lemma connected_eq_components_subset_sing: "connected S \<longleftrightarrow> components S \<subseteq> {S}"
  by (metis components_eq_empty components_eq_sing_iff connected_empty subset_singleton_iff)

lemma connected_eq_components_subset_sing_exists: "connected S \<longleftrightarrow> (\<exists>a. components S \<subseteq> {a})"
  by (metis components_eq_sing_exists connected_eq_components_subset_sing subset_singleton_iff)

lemma in_components_self: "S \<in> components S \<longleftrightarrow> connected S \<and> S \<noteq> {}"
  by (metis components_empty components_eq_sing_iff empty_iff in_components_connected insertI1)

lemma components_maximal: "\<lbrakk>C \<in> components S; connected T; T \<subseteq> S; C \<inter> T \<noteq> {}\<rbrakk> \<Longrightarrow> T \<subseteq> C"
  by (smt (verit, best) Int_Un_eq(4) Un_upper1 bot_eq_sup_iff connected_Un in_components_maximal inf.orderE sup.mono sup.orderI)

lemma exists_component_superset: "\<lbrakk>T \<subseteq> S; S \<noteq> {}; connected T\<rbrakk> \<Longrightarrow> \<exists>C. C \<in> components S \<and> T \<subseteq> C"
  by (meson componentsI connected_component_maximal equals0I subset_eq)

lemma components_intermediate_subset: "\<lbrakk>S \<in> components U; S \<subseteq> T; T \<subseteq> U\<rbrakk> \<Longrightarrow> S \<in> components T"
  by (smt (verit, best) dual_order.trans in_components_maximal)

lemma in_components_unions_complement: "C \<in> components S \<Longrightarrow> S - C = \<Union>(components S - {C})"
  by (metis complement_connected_component_unions components_def components_iff)

lemma connected_intermediate_closure:
  assumes cs: "connected S" and st: "S \<subseteq> T" and ts: "T \<subseteq> closure S"
  shows "connected T"
  using assms unfolding connected_def
  by (smt (verit) Int_assoc inf.absorb_iff2 inf_bot_left open_Int_closure_eq_empty)

lemma closedin_connected_component: "closedin (top_of_set S) (connected_component_set S x)"
proof (cases "connected_component_set S x = {}")
  case True
  then show ?thesis
    by (metis closedin_empty)
next
  case False
  then obtain y where y: "connected_component S x y" and "x \<in> S"
    using connected_component_eq_empty by blast
  have *: "connected_component_set S x \<subseteq> S \<inter> closure (connected_component_set S x)"
    by (auto simp: closure_def connected_component_in)
  have **: "x \<in> closure (connected_component_set S x)"
    by (simp add: \<open>x \<in> S\<close> closure_def)
  have "S \<inter> closure (connected_component_set S x) \<subseteq> connected_component_set S x" if "connected_component S x y"
  proof (rule connected_component_maximal)
    show "connected (S \<inter> closure (connected_component_set S x))"
      using "*" connected_intermediate_closure by blast
  qed (use \<open>x \<in> S\<close> ** in auto)
  with y * show ?thesis
    by (auto simp: closedin_closed)
qed

lemma closedin_component:
   "C \<in> components S \<Longrightarrow> closedin (top_of_set S) C"
  using closedin_connected_component componentsE by blast


subsection\<^marker>\<open>tag unimportant\<close> \<open>Proving a function is constant on a connected set
  by proving that a level set is open\<close>

lemma continuous_levelset_openin_cases:
  fixes f :: "_ \<Rightarrow> 'b::t1_space"
  shows "connected S \<Longrightarrow> continuous_on S f \<Longrightarrow>
        openin (top_of_set S) {x \<in> S. f x = a}
        \<Longrightarrow> (\<forall>x \<in> S. f x \<noteq> a) \<or> (\<forall>x \<in> S. f x = a)"
  unfolding connected_clopen
  using continuous_closedin_preimage_constant by auto

lemma continuous_levelset_openin:
  fixes f :: "_ \<Rightarrow> 'b::t1_space"
  shows "connected S \<Longrightarrow> continuous_on S f \<Longrightarrow>
        openin (top_of_set S) {x \<in> S. f x = a} \<Longrightarrow>
        (\<exists>x \<in> S. f x = a)  \<Longrightarrow> (\<forall>x \<in> S. f x = a)"
  using continuous_levelset_openin_cases[of S f ]
  by meson

lemma continuous_levelset_open:
  fixes f :: "_ \<Rightarrow> 'b::t1_space"
  assumes S: "connected S" "continuous_on S f"
    and a: "open {x \<in> S. f x = a}" "\<exists>x \<in> S.  f x = a"
  shows "\<forall>x \<in> S. f x = a"
  using a continuous_levelset_openin[OF S, of a, unfolded openin_open]
  by fast


subsection\<^marker>\<open>tag unimportant\<close> \<open>Preservation of Connectedness\<close>

lemma homeomorphic_connectedness:
  assumes "S homeomorphic T"
  shows "connected S \<longleftrightarrow> connected T"
using assms unfolding homeomorphic_def homeomorphism_def by (metis connected_continuous_image)

lemma connected_monotone_quotient_preimage:
  assumes "connected T"
      and contf: "continuous_on S f" and fim: "f ` S = T"
      and opT: "\<And>U. U \<subseteq> T
                 \<Longrightarrow> openin (top_of_set S) (S \<inter> f -` U) \<longleftrightarrow>
                     openin (top_of_set T) U"
      and connT: "\<And>y. y \<in> T \<Longrightarrow> connected (S \<inter> f -` {y})"
    shows "connected S"
proof (rule connectedI)
  fix U V
  assume "open U" and "open V" and "U \<inter> S \<noteq> {}" and "V \<inter> S \<noteq> {}"
    and "U \<inter> V \<inter> S = {}" and "S \<subseteq> U \<union> V"
  moreover
  have disjoint: "f ` (S \<inter> U) \<inter> f ` (S \<inter> V) = {}"
  proof -
    have False if "y \<in> f ` (S \<inter> U) \<inter> f ` (S \<inter> V)" for y
    proof -
      have "y \<in> T"
        using fim that by blast
      show ?thesis
        using connectedD [OF connT [OF \<open>y \<in> T\<close>] \<open>open U\<close> \<open>open V\<close>]
              \<open>S \<subseteq> U \<union> V\<close> \<open>U \<inter> V \<inter> S = {}\<close> that by fastforce
    qed
    then show ?thesis by blast
  qed
  ultimately have UU: "(S \<inter> f -` f ` (S \<inter> U)) = S \<inter> U" and VV: "(S \<inter> f -` f ` (S \<inter> V)) = S \<inter> V"
    by auto
  have opeU: "openin (top_of_set T) (f ` (S \<inter> U))"
    by (metis UU \<open>open U\<close> fim image_Int_subset le_inf_iff opT openin_open_Int)
  have opeV: "openin (top_of_set T) (f ` (S \<inter> V))"
    by (metis opT fim VV \<open>open V\<close> openin_open_Int image_Int_subset inf.bounded_iff)
  have "T \<subseteq> f ` (S \<inter> U) \<union> f ` (S \<inter> V)"
    using \<open>S \<subseteq> U \<union> V\<close> fim by auto
  then show False
    using \<open>connected T\<close> disjoint opeU opeV \<open>U \<inter> S \<noteq> {}\<close> \<open>V \<inter> S \<noteq> {}\<close>
    by (auto simp: connected_openin)
qed

lemma connected_open_monotone_preimage:
  assumes contf: "continuous_on S f" and fim: "f ` S = T"
    and ST: "\<And>C. openin (top_of_set S) C \<Longrightarrow> openin (top_of_set T) (f ` C)"
    and connT: "\<And>y. y \<in> T \<Longrightarrow> connected (S \<inter> f -` {y})"
    and "connected C" "C \<subseteq> T"
  shows "connected (S \<inter> f -` C)"
proof -
  have contf': "continuous_on (S \<inter> f -` C) f"
    by (meson contf continuous_on_subset inf_le1)
  have eqC: "f ` (S \<inter> f -` C) = C"
    using \<open>C \<subseteq> T\<close> fim by blast
  show ?thesis
  proof (rule connected_monotone_quotient_preimage [OF \<open>connected C\<close> contf' eqC])
    show "connected (S \<inter> f -` C \<inter> f -` {y})" if "y \<in> C" for y
      by (metis Int_assoc Int_empty_right Int_insert_right_if1 assms(6) connT in_mono that vimage_Int)
    have "\<And>U. openin (top_of_set (S \<inter> f -` C)) U
               \<Longrightarrow> openin (top_of_set C) (f ` U)"
      using open_map_restrict [OF _ ST \<open>C \<subseteq> T\<close>] by metis
    then show "\<And>D. D \<subseteq> C
          \<Longrightarrow> openin (top_of_set (S \<inter> f -` C)) (S \<inter> f -` C \<inter> f -` D) =
              openin (top_of_set C) D"
      using open_map_imp_quotient_map [of "(S \<inter> f -` C)" f] contf' by (simp add: eqC)
  qed
qed


lemma connected_closed_monotone_preimage:
  assumes contf: "continuous_on S f" and fim: "f ` S = T"
    and ST: "\<And>C. closedin (top_of_set S) C \<Longrightarrow> closedin (top_of_set T) (f ` C)"
    and connT: "\<And>y. y \<in> T \<Longrightarrow> connected (S \<inter> f -` {y})"
    and "connected C" "C \<subseteq> T"
  shows "connected (S \<inter> f -` C)"
proof -
  have contf': "continuous_on (S \<inter> f -` C) f"
    by (meson contf continuous_on_subset inf_le1)
  have eqC: "f ` (S \<inter> f -` C) = C"
    using \<open>C \<subseteq> T\<close> fim by blast
  show ?thesis
  proof (rule connected_monotone_quotient_preimage [OF \<open>connected C\<close> contf' eqC])
    show "connected (S \<inter> f -` C \<inter> f -` {y})" if "y \<in> C" for y
      by (metis Int_assoc Int_empty_right Int_insert_right_if1 \<open>C \<subseteq> T\<close> connT subsetD that vimage_Int)
    have "\<And>U. closedin (top_of_set (S \<inter> f -` C)) U
               \<Longrightarrow> closedin (top_of_set C) (f ` U)"
      using closed_map_restrict [OF _ ST \<open>C \<subseteq> T\<close>] by metis
    then show "\<And>D. D \<subseteq> C
          \<Longrightarrow> openin (top_of_set (S \<inter> f -` C)) (S \<inter> f -` C \<inter> f -` D) =
              openin (top_of_set C) D"
      using closed_map_imp_quotient_map [of "(S \<inter> f -` C)" f] contf' by (simp add: eqC)
  qed
qed


subsection\<open>Lemmas about components\<close>

text  \<open>See Newman IV, 3.3 and 3.4.\<close>

lemma connected_Un_clopen_in_complement:
  fixes S U :: "'a::metric_space set"
  assumes "connected S" "connected U" "S \<subseteq> U" 
      and opeT: "openin (top_of_set (U - S)) T" 
      and cloT: "closedin (top_of_set (U - S)) T"
    shows "connected (S \<union> T)"
proof -
  have *: "\<lbrakk>\<And>x y. P x y \<longleftrightarrow> P y x; \<And>x y. P x y \<Longrightarrow> S \<subseteq> x \<or> S \<subseteq> y;
            \<And>x y. \<lbrakk>P x y; S \<subseteq> x\<rbrakk> \<Longrightarrow> False\<rbrakk> \<Longrightarrow> \<not>(\<exists>x y. (P x y))" for P
    by metis
  show ?thesis
    unfolding connected_closedin_eq
  proof (rule *)
    fix H1 H2
    assume H: "closedin (top_of_set (S \<union> T)) H1 \<and> 
               closedin (top_of_set (S \<union> T)) H2 \<and>
               H1 \<union> H2 = S \<union> T \<and> H1 \<inter> H2 = {} \<and> H1 \<noteq> {} \<and> H2 \<noteq> {}"
    then have clo: "closedin (top_of_set S) (S \<inter> H1)"
                   "closedin (top_of_set S) (S \<inter> H2)"
      by (metis Un_upper1 closedin_closed_subset inf_commute)+
    moreover have "S \<inter> ((S \<union> T) \<inter> H1) \<union> S \<inter> ((S \<union> T) \<inter> H2) = S"
      using H by blast
    moreover have "H1 \<inter> (S \<inter> ((S \<union> T) \<inter> H2)) = {}"
      using H by blast
    ultimately have "S \<inter> H1 = {} \<or> S \<inter> H2 = {}"
      by (smt (verit) Int_assoc \<open>connected S\<close> connected_closedin_eq inf_commute inf_sup_absorb)
    then show "S \<subseteq> H1 \<or> S \<subseteq> H2"
      using H \<open>connected S\<close> unfolding connected_closedin by blast
  next
    fix H1 H2
    assume H: "closedin (top_of_set (S \<union> T)) H1 \<and>
               closedin (top_of_set (S \<union> T)) H2 \<and>
               H1 \<union> H2 = S \<union> T \<and> H1 \<inter> H2 = {} \<and> H1 \<noteq> {} \<and> H2 \<noteq> {}" 
       and "S \<subseteq> H1"
    then have H2T: "H2 \<subseteq> T"
      by auto
    have "T \<subseteq> U"
      using Diff_iff opeT openin_imp_subset by auto
    with \<open>S \<subseteq> U\<close> have Ueq: "U = (U - S) \<union> (S \<union> T)" 
      by auto
    have "openin (top_of_set ((U - S) \<union> (S \<union> T))) H2"
    proof (rule openin_subtopology_Un)
      show "openin (top_of_set (S \<union> T)) H2"
        by (metis Diff_cancel H Un_Diff Un_Diff_Int closedin_subset openin_closedin_eq topspace_euclidean_subtopology)
      then show "openin (top_of_set (U - S)) H2"
        by (meson H2T Un_upper2 opeT openin_subset_trans openin_trans)
    qed
    moreover have "closedin (top_of_set ((U - S) \<union> (S \<union> T))) H2"
    proof (rule closedin_subtopology_Un)
      show "closedin (top_of_set (U - S)) H2"
        using H H2T cloT closedin_subset_trans 
        by (blast intro: closedin_subtopology_Un closedin_trans)
    qed (simp add: H)
    ultimately have H2: "H2 = {} \<or> H2 = U"
      using Ueq \<open>connected U\<close> unfolding connected_clopen by metis   
    then have "H2 \<subseteq> S"
      by (metis Diff_partition H Un_Diff_cancel Un_subset_iff \<open>H2 \<subseteq> T\<close> assms(3) inf.orderE opeT openin_imp_subset)
    moreover have "T \<subseteq> H2 - S"
      by (metis (no_types) H2 H opeT openin_closedin_eq topspace_euclidean_subtopology)
    ultimately show False
      using H \<open>S \<subseteq> H1\<close> by blast
  qed blast
qed


proposition component_diff_connected:
  fixes S :: "'a::metric_space set"
  assumes "connected S" "connected U" "S \<subseteq> U" and C: "C \<in> components (U - S)"
  shows "connected(U - C)"
  using \<open>connected S\<close> unfolding connected_closedin_eq not_ex de_Morgan_conj
proof clarify
  fix H3 H4 
  assume clo3: "closedin (top_of_set (U - C)) H3" 
    and clo4: "closedin (top_of_set (U - C)) H4" 
    and H34: "H3 \<union> H4 = U - C" "H3 \<inter> H4 = {}" and "H3 \<noteq> {}" and "H4 \<noteq> {}"
    and * [rule_format]:
    "\<forall>H1 H2. \<not> closedin (top_of_set S) H1 \<or>
                      \<not> closedin (top_of_set S) H2 \<or>
                      H1 \<union> H2 \<noteq> S \<or> H1 \<inter> H2 \<noteq> {} \<or> \<not> H1 \<noteq> {} \<or> \<not> H2 \<noteq> {}"
  then have "H3 \<subseteq> U-C" and ope3: "openin (top_of_set (U - C)) (U - C - H3)"
    and "H4 \<subseteq> U-C" and ope4: "openin (top_of_set (U - C)) (U - C - H4)"
    by (auto simp: closedin_def)
  have "C \<noteq> {}" "C \<subseteq> U-S" "connected C"
    using C in_components_nonempty in_components_subset in_components_maximal by blast+
  have cCH3: "connected (C \<union> H3)"
  proof (rule connected_Un_clopen_in_complement [OF \<open>connected C\<close> \<open>connected U\<close> _ _ clo3])
    show "openin (top_of_set (U - C)) H3"
      by (metis Diff_cancel Un_Diff Un_Diff_Int \<open>H3 \<inter> H4 = {}\<close> \<open>H3 \<union> H4 = U - C\<close> ope4)
  qed (use clo3 \<open>C \<subseteq> U - S\<close> in auto)
  have cCH4: "connected (C \<union> H4)"
  proof (rule connected_Un_clopen_in_complement [OF \<open>connected C\<close> \<open>connected U\<close> _ _ clo4])
    show "openin (top_of_set (U - C)) H4"
      by (metis Diff_cancel Diff_triv Int_Un_eq(2) Un_Diff H34 inf_commute ope3)
  qed (use clo4 \<open>C \<subseteq> U - S\<close> in auto)
  have "closedin (top_of_set S) (S \<inter> H3)" "closedin (top_of_set S) (S \<inter> H4)"
    using clo3 clo4 \<open>S \<subseteq> U\<close> \<open>C \<subseteq> U - S\<close> by (auto simp: closedin_closed)
  moreover have "S \<inter> H3 \<noteq> {}"      
    using components_maximal [OF C cCH3] \<open>C \<noteq> {}\<close> \<open>C \<subseteq> U - S\<close> \<open>H3 \<noteq> {}\<close> \<open>H3 \<subseteq> U - C\<close> by auto
  moreover have "S \<inter> H4 \<noteq> {}"
    using components_maximal [OF C cCH4] \<open>C \<noteq> {}\<close> \<open>C \<subseteq> U - S\<close> \<open>H4 \<noteq> {}\<close> \<open>H4 \<subseteq> U - C\<close> by auto
  ultimately show False
    using * [of "S \<inter> H3" "S \<inter> H4"] \<open>H3 \<inter> H4 = {}\<close> \<open>C \<subseteq> U - S\<close> \<open>H3 \<union> H4 = U - C\<close> \<open>S \<subseteq> U\<close> 
    by auto
qed


subsection\<^marker>\<open>tag unimportant\<close>\<open>Constancy of a function from a connected set into a finite, disconnected or discrete set\<close>

text\<open>Still missing: versions for a set that is smaller than R, or countable.\<close>

lemma continuous_disconnected_range_constant:
  assumes S: "connected S"
      and conf: "continuous_on S f"
      and fim: "f \<in> S \<rightarrow> T"
      and cct: "\<And>y. y \<in> T \<Longrightarrow> connected_component_set T y = {y}"
    shows "f constant_on S"
proof (cases "S = {}")
  case True then show ?thesis
    by (simp add: constant_on_def)
next
  case False
  then have "f ` S \<subseteq> {f x}" if "x \<in> S" for x
    by (metis PiE S cct connected_component_maximal connected_continuous_image [OF conf] fim image_eqI 
        image_subset_iff that)
  with False show ?thesis
    unfolding constant_on_def by blast
qed


text\<open>This proof requires the existence of two separate values of the range type.\<close>
lemma finite_range_constant_imp_connected:
  assumes "\<And>f::'a::topological_space \<Rightarrow> 'b::real_normed_algebra_1.
              \<lbrakk>continuous_on S f; finite(f ` S)\<rbrakk> \<Longrightarrow> f constant_on S"
    shows "connected S"
proof -
  { fix T U
    assume clt: "closedin (top_of_set S) T"
       and clu: "closedin (top_of_set S) U"
       and tue: "T \<inter> U = {}" and tus: "T \<union> U = S"
    have "continuous_on (T \<union> U) (\<lambda>x. if x \<in> T then 0 else 1)"
      using clt clu tue by (intro continuous_on_cases_local) (auto simp: tus)
    then have conif: "continuous_on S (\<lambda>x. if x \<in> T then 0 else 1)"
      using tus by blast
    have fi: "finite ((\<lambda>x. if x \<in> T then 0 else 1) ` S)"
      by (rule finite_subset [of _ "{0,1}"]) auto
    have "T = {} \<or> U = {}"
      using assms [OF conif fi] tus [symmetric]
      by (auto simp: Ball_def constant_on_def) (metis IntI empty_iff one_neq_zero tue)
  }
  then show ?thesis
    by (simp add: connected_closedin_eq)
qed

end