Quelle Path_Connected.thy Sprache: Isabelle

(*  Title:      HOL/Analysis/Path_Connected.thy
    Authors:    LC Paulson and Robert Himmelmann (TU Muenchen), based on material from HOL Light
*)

section \<open>Path-Connectedness\<close>

theory Path_Connected
imports
  Starlike
  T1_Spaces
begin

subsection \<open>Paths and Arcs\<close>

definition\<^marker>\<open>tag important\<close> path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
  where "path g \ continuous_on {0..1} g"

definition\<^marker>\<open>tag important\<close> pathstart :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"
  where "pathstart g \ g 0"

definition\<^marker>\<open>tag important\<close> pathfinish :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"
  where "pathfinish g \ g 1"

definition\<^marker>\<open>tag important\<close> path_image :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a set"
  where "path_image g \ g ` {0 .. 1}"

definition\<^marker>\<open>tag important\<close> reversepath :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> real \<Rightarrow> 'a"
  where "reversepath g \ (\x. g(1 - x))"

definition\<^marker>\<open>tag important\<close> joinpaths :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> (real \<Rightarrow> 'a) \<Rightarrow> real \<Rightarrow> 'a"
    (infixr \<open>+++\<close> 75)
  where "g1 +++ g2 \ (\x. if x \ 1/2 then g1 (2 * x) else g2 (2 * x - 1))"

definition\<^marker>\<open>tag important\<close> loop_free :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
  where "loop_free g \ \x\{0..1}. \y\{0..1}. g x = g y \ x = y \ x = 0 \ y = 1 \ x = 1 \ y = 0"

definition\<^marker>\<open>tag important\<close> simple_path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
  where "simple_path g \ path g \ loop_free g"

definition\<^marker>\<open>tag important\<close> arc :: "(real \<Rightarrow> 'a :: topological_space) \<Rightarrow> bool"
  where "arc g \ path g \ inj_on g {0..1}"

subsection\<^marker>\<open>tag unimportant\<close>\<open>Invariance theorems\<close>

lemma path_eq: "path p \ (\t. t \ {0..1} \ p t = q t) \ path q"
  using continuous_on_eq path_def by blast

lemma path_continuous_image: "path g \ continuous_on (path_image g) f \ path(f \ g)"
  unfolding path_def path_image_def
  using continuous_on_compose by blast

lemma path_translation_eq:
  fixes g :: "real \ 'a :: real_normed_vector"
  shows "path((\x. a + x) \ g) = path g"
  using continuous_on_translation_eq path_def by blast

lemma path_linear_image_eq:
  fixes f :: "'a::euclidean_space \ 'b::euclidean_space"
   assumes "linear f" "inj f"
     shows "path(f \ g) = path g"
proof -
  from linear_injective_left_inverse [OF assms]
  obtain h where h: "linear h" "h \ f = id"
    by blast
  with assms show ?thesis
    by (metis comp_assoc id_comp linear_continuous_on linear_linear path_continuous_image)
qed

lemma pathstart_translation: "pathstart((\x. a + x) \ g) = a + pathstart g"
  by (simp add: pathstart_def)

lemma pathstart_linear_image_eq: "linear f \ pathstart(f \ g) = f(pathstart g)"
  by (simp add: pathstart_def)

lemma pathfinish_translation: "pathfinish((\x. a + x) \ g) = a + pathfinish g"
  by (simp add: pathfinish_def)

lemma pathfinish_linear_image: "linear f \ pathfinish(f \ g) = f(pathfinish g)"
  by (simp add: pathfinish_def)

lemma path_image_translation: "path_image((\x. a + x) \ g) = (\x. a + x) ` (path_image g)"
  by (simp add: image_comp path_image_def)

lemma path_image_linear_image: "linear f \ path_image(f \ g) = f ` (path_image g)"
  by (simp add: image_comp path_image_def)

lemma reversepath_translation: "reversepath((\x. a + x) \ g) = (\x. a + x) \ reversepath g"
  by (rule ext) (simp add: reversepath_def)

lemma reversepath_linear_image: "linear f \ reversepath(f \ g) = f \ reversepath g"
  by (rule ext) (simp add: reversepath_def)

lemma joinpaths_translation:
    "((\x. a + x) \ g1) +++ ((\x. a + x) \ g2) = (\x. a + x) \ (g1 +++ g2)"
  by (rule ext) (simp add: joinpaths_def)

lemma joinpaths_linear_image: "linear f \ (f \ g1) +++ (f \ g2) = f \ (g1 +++ g2)"
  by (rule ext) (simp add: joinpaths_def)

lemma loop_free_translation_eq:
  fixes g :: "real \ 'a::euclidean_space"
  shows "loop_free((\x. a + x) \ g) = loop_free g"
  by (simp add: loop_free_def)

lemma simple_path_translation_eq:
  fixes g :: "real \ 'a::euclidean_space"
  shows "simple_path((\x. a + x) \ g) = simple_path g"
  by (simp add: simple_path_def loop_free_translation_eq path_translation_eq)

lemma loop_free_linear_image_eq:
  fixes f :: "'a::euclidean_space \ 'b::euclidean_space"
  assumes "linear f" "inj f"
    shows "loop_free(f \ g) = loop_free g"
  using assms inj_on_eq_iff [of f] by (auto simp: loop_free_def)

lemma simple_path_linear_image_eq:
  fixes f :: "'a::euclidean_space \ 'b::euclidean_space"
  assumes "linear f" "inj f"
    shows "simple_path(f \ g) = simple_path g"
  using assms
  by (simp add: loop_free_linear_image_eq path_linear_image_eq simple_path_def)

lemma simple_pathI [intro?]:
  assumes "path p"
  assumes "\x y. 0 \ x \ x < y \ y \ 1 \ p x = p y \ x = 0 \ y = 1"
  shows   "simple_path p"
  unfolding simple_path_def loop_free_def
proof (intro ballI conjI impI)
  fix x y assume "x \ {0..1}" "y \ {0..1}" "p x = p y"
  thus "x = y \ x = 0 \ y = 1 \ x = 1 \ y = 0"
    by (metis assms(2) atLeastAtMost_iff linorder_less_linear)
qed fact+

lemma arcD: "arc p \ p x = p y \ x \ {0..1} \ y \ {0..1} \ x = y"
  by (auto simp: arc_def inj_on_def)

lemma arc_translation_eq:
  fixes g :: "real \ 'a::euclidean_space"
  shows "arc((\x. a + x) \ g) \ arc g"
  by (auto simp: arc_def inj_on_def path_translation_eq)

lemma arc_linear_image_eq:
  fixes f :: "'a::euclidean_space \ 'b::euclidean_space"
   assumes "linear f" "inj f"
     shows  "arc(f \ g) = arc g"
  using assms inj_on_eq_iff [of f]
  by (auto simp: arc_def inj_on_def path_linear_image_eq)

subsection\<^marker>\<open>tag unimportant\<close>\<open>Basic lemmas about paths\<close>

lemma path_of_real: "path complex_of_real"
  unfolding path_def by (intro continuous_intros)

lemma path_const: "path (\t. a)" for a::"'a::real_normed_vector"
  unfolding path_def by (intro continuous_intros)

lemma path_minus: "path g \ path (\t. - g t)" for g::"real\'a::real_normed_vector"
  unfolding path_def by (intro continuous_intros)

lemma path_add: "\path f; path g\ \ path (\t. f t + g t)" for f::"real\'a::real_normed_vector"
  unfolding path_def by (intro continuous_intros)

lemma path_diff: "\path f; path g\ \ path (\t. f t - g t)" for f::"real\'a::real_normed_vector"
  unfolding path_def by (intro continuous_intros)

lemma path_mult: "\path f; path g\ \ path (\t. f t * g t)" for f::"real\'a::real_normed_field"
  unfolding path_def by (intro continuous_intros)

lemma pathin_iff_path_real [simp]: "pathin euclideanreal g \ path g"
  by (simp add: pathin_def path_def)

lemma continuous_on_path: "path f \ t \ {0..1} \ continuous_on t f"
  using continuous_on_subset path_def by blast

lemma inj_on_imp_loop_free: "inj_on g {0..1} \ loop_free g"
  by (simp add: inj_onD loop_free_def)

lemma arc_imp_simple_path: "arc g \ simple_path g"
  by (simp add: arc_def inj_on_imp_loop_free simple_path_def)

lemma arc_imp_path: "arc g \ path g"
  using arc_def by blast

lemma arc_imp_inj_on: "arc g \ inj_on g {0..1}"
  by (auto simp: arc_def)

lemma simple_path_imp_path: "simple_path g \ path g"
  using simple_path_def by blast

lemma loop_free_cases: "loop_free g \ inj_on g {0..1} \ pathfinish g = pathstart g"
  by (force simp: inj_on_def loop_free_def pathfinish_def pathstart_def)

lemma simple_path_cases: "simple_path g \ arc g \ pathfinish g = pathstart g"
  using arc_def loop_free_cases simple_path_def by blast

lemma simple_path_imp_arc: "simple_path g \ pathfinish g \ pathstart g \ arc g"
  using simple_path_cases by auto

lemma arc_distinct_ends: "arc g \ pathfinish g \ pathstart g"
  unfolding arc_def inj_on_def pathfinish_def pathstart_def
  by fastforce

lemma arc_simple_path: "arc g \ simple_path g \ pathfinish g \ pathstart g"
  using arc_distinct_ends arc_imp_simple_path simple_path_cases by blast

lemma simple_path_eq_arc: "pathfinish g \ pathstart g \ (simple_path g = arc g)"
  by (simp add: arc_simple_path)

lemma path_image_const [simp]: "path_image (\t. a) = {a}"
  by (force simp: path_image_def)

lemma path_image_nonempty [simp]: "path_image g \ {}"
  unfolding path_image_def image_is_empty box_eq_empty
  by auto

lemma pathstart_in_path_image[intro]: "pathstart g \ path_image g"
  unfolding pathstart_def path_image_def
  by auto

lemma pathfinish_in_path_image[intro]: "pathfinish g \ path_image g"
  unfolding pathfinish_def path_image_def
  by auto

lemma connected_path_image[intro]: "path g \ connected (path_image g)"
  unfolding path_def path_image_def
  using connected_continuous_image connected_Icc by blast

lemma compact_path_image[intro]: "path g \ compact (path_image g)"
  unfolding path_def path_image_def
  using compact_continuous_image connected_Icc by blast

lemma reversepath_reversepath[simp]: "reversepath (reversepath g) = g"
  unfolding reversepath_def
  by auto

lemma pathstart_reversepath[simp]: "pathstart (reversepath g) = pathfinish g"
  unfolding pathstart_def reversepath_def pathfinish_def
  by auto

lemma pathfinish_reversepath[simp]: "pathfinish (reversepath g) = pathstart g"
  unfolding pathstart_def reversepath_def pathfinish_def
  by auto

lemma reversepath_o: "reversepath g = g \ (-)1"
  by (auto simp: reversepath_def)

lemma pathstart_join[simp]: "pathstart (g1 +++ g2) = pathstart g1"
  unfolding pathstart_def joinpaths_def pathfinish_def
  by auto

lemma pathfinish_join[simp]: "pathfinish (g1 +++ g2) = pathfinish g2"
  unfolding pathstart_def joinpaths_def pathfinish_def
  by auto

lemma path_image_reversepath[simp]: "path_image (reversepath g) = path_image g"
  by (metis cancel_comm_monoid_add_class.diff_cancel diff_zero image_comp
      image_diff_atLeastAtMost path_image_def reversepath_o)

lemma path_reversepath [simp]: "path (reversepath g) \ path g"
  by (metis continuous_on_compose continuous_on_op_minus image_comp image_ident path_def path_image_def path_image_reversepath reversepath_o reversepath_reversepath)

lemma arc_reversepath:
  assumes "arc g" shows "arc(reversepath g)"
proof -
  have injg: "inj_on g {0..1}"
    using assms
    by (simp add: arc_def)
  have **: "\x y::real. 1-x = 1-y \ x = y"
    by simp
  show ?thesis
    using assms  by (clarsimp simp: arc_def intro!: inj_onI) (simp add: inj_onD reversepath_def **)
qed

lemma loop_free_reversepath:
  assumes "loop_free g" shows "loop_free(reversepath g)"
  using assms by (simp add: reversepath_def loop_free_def Ball_def) (smt (verit))

lemma simple_path_reversepath: "simple_path g \ simple_path (reversepath g)"
  by (simp add: loop_free_reversepath simple_path_def)

lemmas reversepath_simps =
  path_reversepath path_image_reversepath pathstart_reversepath pathfinish_reversepath

lemma path_join[simp]:
  assumes "pathfinish g1 = pathstart g2"
  shows "path (g1 +++ g2) \ path g1 \ path g2"
  unfolding path_def pathfinish_def pathstart_def
proof safe
  assume cont: "continuous_on {0..1} (g1 +++ g2)"
  have g1: "continuous_on {0..1} g1 \ continuous_on {0..1} ((g1 +++ g2) \ (\x. x / 2))"
    by (intro continuous_on_cong refl) (auto simp: joinpaths_def)
  have g2: "continuous_on {0..1} g2 \ continuous_on {0..1} ((g1 +++ g2) \ (\x. x / 2 + 1/2))"
    using assms
    by (intro continuous_on_cong refl) (auto simp: joinpaths_def pathfinish_def pathstart_def)
  show "continuous_on {0..1} g1" and "continuous_on {0..1} g2"
    unfolding g1 g2
    by (auto intro!: continuous_intros continuous_on_subset[OF cont] simp del: o_apply)
next
  assume g1g2: "continuous_on {0..1} g1" "continuous_on {0..1} g2"
  have 01: "{0 .. 1} = {0..1/2} \ {1/2 .. 1::real}"
    by auto
  {
    fix x :: real
    assume "0 \ x" and "x \ 1"
    then have "x \ (\x. x * 2) ` {0..1 / 2}"
      by (intro image_eqI[where x="x/2"]) auto
  }
  note 1 = this
  {
    fix x :: real
    assume "0 \ x" and "x \ 1"
    then have "x \ (\x. x * 2 - 1) ` {1 / 2..1}"
      by (intro image_eqI[where x="x/2 + 1/2"]) auto
  }
  note 2 = this
  show "continuous_on {0..1} (g1 +++ g2)"
    using assms
    unfolding joinpaths_def 01
    apply (intro continuous_on_cases closed_atLeastAtMost g1g2[THEN continuous_on_compose2] continuous_intros)
    apply (auto simp: field_simps pathfinish_def pathstart_def intro!: 1 2)
    done
qed

subsection\<^marker>\<open>tag unimportant\<close> \<open>Path Images\<close>

lemma bounded_path_image: "path g \ bounded(path_image g)"
  by (simp add: compact_imp_bounded compact_path_image)

lemma closed_path_image:
  fixes g :: "real \ 'a::t2_space"
  shows "path g \ closed(path_image g)"
  by (metis compact_path_image compact_imp_closed)

lemma connected_simple_path_image: "simple_path g \ connected(path_image g)"
  by (metis connected_path_image simple_path_imp_path)

lemma compact_simple_path_image: "simple_path g \ compact(path_image g)"
  by (metis compact_path_image simple_path_imp_path)

lemma bounded_simple_path_image: "simple_path g \ bounded(path_image g)"
  by (metis bounded_path_image simple_path_imp_path)

lemma closed_simple_path_image:
  fixes g :: "real \ 'a::t2_space"
  shows "simple_path g \ closed(path_image g)"
  by (metis closed_path_image simple_path_imp_path)

lemma connected_arc_image: "arc g \ connected(path_image g)"
  by (metis connected_path_image arc_imp_path)

lemma compact_arc_image: "arc g \ compact(path_image g)"
  by (metis compact_path_image arc_imp_path)

lemma bounded_arc_image: "arc g \ bounded(path_image g)"
  by (metis bounded_path_image arc_imp_path)

lemma closed_arc_image:
  fixes g :: "real \ 'a::t2_space"
  shows "arc g \ closed(path_image g)"
  by (metis closed_path_image arc_imp_path)

lemma path_image_join_subset: "path_image (g1 +++ g2) \ path_image g1 \ path_image g2"
  unfolding path_image_def joinpaths_def
  by auto

lemma subset_path_image_join:
  assumes "path_image g1 \ S" and "path_image g2 \ S"
  shows "path_image (g1 +++ g2) \ S"
  using path_image_join_subset[of g1 g2] and assms
  by auto

lemma path_image_join:
  assumes "pathfinish g1 = pathstart g2"
  shows "path_image(g1 +++ g2) = path_image g1 \ path_image g2"
proof -
  have "path_image g1 \ path_image (g1 +++ g2)"
  proof (clarsimp simp: path_image_def joinpaths_def)
    fix u::real
    assume "0 \ u" "u \ 1"
    then show "g1 u \ (\x. g1 (2 * x)) ` ({0..1} \ {x. x * 2 \ 1})"
      by (rule_tac x="u/2" in image_eqI) auto
  qed
  moreover
  have \<section>: "g2 u \<in> (\<lambda>x. g2 (2 * x - 1)) ` ({0..1} \<inter> {x. \<not> x * 2 \<le> 1})"
    if "0 < u" "u \ 1" for u
    using that assms
    by (rule_tac x="(u+1)/2" in image_eqI) (auto simp: field_simps pathfinish_def pathstart_def)
  have "g2 0 \ (\x. g1 (2 * x)) ` ({0..1} \ {x. x * 2 \ 1})"
    using assms
    by (rule_tac x="1/2" in image_eqI) (auto simp: pathfinish_def pathstart_def)
  then have "path_image g2 \ path_image (g1 +++ g2)"
    by (auto simp: path_image_def joinpaths_def intro!: \<section>)
  ultimately show ?thesis
    using path_image_join_subset by blast
qed

lemma not_in_path_image_join:
  assumes "x \ path_image g1" and "x \ path_image g2"
  shows "x \ path_image (g1 +++ g2)"
  using assms and path_image_join_subset[of g1 g2]
  by auto

lemma pathstart_compose: "pathstart(f \ p) = f(pathstart p)"
  by (simp add: pathstart_def)

lemma pathfinish_compose: "pathfinish(f \ p) = f(pathfinish p)"
  by (simp add: pathfinish_def)

lemma path_image_compose: "path_image (f \ p) = f ` (path_image p)"
  by (simp add: image_comp path_image_def)

lemma path_compose_join: "f \ (p +++ q) = (f \ p) +++ (f \ q)"
  by (rule ext) (simp add: joinpaths_def)

lemma path_compose_reversepath: "f \ reversepath p = reversepath(f \ p)"
  by (rule ext) (simp add: reversepath_def)

lemma joinpaths_eq:
  "(\t. t \ {0..1} \ p t = p' t) \
   (\<And>t. t \<in> {0..1} \<Longrightarrow> q t = q' t)
   \<Longrightarrow>  t \<in> {0..1} \<Longrightarrow> (p +++ q) t = (p' +++ q') t"
  by (auto simp: joinpaths_def)

lemma loop_free_inj_on: "loop_free g \ inj_on g {0<..<1}"
  by (force simp: inj_on_def loop_free_def)

lemma simple_path_inj_on: "simple_path g \ inj_on g {0<..<1}"
  using loop_free_inj_on simple_path_def by auto

subsection\<^marker>\<open>tag unimportant\<close>\<open>Simple paths with the endpoints removed\<close>

lemma simple_path_endless:
  assumes "simple_path c"
  shows "path_image c - {pathstart c,pathfinish c} = c ` {0<..<1}" (is "?lhs = ?rhs")
proof
  show "?lhs \ ?rhs"
    using less_eq_real_def by (auto simp: path_image_def pathstart_def pathfinish_def)
  show "?rhs \ ?lhs"
    using assms
    apply (simp add: image_subset_iff path_image_def pathstart_def pathfinish_def simple_path_def loop_free_def Ball_def)
    by (smt (verit))
qed

lemma connected_simple_path_endless:
  assumes "simple_path c"
  shows "connected(path_image c - {pathstart c,pathfinish c})"
proof -
  have "continuous_on {0<..<1} c"
    using assms by (simp add: simple_path_def continuous_on_path path_def subset_iff)
  then have "connected (c ` {0<..<1})"
    using connected_Ioo connected_continuous_image by blast
  then show ?thesis
    using assms by (simp add: simple_path_endless)
qed

lemma nonempty_simple_path_endless:
    "simple_path c \ path_image c - {pathstart c,pathfinish c} \ {}"
  by (simp add: simple_path_endless)

lemma simple_path_continuous_image:
  assumes "simple_path f" "continuous_on (path_image f) g" "inj_on g (path_image f)"
  shows   "simple_path (g \ f)"
  unfolding simple_path_def
proof
  show "path (g \ f)"
    using assms unfolding simple_path_def by (intro path_continuous_image) auto
  from assms have [simp]: "g (f x) = g (f y) \ f x = f y" if "x \ {0..1}" "y \ {0..1}" for x y
    unfolding inj_on_def path_image_def using that by fastforce
  show "loop_free (g \ f)"
    using assms(1) by (auto simp: loop_free_def simple_path_def)
qed

subsection\<^marker>\<open>tag unimportant\<close>\<open>The operations on paths\<close>

lemma path_image_subset_reversepath: "path_image(reversepath g) \ path_image g"
  by simp

lemma path_imp_reversepath: "path g \ path(reversepath g)"
  by simp

lemma half_bounded_equal: "1 \ x * 2 \ x * 2 \ 1 \ x = (1/2::real)"
  by simp

lemma continuous_on_joinpaths:
  assumes "continuous_on {0..1} g1" "continuous_on {0..1} g2" "pathfinish g1 = pathstart g2"
    shows "continuous_on {0..1} (g1 +++ g2)"
  using assms path_def path_join by blast

lemma path_join_imp: "\path g1; path g2; pathfinish g1 = pathstart g2\ \ path(g1 +++ g2)"
  by simp

lemma arc_join:
  assumes "arc g1" "arc g2"
          "pathfinish g1 = pathstart g2"
          "path_image g1 \ path_image g2 \ {pathstart g2}"
    shows "arc(g1 +++ g2)"
proof -
  have injg1: "inj_on g1 {0..1}" and injg2: "inj_on g2 {0..1}"
     and g11: "g1 1 = g2 0" and sb: "g1 ` {0..1} \ g2 ` {0..1} \ {g2 0}"
    using assms
    by (auto simp: arc_def pathfinish_def pathstart_def path_image_def)
  { fix x and y::real
    assume xy: "g2 (2 * x - 1) = g1 (2 * y)" "x \ 1" "0 \ y" " y * 2 \ 1" "\ x * 2 \ 1"
    then have "g1 (2 * y) = g2 0"
      using sb by force
    then have False
      using xy inj_onD injg2 by fastforce
   } note * = this
  have "inj_on (g1 +++ g2) {0..1}"
    using inj_onD [OF injg1] inj_onD [OF injg2] *
    by (simp add: inj_on_def joinpaths_def Ball_def) (smt (verit))
  then show ?thesis
    using arc_def assms path_join_imp by blast
qed

lemma simple_path_join_loop:
  assumes "arc g1" "arc g2"
          "pathfinish g1 = pathstart g2"  "pathfinish g2 = pathstart g1"
          "path_image g1 \ path_image g2 \ {pathstart g1, pathstart g2}"
        shows "simple_path(g1 +++ g2)"
proof -
  have injg1: "inj_on g1 {0..1}" and injg2: "inj_on g2 {0..1}"
    using assms by (auto simp add: arc_def)
  have g12: "g1 1 = g2 0"
   and g21: "g2 1 = g1 0"
   and sb:  "g1 ` {0..1} \ g2 ` {0..1} \ {g1 0, g2 0}"
    using assms
    by (simp_all add: arc_def pathfinish_def pathstart_def path_image_def)
  { fix x and y::real
    assume g2_eq: "g2 (2 * x - 1) = g1 (2 * y)"
      and xyI: "x \ 1 \ y \ 0"
      and xy: "x \ 1" "0 \ y" " y * 2 \ 1" "\ x * 2 \ 1"
    then consider "g1 (2 * y) = g1 0" | "g1 (2 * y) = g2 0"
      using sb by force
    then have False
    proof cases
      case 1
      then have "y = 0"
        using xy g2_eq by (auto dest!: inj_onD [OF injg1])
      then show ?thesis
        using xy g2_eq xyI by (auto dest: inj_onD [OF injg2] simp flip: g21)
    next
      case 2
      then have "2*x = 1"
        using g2_eq g12 inj_onD [OF injg2] atLeastAtMost_iff xy(1) xy(4) by fastforce
      with xy show False by auto
    qed
  } note * = this
  have "loop_free(g1 +++ g2)"
    using inj_onD [OF injg1] inj_onD [OF injg2] *
    by (simp add: loop_free_def joinpaths_def Ball_def) (smt (verit))
  then show ?thesis
    by (simp add: arc_imp_path assms simple_path_def)
qed

lemma reversepath_joinpaths:
    "pathfinish g1 = pathstart g2 \ reversepath(g1 +++ g2) = reversepath g2 +++ reversepath g1"
  unfolding reversepath_def pathfinish_def pathstart_def joinpaths_def
  by (rule ext) (auto simp: mult.commute)

subsection\<^marker>\<open>tag unimportant\<close>\<open>Some reversed and "if and only if" versions of joining theorems\<close>

lemma path_join_path_ends:
  fixes g1 :: "real \ 'a::metric_space"
  assumes "path(g1 +++ g2)" "path g2"
    shows "pathfinish g1 = pathstart g2"
proof (rule ccontr)
  define e where "e = dist (g1 1) (g2 0)"
  assume Neg: "pathfinish g1 \ pathstart g2"
  then have "0 < dist (pathfinish g1) (pathstart g2)"
    by auto
  then have "e > 0"
    by (metis e_def pathfinish_def pathstart_def)
  then have "\e>0. \d>0. \x'\{0..1}. dist x' 0 < d \ dist (g2 x') (g2 0) < e"
    using \<open>path g2\<close> atLeastAtMost_iff zero_le_one unfolding path_def continuous_on_iff
    by blast
  then obtain d1 where "d1 > 0"
       and d1: "\x'. \x'\{0..1}; norm x' < d1\ \ dist (g2 x') (g2 0) < e/2"
    by (metis \<open>0 < e\<close> half_gt_zero_iff norm_conv_dist)
  obtain d2 where "d2 > 0"
       and d2: "\x'. \x'\{0..1}; dist x' (1/2) < d2\
                      \<Longrightarrow> dist ((g1 +++ g2) x') (g1 1) < e/2"
    using assms(1) \<open>e > 0\<close> unfolding path_def continuous_on_iff
    apply (drule_tac x="1/2" in bspec, simp)
    apply (drule_tac x="e/2" in spec, force simp: joinpaths_def)
    done
  have int01_1: "min (1/2) (min d1 d2) / 2 \ {0..1}"
    using \<open>d1 > 0\<close> \<open>d2 > 0\<close> by (simp add: min_def)
  have dist1: "norm (min (1 / 2) (min d1 d2) / 2) < d1"
    using \<open>d1 > 0\<close> \<open>d2 > 0\<close> by (simp add: min_def dist_norm)
  have int01_2: "1/2 + min (1/2) (min d1 d2) / 4 \ {0..1}"
    using \<open>d1 > 0\<close> \<open>d2 > 0\<close> by (simp add: min_def)
  have dist2: "dist (1 / 2 + min (1 / 2) (min d1 d2) / 4) (1 / 2) < d2"
    using \<open>d1 > 0\<close> \<open>d2 > 0\<close> by (simp add: min_def dist_norm)
  have [simp]: "\ min (1 / 2) (min d1 d2) \ 0"
    using \<open>d1 > 0\<close> \<open>d2 > 0\<close> by (simp add: min_def)
  have "dist (g2 (min (1 / 2) (min d1 d2) / 2)) (g1 1) < e/2"
       "dist (g2 (min (1 / 2) (min d1 d2) / 2)) (g2 0) < e/2"
    using d1 [OF int01_1 dist1] d2 [OF int01_2 dist2] by (simp_all add: joinpaths_def)
  then have "dist (g1 1) (g2 0) < e/2 + e/2"
    using dist_triangle_half_r e_def by blast
  then show False
    by (simp add: e_def [symmetric])
qed

lemma path_join_eq [simp]:
  fixes g1 :: "real \ 'a::metric_space"
  assumes "path g1" "path g2"
    shows "path(g1 +++ g2) \ pathfinish g1 = pathstart g2"
  using assms by (metis path_join_path_ends path_join_imp)

lemma simple_path_joinE:
  assumes "simple_path(g1 +++ g2)" and "pathfinish g1 = pathstart g2"
  obtains "arc g1" "arc g2"
          "path_image g1 \ path_image g2 \ {pathstart g1, pathstart g2}"
proof -
  have *: "\x y. \0 \ x; x \ 1; 0 \ y; y \ 1; (g1 +++ g2) x = (g1 +++ g2) y\
               \<Longrightarrow> x = y \<or> x = 0 \<and> y = 1 \<or> x = 1 \<and> y = 0"
    using assms by (simp add: simple_path_def loop_free_def)
  have "path g1"
    using assms path_join simple_path_imp_path by blast
  moreover have "inj_on g1 {0..1}"
  proof (clarsimp simp: inj_on_def)
    fix x y
    assume "g1 x = g1 y" "0 \ x" "x \ 1" "0 \ y" "y \ 1"
    then show "x = y"
      using * [of "x/2" "y/2"] by (simp add: joinpaths_def split_ifs)
  qed
  ultimately have "arc g1"
    using assms  by (simp add: arc_def)
  have [simp]: "g2 0 = g1 1"
    using assms by (metis pathfinish_def pathstart_def)
  have "path g2"
    using assms path_join simple_path_imp_path by blast
  moreover have "inj_on g2 {0..1}"
  proof (clarsimp simp: inj_on_def)
    fix x y
    assume "g2 x = g2 y" "0 \ x" "x \ 1" "0 \ y" "y \ 1"
    then show "x = y"
      using * [of "(x+1) / 2" "(y+1) / 2"]
      by (force simp: joinpaths_def split_ifs field_split_simps)
  qed
  ultimately have "arc g2"
    using assms  by (simp add: arc_def)
  have "g2 y = g1 0 \ g2 y = g1 1"
       if "g1 x = g2 y" "0 \ x" "x \ 1" "0 \ y" "y \ 1" for x y
      using * [of "x / 2" "(y + 1) / 2"] that
      by (auto simp: joinpaths_def split_ifs field_split_simps)
  then have "path_image g1 \ path_image g2 \ {pathstart g1, pathstart g2}"
    by (fastforce simp: pathstart_def pathfinish_def path_image_def)
  with \<open>arc g1\<close> \<open>arc g2\<close> show ?thesis using that by blast
qed

lemma simple_path_join_loop_eq:
  assumes "pathfinish g2 = pathstart g1" "pathfinish g1 = pathstart g2"
  shows "simple_path(g1 +++ g2) \
             arc g1 \<and> arc g2 \<and> path_image g1 \<inter> path_image g2 \<subseteq> {pathstart g1, pathstart g2}"
  by (metis assms simple_path_joinE simple_path_join_loop)

lemma arc_join_eq:
  assumes "pathfinish g1 = pathstart g2"
    shows "arc(g1 +++ g2) \
           arc g1 \<and> arc g2 \<and> path_image g1 \<inter> path_image g2 \<subseteq> {pathstart g2}"
           (is "?lhs = ?rhs")
proof
  assume ?lhs then show ?rhs
    using reversepath_simps assms
    by (smt (verit, best) Int_commute arc_reversepath arc_simple_path in_mono insertE pathstart_join
          reversepath_joinpaths simple_path_joinE subsetI)
next
  assume ?rhs then show ?lhs
    using assms
    by (fastforce simp: pathfinish_def pathstart_def intro!: arc_join)
qed

lemma arc_join_eq_alt:
  "pathfinish g1 = pathstart g2
   \<Longrightarrow> arc(g1 +++ g2) \<longleftrightarrow> arc g1 \<and> arc g2 \<and> path_image g1 \<inter> path_image g2 = {pathstart g2}"
  using pathfinish_in_path_image by (fastforce simp: arc_join_eq)

subsubsection\<^marker>\<open>tag unimportant\<close>\<open>Symmetry and loops\<close>

lemma path_sym:
  "\pathfinish p = pathstart q; pathfinish q = pathstart p\ \ path(p +++ q) \ path(q +++ p)"
  by auto

lemma simple_path_sym:
  "\pathfinish p = pathstart q; pathfinish q = pathstart p\
     \<Longrightarrow> simple_path(p +++ q) \<longleftrightarrow> simple_path(q +++ p)"
  by (metis (full_types) inf_commute insert_commute simple_path_joinE simple_path_join_loop)

lemma path_image_sym:
  "\pathfinish p = pathstart q; pathfinish q = pathstart p\
     \<Longrightarrow> path_image(p +++ q) = path_image(q +++ p)"
  by (simp add: path_image_join sup_commute)

lemma simple_path_joinI:
  assumes "arc p1" "arc p2" "pathfinish p1 = pathstart p2"
  assumes "path_image p1 \ path_image p2
        \<subseteq> insert (pathstart p2) (if pathstart p1 = pathfinish p2 then {pathstart p1} else {})"
  shows   "simple_path (p1 +++ p2)"
  by (smt (verit, best) Int_commute arc_imp_simple_path arc_join assms insert_commute simple_path_join_loop simple_path_sym)

lemma simple_path_join3I:
  assumes "arc p1" "arc p2" "arc p3"
  assumes "path_image p1 \ path_image p2 \ {pathstart p2}"
  assumes "path_image p2 \ path_image p3 \ {pathstart p3}"
  assumes "path_image p1 \ path_image p3 \ {pathstart p1} \ {pathfinish p3}"
  assumes "pathfinish p1 = pathstart p2" "pathfinish p2 = pathstart p3"
  shows   "simple_path (p1 +++ p2 +++ p3)"
  using assms by (intro simple_path_joinI arc_join) (auto simp: path_image_join)

subsection\<^marker>\<open>tag unimportant\<close>\<open>The joining of paths is associative\<close>

lemma path_assoc:
  "\pathfinish p = pathstart q; pathfinish q = pathstart r\
     \<Longrightarrow> path(p +++ (q +++ r)) \<longleftrightarrow> path((p +++ q) +++ r)"
  by simp

lemma simple_path_assoc:
  assumes "pathfinish p = pathstart q" "pathfinish q = pathstart r"
    shows "simple_path (p +++ (q +++ r)) \ simple_path ((p +++ q) +++ r)"
proof (cases "pathstart p = pathfinish r")
  case True show ?thesis
  proof
    assume "simple_path (p +++ q +++ r)"
    with assms True show "simple_path ((p +++ q) +++ r)"
      by (fastforce simp add: simple_path_join_loop_eq arc_join_eq path_image_join
                    dest: arc_distinct_ends [of r])
  next
    assume 0: "simple_path ((p +++ q) +++ r)"
    with assms True have q: "pathfinish r \ path_image q"
      using arc_distinct_ends
      by (fastforce simp add: simple_path_join_loop_eq arc_join_eq path_image_join)
    have "pathstart r \ path_image p"
      using assms
      by (metis 0 IntI arc_distinct_ends arc_join_eq_alt empty_iff insert_iff
              pathfinish_in_path_image pathfinish_join simple_path_joinE)
    with assms 0 q True show "simple_path (p +++ q +++ r)"
      by (auto simp: simple_path_join_loop_eq arc_join_eq path_image_join
               dest!: subsetD [OF _ IntI])
  qed
next
  case False
  { fix x :: 'a
    assume a: "path_image p \ path_image q \ {pathstart q}"
              "(path_image p \ path_image q) \ path_image r \ {pathstart r}"
              "x \ path_image p" "x \ path_image r"
    have "pathstart r \ path_image q"
      by (metis assms(2) pathfinish_in_path_image)
    with a have "x = pathstart q"
      by blast
  }
  with False assms show ?thesis
    by (auto simp: simple_path_eq_arc simple_path_join_loop_eq arc_join_eq path_image_join)
qed

lemma arc_assoc:
     "\pathfinish p = pathstart q; pathfinish q = pathstart r\
      \<Longrightarrow> arc(p +++ (q +++ r)) \<longleftrightarrow> arc((p +++ q) +++ r)"
by (simp add: arc_simple_path simple_path_assoc)

subsection\<open>Subpath\<close>

definition\<^marker>\<open>tag important\<close> subpath :: "real \<Rightarrow> real \<Rightarrow> (real \<Rightarrow> 'a) \<Rightarrow> real \<Rightarrow> 'a::real_normed_vector"
  where "subpath a b g \ \x. g((b - a) * x + a)"

lemma path_image_subpath_gen:
  fixes g :: "_ \ 'a::real_normed_vector"
  shows "path_image(subpath u v g) = g ` (closed_segment u v)"
  by (auto simp add: closed_segment_real_eq path_image_def subpath_def)

lemma path_image_subpath:
  fixes g :: "real \ 'a::real_normed_vector"
  shows "path_image(subpath u v g) = (if u \ v then g ` {u..v} else g ` {v..u})"
  by (simp add: path_image_subpath_gen closed_segment_eq_real_ivl)

lemma path_image_subpath_commute:
  fixes g :: "real \ 'a::real_normed_vector"
  shows "path_image(subpath u v g) = path_image(subpath v u g)"
  by (simp add: path_image_subpath_gen closed_segment_eq_real_ivl)

lemma path_subpath [simp]:
  fixes g :: "real \ 'a::real_normed_vector"
  assumes "path g" "u \ {0..1}" "v \ {0..1}"
    shows "path(subpath u v g)"
proof -
  have "continuous_on {u..v} g" "continuous_on {v..u} g"
    using assms continuous_on_path by fastforce+
  then have "continuous_on {0..1} (g \ (\x. ((v-u) * x+ u)))"
    by (intro continuous_intros; simp add: image_affinity_atLeastAtMost [where c=u])
  then show ?thesis
    by (simp add: path_def subpath_def)
qed

lemma pathstart_subpath [simp]: "pathstart(subpath u v g) = g(u)"
  by (simp add: pathstart_def subpath_def)

lemma pathfinish_subpath [simp]: "pathfinish(subpath u v g) = g(v)"
  by (simp add: pathfinish_def subpath_def)

lemma subpath_trivial [simp]: "subpath 0 1 g = g"
  by (simp add: subpath_def)

lemma subpath_reversepath: "subpath 1 0 g = reversepath g"
  by (simp add: reversepath_def subpath_def)

lemma reversepath_subpath: "reversepath(subpath u v g) = subpath v u g"
  by (simp add: reversepath_def subpath_def algebra_simps)

lemma subpath_translation: "subpath u v ((\x. a + x) \ g) = (\x. a + x) \ subpath u v g"
  by (rule ext) (simp add: subpath_def)

lemma subpath_image: "subpath u v (f \ g) = f \ subpath u v g"
  by (rule ext) (simp add: subpath_def)

lemma affine_ineq:
  fixes x :: "'a::linordered_idom"
  assumes "x \ 1" "v \ u"
    shows "v + x * u \ u + x * v"
proof -
  have "(1-x)*(u-v) \ 0"
    using assms by auto
  then show ?thesis
    by (simp add: algebra_simps)
qed

lemma sum_le_prod1:
  fixes a::real shows "\a \ 1; b \ 1\ \ a + b \ 1 + a * b"
  by (metis add.commute affine_ineq mult.right_neutral)

lemma simple_path_subpath_eq:
  "simple_path(subpath u v g) \
     path(subpath u v g) \<and> u\<noteq>v \<and>
     (\<forall>x y. x \<in> closed_segment u v \<and> y \<in> closed_segment u v \<and> g x = g y
                \<longrightarrow> x = y \<or> x = u \<and> y = v \<or> x = v \<and> y = u)"
    (is "?lhs = ?rhs")
proof
  assume ?lhs
  then have p: "path (\x. g ((v - u) * x + u))"
        and sim: "(\x y. \x\{0..1}; y\{0..1}; g ((v - u) * x + u) = g ((v - u) * y + u)\
                  \<Longrightarrow> x = y \<or> x = 0 \<and> y = 1 \<or> x = 1 \<and> y = 0)"
    by (auto simp: simple_path_def loop_free_def subpath_def)
  { fix x y
    assume "x \ closed_segment u v" "y \ closed_segment u v" "g x = g y"
    then have "x = y \ x = u \ y = v \ x = v \ y = u"
      using sim [of "(x-u)/(v-u)" "(y-u)/(v-u)"] p
      by (auto split: if_split_asm simp add: closed_segment_real_eq image_affinity_atLeastAtMost)
        (simp_all add: field_split_simps)
  } moreover
  have "path(subpath u v g) \ u\v"
    using sim [of "1/3" "2/3"] p
    by (auto simp: subpath_def)
  ultimately show ?rhs
    by metis
next
  assume ?rhs
  then
  have d1: "\x y. \g x = g y; u \ x; x \ v; u \ y; y \ v\ \ x = y \ x = u \ y = v \ x = v \ y = u"
   and d2: "\x y. \g x = g y; v \ x; x \ u; v \ y; y \ u\ \ x = y \ x = u \ y = v \ x = v \ y = u"
   and ne: "u < v \ v < u"
   and psp: "path (subpath u v g)"
    by (auto simp: closed_segment_real_eq image_affinity_atLeastAtMost)
  have [simp]: "\x. u + x * v = v + x * u \ u=v \ x=1"
    by algebra
  show ?lhs using psp ne
    unfolding simple_path_def loop_free_def subpath_def
    by (fastforce simp add: algebra_simps affine_ineq mult_left_mono crossproduct_eq dest: d1 d2)
qed

lemma arc_subpath_eq:
  "arc(subpath u v g) \ path(subpath u v g) \ u\v \ inj_on g (closed_segment u v)"
  by (smt (verit, best) arc_simple_path closed_segment_commute ends_in_segment(2) inj_on_def pathfinish_subpath pathstart_subpath simple_path_subpath_eq)

lemma simple_path_subpath:
  assumes "simple_path g" "u \ {0..1}" "v \ {0..1}" "u \ v"
  shows "simple_path(subpath u v g)"
  using assms
  unfolding simple_path_subpath_eq
  by (force simp:  simple_path_def loop_free_def closed_segment_real_eq image_affinity_atLeastAtMost)

lemma arc_simple_path_subpath:
    "\simple_path g; u \ {0..1}; v \ {0..1}; g u \ g v\ \ arc(subpath u v g)"
  by (force intro: simple_path_subpath simple_path_imp_arc)

lemma arc_subpath_arc:
    "\arc g; u \ {0..1}; v \ {0..1}; u \ v\ \ arc(subpath u v g)"
  by (meson arc_def arc_imp_simple_path arc_simple_path_subpath inj_onD)

lemma arc_simple_path_subpath_interior:
    "\simple_path g; u \ {0..1}; v \ {0..1}; u \ v; \u-v\ < 1\ \ arc(subpath u v g)"
  by (force simp: simple_path_def loop_free_def intro: arc_simple_path_subpath)

lemma path_image_subpath_subset:
    "\u \ {0..1}; v \ {0..1}\ \ path_image(subpath u v g) \ path_image g"
  by (metis atLeastAtMost_iff atLeastatMost_subset_iff path_image_def path_image_subpath subset_image_iff)

lemma join_subpaths_middle: "subpath (0) ((1 / 2)) p +++ subpath ((1 / 2)) 1 p = p"
  by (rule ext) (simp add: joinpaths_def subpath_def field_split_simps)

subsection\<^marker>\<open>tag unimportant\<close>\<open>There is a subpath to the frontier\<close>

lemma subpath_to_frontier_explicit:
    fixes S :: "'a::metric_space set"
    assumes g: "path g" and "pathfinish g \ S"
    obtains u where "0 \ u" "u \ 1"
                "\x. 0 \ x \ x < u \ g x \ interior S"
                "(g u \ interior S)" "(u = 0 \ g u \ closure S)"
proof -
  have gcon: "continuous_on {0..1} g"
    using g by (simp add: path_def)
  moreover have "bounded ({u. g u \ closure (- S)} \ {0..1})"
    using compact_eq_bounded_closed by fastforce
  ultimately have com: "compact ({0..1} \ {u. g u \ closure (- S)})"
    using closed_vimage_Int
    by (metis (full_types) Int_commute closed_atLeastAtMost closed_closure compact_eq_bounded_closed vimage_def)
  have "1 \ {u. g u \ closure (- S)}"
    using assms by (simp add: pathfinish_def closure_def)
  then have dis: "{0..1} \ {u. g u \ closure (- S)} \ {}"
    using atLeastAtMost_iff zero_le_one by blast
  then obtain u where "0 \ u" "u \ 1" and gu: "g u \ closure (- S)"
                  and umin: "\t. \0 \ t; t \ 1; g t \ closure (- S)\ \ u \ t"
    using compact_attains_inf [OF com dis] by fastforce
  then have umin': "\t. \0 \ t; t \ 1; t < u\ \ g t \ S"
    using closure_def by fastforce
  have \<section>: "g u \<in> closure S" if "u \<noteq> 0"
  proof -
    have "u > 0" using that \<open>0 \<le> u\<close> by auto
    { fix e::real assume "e > 0"
      obtain d where "d>0" and d: "\x'. \x' \ {0..1}; dist x' u \ d\ \ dist (g x') (g u) < e"
        using continuous_onE [OF gcon _ \<open>e > 0\<close>] \<open>0 \<le> _\<close> \<open>_ \<le> 1\<close> atLeastAtMost_iff by auto
      have *: "dist (max 0 (u - d / 2)) u \ d"
        using \<open>0 \<le> u\<close> \<open>u \<le> 1\<close> \<open>d > 0\<close> by (simp add: dist_real_def)
      have "\y\S. dist y (g u) < e"
        using \<open>0 < u\<close> \<open>u \<le> 1\<close> \<open>d > 0\<close>
        by (force intro: d [OF _ *] umin')
    }
    then show ?thesis
      by (simp add: frontier_def closure_approachable)
  qed
  show ?thesis
  proof
    show "\x. 0 \ x \ x < u \ g x \ interior S"
      using \<open>u \<le> 1\<close> interior_closure umin by fastforce
    show "g u \ interior S"
      by (simp add: gu interior_closure)
  qed (use \<open>0 \<le> u\<close> \<open>u \<le> 1\<close> \<section> in auto)
qed

lemma subpath_to_frontier_strong:
    assumes g: "path g" and "pathfinish g \ S"
    obtains u where "0 \ u" "u \ 1" "g u \ interior S"
                    "u = 0 \ (\x. 0 \ x \ x < 1 \ subpath 0 u g x \ interior S) \ g u \ closure S"
proof -
  obtain u where "0 \ u" "u \ 1"
             and gxin: "\x. 0 \ x \ x < u \ g x \ interior S"
             and gunot: "(g u \ interior S)" and u0: "(u = 0 \ g u \ closure S)"
    using subpath_to_frontier_explicit [OF assms] by blast
  show ?thesis
  proof
    show "g u \ interior S"
      using gunot by blast
  qed (use \<open>0 \<le> u\<close> \<open>u \<le> 1\<close> u0 in \<open>(force simp: subpath_def gxin)+\<close>)
qed

lemma subpath_to_frontier:
    assumes g: "path g" and g0: "pathstart g \ closure S" and g1: "pathfinish g \ S"
    obtains u where "0 \ u" "u \ 1" "g u \ frontier S" "path_image(subpath 0 u g) - {g u} \ interior S"
proof -
  obtain u where "0 \ u" "u \ 1"
             and notin: "g u \ interior S"
             and disj: "u = 0 \
                        (\<forall>x. 0 \<le> x \<and> x < 1 \<longrightarrow> subpath 0 u g x \<in> interior S) \<and> g u \<in> closure S"
                       (is "_ \ ?P")
    using subpath_to_frontier_strong [OF g g1] by blast
  show ?thesis
  proof
    show "g u \ frontier S"
      by (metis DiffI disj frontier_def g0 notin pathstart_def)
    show "path_image (subpath 0 u g) - {g u} \ interior S"
      using disj
    proof
      assume "u = 0"
      then show ?thesis
        by (simp add: path_image_subpath)
    next
      assume P: ?P
      show ?thesis
      proof (clarsimp simp add: path_image_subpath_gen)
        fix y
        assume y: "y \ closed_segment 0 u" "g y \ interior S"
        with \<open>0 \<le> u\<close> have "0 \<le> y" "y \<le> u"
          by (auto simp: closed_segment_eq_real_ivl split: if_split_asm)
        then have "y=u \ subpath 0 u g (y/u) \ interior S"
          using P less_eq_real_def by force
        then show "g y = g u"
          using y by (auto simp: subpath_def split: if_split_asm)
      qed
    qed
  qed (use \<open>0 \<le> u\<close> \<open>u \<le> 1\<close> in auto)
qed

lemma exists_path_subpath_to_frontier:
    fixes S :: "'a::real_normed_vector set"
    assumes "path g" "pathstart g \ closure S" "pathfinish g \ S"
    obtains h where "path h" "pathstart h = pathstart g" "path_image h \ path_image g"
                    "path_image h - {pathfinish h} \ interior S"
                    "pathfinish h \ frontier S"
proof -
  obtain u where u: "0 \ u" "u \ 1" "g u \ frontier S" "(path_image(subpath 0 u g) - {g u}) \ interior S"
    using subpath_to_frontier [OF assms] by blast
  show ?thesis
  proof
    show "path_image (subpath 0 u g) \ path_image g"
      by (simp add: path_image_subpath_subset u)
    show "pathstart (subpath 0 u g) = pathstart g"
      by (metis pathstart_def pathstart_subpath)
  qed (use assms u in \<open>auto simp: path_image_subpath\<close>)
qed

lemma exists_path_subpath_to_frontier_closed:
    fixes S :: "'a::real_normed_vector set"
    assumes S: "closed S" and g: "path g" and g0: "pathstart g \ S" and g1: "pathfinish g \ S"
    obtains h where "path h" "pathstart h = pathstart g" "path_image h \ path_image g \ S"
                    "pathfinish h \ frontier S"
  by (smt (verit, del_insts) Diff_iff Int_iff S closure_closed exists_path_subpath_to_frontier
      frontier_def g g0 g1 interior_subset singletonD subset_eq)

subsection \<open>Shift Path to Start at Some Given Point\<close>

definition\<^marker>\<open>tag important\<close> shiftpath :: "real \<Rightarrow> (real \<Rightarrow> 'a::topological_space) \<Rightarrow> real \<Rightarrow> 'a"
  where "shiftpath a f = (\x. if (a + x) \ 1 then f (a + x) else f (a + x - 1))"

lemma shiftpath_alt_def: "shiftpath a f = (\x. if x \ 1-a then f (a + x) else f (a + x - 1))"
  by (auto simp: shiftpath_def)

lemma pathstart_shiftpath: "a \ 1 \ pathstart (shiftpath a g) = g a"
  unfolding pathstart_def shiftpath_def by auto

lemma pathfinish_shiftpath:
  assumes "0 \ a"
    and "pathfinish g = pathstart g"
  shows "pathfinish (shiftpath a g) = g a"
  using assms
  unfolding pathstart_def pathfinish_def shiftpath_def
  by auto

lemma endpoints_shiftpath:
  assumes "pathfinish g = pathstart g"
    and "a \ {0 .. 1}"
  shows "pathfinish (shiftpath a g) = g a"
    and "pathstart (shiftpath a g) = g a"
  using assms
  by (simp_all add: pathstart_shiftpath pathfinish_shiftpath)

lemma closed_shiftpath:
  assumes "pathfinish g = pathstart g"
    and "a \ {0..1}"
  shows "pathfinish (shiftpath a g) = pathstart (shiftpath a g)"
  using endpoints_shiftpath[OF assms]
  by auto

lemma path_shiftpath:
  assumes "path g"
    and "pathfinish g = pathstart g"
    and "a \ {0..1}"
  shows "path (shiftpath a g)"
proof -
  have *: "{0 .. 1} = {0 .. 1-a} \ {1-a .. 1}"
    using assms(3) by auto
  have **: "\x. x + a = 1 \ g (x + a - 1) = g (x + a)"
    by (smt (verit, best) assms(2) pathfinish_def pathstart_def)
  show ?thesis
    unfolding path_def shiftpath_def *
  proof (rule continuous_on_closed_Un)
    have contg: "continuous_on {0..1} g"
      using \<open>path g\<close> path_def by blast
    show "continuous_on {0..1-a} (\x. if a + x \ 1 then g (a + x) else g (a + x - 1))"
    proof (rule continuous_on_eq)
      show "continuous_on {0..1-a} (g \ (+) a)"
        by (intro continuous_intros continuous_on_subset [OF contg]) (use \<open>a \<in> {0..1}\<close> in auto)
    qed auto
    show "continuous_on {1-a..1} (\x. if a + x \ 1 then g (a + x) else g (a + x - 1))"
    proof (rule continuous_on_eq)
      show "continuous_on {1-a..1} (g \ (+) (a - 1))"
        by (intro continuous_intros continuous_on_subset [OF contg]) (use \<open>a \<in> {0..1}\<close> in auto)
    qed (auto simp: "**" add.commute add_diff_eq)
  qed auto
qed

lemma shiftpath_shiftpath:
  assumes "pathfinish g = pathstart g"
    and "a \ {0..1}"
    and "x \ {0..1}"
  shows "shiftpath (1 - a) (shiftpath a g) x = g x"
  using assms
  unfolding pathfinish_def pathstart_def shiftpath_def
  by auto

lemma path_image_shiftpath:
  assumes a: "a \ {0..1}"
    and "pathfinish g = pathstart g"
  shows "path_image (shiftpath a g) = path_image g"
proof -
  { fix x
    assume g: "g 1 = g 0" "x \ {0..1::real}" and gne: "\y. y\{0..1} \ {x. \ a + x \ 1} \ g x \ g (a + y - 1)"
    then have "\y\{0..1} \ {x. a + x \ 1}. g x = g (a + y)"
    proof (cases "a \ x")
      case False
      then show ?thesis
        apply (rule_tac x="1 + x - a" in bexI)
        using g gne[of "1 + x - a"] a by (force simp: field_simps)+
    next
      case True
      then show ?thesis
        using g a  by (rule_tac x="x - a" in bexI) (auto simp: field_simps)
    qed
  }
  then show ?thesis
    using assms
    unfolding shiftpath_def path_image_def pathfinish_def pathstart_def
    by (auto simp: image_iff)
qed

lemma loop_free_shiftpath:
  assumes "loop_free g" "pathfinish g = pathstart g" and a: "0 \ a" "a \ 1"
    shows "loop_free (shiftpath a g)"
  unfolding loop_free_def
proof (intro conjI impI ballI)
  show "x = y \ x = 0 \ y = 1 \ x = 1 \ y = 0"
    if "x \ {0..1}" "y \ {0..1}" "shiftpath a g x = shiftpath a g y" for x y
    using that a assms unfolding shiftpath_def loop_free_def
    by (smt (verit, ccfv_threshold) atLeastAtMost_iff)
qed

lemma simple_path_shiftpath:
  assumes "simple_path g" "pathfinish g = pathstart g" and a: "0 \ a" "a \ 1"
  shows "simple_path (shiftpath a g)"
  using assms loop_free_shiftpath path_shiftpath simple_path_def by fastforce

subsection \<open>Straight-Line Paths\<close>

definition\<^marker>\<open>tag important\<close> linepath :: "'a::real_normed_vector \<Rightarrow> 'a \<Rightarrow> real \<Rightarrow> 'a"
  where "linepath a b = (\x. (1 - x) *\<^sub>R a + x *\<^sub>R b)"

lemma pathstart_linepath[simp]: "pathstart (linepath a b) = a"
  unfolding pathstart_def linepath_def
  by auto

lemma pathfinish_linepath[simp]: "pathfinish (linepath a b) = b"
  unfolding pathfinish_def linepath_def
  by auto

lemma linepath_inner: "linepath a b x \ v = linepath (a \ v) (b \ v) x"
  by (simp add: linepath_def algebra_simps)

lemma Re_linepath': "Re (linepath a b x) = linepath (Re a) (Re b) x"
  by (simp add: linepath_def)

lemma Im_linepath': "Im (linepath a b x) = linepath (Im a) (Im b) x"
  by (simp add: linepath_def)

lemma linepath_0': "linepath a b 0 = a"
  by (simp add: linepath_def)

lemma linepath_1': "linepath a b 1 = b"
  by (simp add: linepath_def)

lemma continuous_linepath_at[intro]: "continuous (at x) (linepath a b)"
  unfolding linepath_def
  by (intro continuous_intros)

lemma continuous_on_linepath [intro,continuous_intros]: "continuous_on s (linepath a b)"
  using continuous_linepath_at
  by (auto intro!: continuous_at_imp_continuous_on)

lemma path_linepath[iff]: "path (linepath a b)"
  unfolding path_def
  by (rule continuous_on_linepath)

lemma path_image_linepath[simp]: "path_image (linepath a b) = closed_segment a b"
  unfolding path_image_def segment linepath_def
  by auto

lemma reversepath_linepath[simp]: "reversepath (linepath a b) = linepath b a"
  unfolding reversepath_def linepath_def
  by auto

lemma linepath_0 [simp]: "linepath 0 b x = x *\<^sub>R b"
  by (simp add: linepath_def)

lemma linepath_cnj: "cnj (linepath a b x) = linepath (cnj a) (cnj b) x"
  by (simp add: linepath_def)

lemma arc_linepath:
  assumes "a \ b" shows [simp]: "arc (linepath a b)"
proof -
  {
    fix x y :: "real"
    assume "x *\<^sub>R b + y *\<^sub>R a = x *\<^sub>R a + y *\<^sub>R b"
    then have "(x - y) *\<^sub>R a = (x - y) *\<^sub>R b"
      by (simp add: algebra_simps)
    with assms have "x = y"
      by simp
  }
  then show ?thesis
    unfolding arc_def inj_on_def
    by (fastforce simp: algebra_simps linepath_def)
qed

lemma simple_path_linepath[intro]: "a \ b \ simple_path (linepath a b)"
  by (simp add: arc_imp_simple_path)

lemma linepath_trivial [simp]: "linepath a a x = a"
  by (simp add: linepath_def real_vector.scale_left_diff_distrib)

lemma linepath_refl: "linepath a a = (\x. a)"
  by auto

lemma subpath_refl: "subpath a a g = linepath (g a) (g a)"
  by (simp add: subpath_def linepath_def algebra_simps)

lemma linepath_of_real: "(linepath (of_real a) (of_real b) x) = of_real ((1 - x)*a + x*b)"
  by (simp add: scaleR_conv_of_real linepath_def)

lemma of_real_linepath: "of_real (linepath a b x) = linepath (of_real a) (of_real b) x"
  by (metis linepath_of_real mult.right_neutral of_real_def real_scaleR_def)

lemma inj_on_linepath:
  assumes "a \ b" shows "inj_on (linepath a b) {0..1}"
  using arc_imp_inj_on arc_linepath assms by blast

lemma linepath_le_1:
  fixes a::"'a::linordered_idom" shows "\a \ 1; b \ 1; 0 \ u; u \ 1\ \ (1 - u) * a + u * b \ 1"
  using mult_left_le [of a "1-u"] mult_left_le [of b u] by auto

lemma linepath_in_path:
  shows "x \ {0..1} \ linepath a b x \ closed_segment a b"
  by (auto simp: segment linepath_def)

lemma linepath_image_01: "linepath a b ` {0..1} = closed_segment a b"
  by (auto simp: segment linepath_def)

lemma linepath_in_convex_hull:
  fixes x::real
  assumes "a \ convex hull S"
    and "b \ convex hull S"
    and "0\x" "x\1"
  shows "linepath a b x \ convex hull S"
  by (meson assms atLeastAtMost_iff convex_contains_segment convex_convex_hull linepath_in_path subset_eq)

lemma Re_linepath: "Re(linepath (of_real a) (of_real b) x) = (1 - x)*a + x*b"
  by (simp add: linepath_def)

lemma Im_linepath: "Im(linepath (of_real a) (of_real b) x) = 0"
  by (simp add: linepath_def)

lemma
  assumes "x \ closed_segment y z"
  shows in_closed_segment_imp_Re_in_closed_segment: "Re x \ closed_segment (Re y) (Re z)" (is ?th1)
    and in_closed_segment_imp_Im_in_closed_segment: "Im x \ closed_segment (Im y) (Im z)" (is ?th2)
proof -
  from assms obtain t where t: "t \ {0..1}" "x = linepath y z t"
    by (metis imageE linepath_image_01)
  have "Re x = linepath (Re y) (Re z) t" "Im x = linepath (Im y) (Im z) t"
    by (simp_all add: t Re_linepath' Im_linepath')
  with t(1) show ?th1 ?th2
    using linepath_in_path[of t "Re y" "Re z"] linepath_in_path[of t "Im y" "Im z"] by simp_all
qed

lemma linepath_in_open_segment: "t \ {0<..<1} \ x \ y \ linepath x y t \ open_segment x y"
  unfolding greaterThanLessThan_iff by (metis in_segment(2) linepath_def)

lemma in_open_segment_imp_Re_in_open_segment:
  assumes "x \ open_segment y z" "Re y \ Re z"
  shows   "Re x \ open_segment (Re y) (Re z)"
proof -
  from assms obtain t where t: "t \ {0<..<1}" "x = linepath y z t"
    by (metis greaterThanLessThan_iff in_segment(2) linepath_def)
  have "Re x = linepath (Re y) (Re z) t"
    by (simp_all add: t Re_linepath')
  with t(1) show ?thesis
    using linepath_in_open_segment[of t "Re y" "Re z"] assms by auto
qed

lemma in_open_segment_imp_Im_in_open_segment:
  assumes "x \ open_segment y z" "Im y \ Im z"
  shows   "Im x \ open_segment (Im y) (Im z)"
proof -
  from assms obtain t where t: "t \ {0<..<1}" "x = linepath y z t"
    by (metis greaterThanLessThan_iff in_segment(2) linepath_def)
  have "Im x = linepath (Im y) (Im z) t"
    by (simp_all add: t Im_linepath')
  with t(1) show ?thesis
    using linepath_in_open_segment[of t "Im y" "Im z"] assms by auto
qed

lemma bounded_linear_linepath:
  assumes "bounded_linear f"
  shows   "f (linepath a b x) = linepath (f a) (f b) x"
proof -
  interpret f: bounded_linear f by fact
  show ?thesis by (simp add: linepath_def f.add f.scale)
qed

lemma bounded_linear_linepath':
  assumes "bounded_linear f"
  shows   "f \ linepath a b = linepath (f a) (f b)"
  using bounded_linear_linepath[OF assms] by (simp add: fun_eq_iff)

lemma linepath_cnj': "cnj \ linepath a b = linepath (cnj a) (cnj b)"
  by (simp add: linepath_def fun_eq_iff)

lemma differentiable_linepath [intro]: "linepath a b differentiable at x within A"
  by (auto simp: linepath_def)

lemma has_vector_derivative_linepath_within:
    "(linepath a b has_vector_derivative (b - a)) (at x within S)"
  by (force intro: derivative_eq_intros simp add: linepath_def has_vector_derivative_def algebra_simps)

lemma linepath_real_ge_left:
  fixes x y :: real
  assumes "x \ y" "t \ 0"
  shows   "linepath x y t \ x"
proof -
  have "x + 0 \ x + t *\<^sub>R (y - x)"
    using assms by (intro add_left_mono) auto
  also have "\ = linepath x y t"
    by (simp add: linepath_def algebra_simps)
  finally show ?thesis by simp
qed

lemma linepath_real_le_right:
  fixes x y :: real
  assumes "x \ y" "t \ 1"
  shows   "linepath x y t \ y"
proof -
  have "y + 0 \ y + (1 - t) *\<^sub>R (x - y)"
    using assms by (intro add_left_mono) (auto intro: mult_nonneg_nonpos)
  also have "y + (1 - t) *\<^sub>R (x - y) = linepath x y t"
    by (simp add: linepath_def algebra_simps)
  finally show ?thesis by simp
qed

lemma linepath_translate: "(+) c \ linepath a b = linepath (a + c) (b + c)"
  by (auto simp: linepath_def algebra_simps)

subsection\<^marker>\<open>tag unimportant\<close>\<open>Segments via convex hulls\<close>

lemma segments_subset_convex_hull:
    "closed_segment a b \ (convex hull {a,b,c})"
    "closed_segment a c \ (convex hull {a,b,c})"
    "closed_segment b c \ (convex hull {a,b,c})"
    "closed_segment b a \ (convex hull {a,b,c})"
    "closed_segment c a \ (convex hull {a,b,c})"
    "closed_segment c b \ (convex hull {a,b,c})"
by (auto simp: segment_convex_hull linepath_of_real  elim!: rev_subsetD [OF _ hull_mono])

lemma midpoints_in_convex_hull:
  assumes "x \ convex hull s" "y \ convex hull s"
    shows "midpoint x y \ convex hull s"
  using assms closed_segment_subset_convex_hull csegment_midpoint_subset by blast

lemma not_in_interior_convex_hull_3:
  fixes a :: "complex"
  shows "a \ interior(convex hull {a,b,c})"
        "b \ interior(convex hull {a,b,c})"
        "c \ interior(convex hull {a,b,c})"
  by (auto simp: card_insert_le_m1 not_in_interior_convex_hull)

lemma midpoint_in_closed_segment [simp]: "midpoint a b \ closed_segment a b"
  using midpoints_in_convex_hull segment_convex_hull by blast

lemma midpoint_in_open_segment [simp]: "midpoint a b \ open_segment a b \ a \ b"
  by (simp add: open_segment_def)

lemma continuous_IVT_local_extremum:
  fixes f :: "'a::euclidean_space \ real"
  assumes contf: "continuous_on (closed_segment a b) f"
      and ab: "a \ b" "f a = f b"
  obtains z where "z \ open_segment a b"
                  "(\w \ closed_segment a b. (f w) \ (f z)) \
                   (\<forall>w \<in> closed_segment a b. (f z) \<le> (f w))"
proof -
  obtain c where "c \ closed_segment a b" and c: "\y. y \ closed_segment a b \ f y \ f c"
    using continuous_attains_sup [of "closed_segment a b" f] contf by auto
  moreover
  obtain d where "d \ closed_segment a b" and d: "\y. y \ closed_segment a b \ f d \ f y"
    using continuous_attains_inf [of "closed_segment a b" f] contf by auto
  ultimately show ?thesis
    by (smt (verit) UnE ab closed_segment_eq_open empty_iff insert_iff midpoint_in_open_segment that)
qed

text\<open>An injective map into R is also an open map w.r.T. the universe, and conversely. \<close>
proposition injective_eq_1d_open_map_UNIV:
  fixes f :: "real \ real"
  assumes contf: "continuous_on S f" and S: "is_interval S"
    shows "inj_on f S \ (\T. open T \ T \ S \ open(f ` T))"
          (is "?lhs = ?rhs")
proof safe
  fix T
  assume injf: ?lhs and "open T" and "T \ S"
  have "\U. open U \ f x \ U \ U \ f ` T" if "x \ T" for x
  proof -
    obtain \<delta> where "\<delta> > 0" and \<delta>: "cball x \<delta> \<subseteq> T"
      using \<open>open T\<close> \<open>x \<in> T\<close> open_contains_cball_eq by blast
    show ?thesis
    proof (intro exI conjI)
      have "closed_segment (x-\) (x+\) = {x-\..x+\}"
        using \<open>0 < \<delta>\<close> by (auto simp: closed_segment_eq_real_ivl)
      also have "\ \ S"
        using \<delta> \<open>T \<subseteq> S\<close> by (auto simp: dist_norm subset_eq)
      finally have "f ` (open_segment (x-\) (x+\)) = open_segment (f (x-\)) (f (x+\))"
        using continuous_injective_image_open_segment_1
        by (metis continuous_on_subset [OF contf] inj_on_subset [OF injf])
      then show "open (f ` {x-\<..})"
        using \<open>0 < \<delta>\<close> by (simp add: open_segment_eq_real_ivl)
      show "f x \ f ` {x - \<..}"
        by (auto simp: \<open>\<delta> > 0\<close>)
      show "f ` {x - \<..} \ f ` T"
        using \<delta> by (auto simp: dist_norm subset_iff)
    qed
  qed
  with open_subopen show "open (f ` T)"
    by blast
next
  assume R: ?rhs
  have False if xy: "x \ S" "y \ S" and "f x = f y" "x \ y" for x y
  proof -
    have "open (f ` open_segment x y)"
      using R
      by (metis S convex_contains_open_segment is_interval_convex open_greaterThanLessThan open_segment_eq_real_ivl xy)
    moreover
    have "continuous_on (closed_segment x y) f"
      by (meson S closed_segment_subset contf continuous_on_subset is_interval_convex that)
    then obtain \<xi> where "\<xi> \<in> open_segment x y"
                    and \<xi>: "(\<forall>w \<in> closed_segment x y. (f w) \<le> (f \<xi>)) \<or>
                            (\<forall>w \<in> closed_segment x y. (f \<xi>) \<le> (f w))"
      using continuous_IVT_local_extremum [of x y f] \<open>f x = f y\<close> \<open>x \<noteq> y\<close> by blast
    ultimately obtain e where "e>0" and e: "\u. dist u (f \) < e \ u \ f ` open_segment x y"
      using open_dist by (metis image_eqI)
    have fin: "f \ + (e/2) \ f ` open_segment x y" "f \ - (e/2) \ f ` open_segment x y"
      using e [of "f \ + (e/2)"] e [of "f \ - (e/2)"] \e > 0\ by (auto simp: dist_norm)
    show ?thesis
      using \<xi> \<open>0 < e\<close> fin open_closed_segment by fastforce
  qed
  then show ?lhs
    by (force simp: inj_on_def)
qed

subsection\<^marker>\<open>tag unimportant\<close> \<open>Bounding a point away from a path\<close>

lemma not_on_path_ball:
  fixes g :: "real \ 'a::heine_borel"
  assumes "path g"
    and z: "z \ path_image g"
  shows "\e > 0. ball z e \ path_image g = {}"
proof -
  have "closed (path_image g)"
    by (simp add: \<open>path g\<close> closed_path_image)
  then obtain a where "a \ path_image g" "\y \ path_image g. dist z a \ dist z y"
    by (auto intro: distance_attains_inf[OF _ path_image_nonempty, of g z])
  then show ?thesis
    by (rule_tac x="dist z a" in exI) (use dist_commute z in auto)
qed

lemma not_on_path_cball:
  fixes g :: "real \ 'a::heine_borel"
  assumes "path g"
    and "z \ path_image g"
  shows "\e>0. cball z e \ (path_image g) = {}"
  by (smt (verit, ccfv_threshold) open_ball assms centre_in_ball inf.orderE inf_assoc
      inf_bot_right not_on_path_ball open_contains_cball_eq)

subsection \<open>Path component\<close>

text \<open>Original formalization by Tom Hales\<close>

definition\<^marker>\<open>tag important\<close> "path_component S x y \<equiv>
  (\<exists>g. path g \<and> path_image g \<subseteq> S \<and> pathstart g = x \<and> pathfinish g = y)"

abbreviation\<^marker>\<open>tag important\<close>
  "path_component_set S x \ Collect (path_component S x)"

lemmas path_defs = path_def pathstart_def pathfinish_def path_image_def path_component_def

lemma path_component_mem:
  assumes "path_component S x y"
  shows "x \ S" and "y \ S"
  using assms
  unfolding path_defs
  by auto

lemma path_component_refl:
  assumes "x \ S"
  shows "path_component S x x"
  using assms
  unfolding path_defs
  by (metis (full_types) assms continuous_on_const image_subset_iff path_image_def)

lemma path_component_refl_eq: "path_component S x x \ x \ S"
--> --------------------

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Bemerkung:

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