36583

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(* Title: Multivariate_Analysis/Path_Connected.thy


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Author: Robert Himmelmann, TU Muenchen


3 
*)


4 


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header {* Continuous paths and pathconnected sets *}


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theory Path_Connected


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imports Convex_Euclidean_Space


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begin


10 


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subsection {* Paths. *}


12 


13 
definition


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path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"


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where "path g \<longleftrightarrow> continuous_on {0 .. 1} g"


16 


17 
definition


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pathstart :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"


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where "pathstart g = g 0"


20 


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definition


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pathfinish :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"


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where "pathfinish g = g 1"


24 


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definition


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path_image :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a set"


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where "path_image g = g ` {0 .. 1}"


28 


29 
definition


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reversepath :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> (real \<Rightarrow> 'a)"


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where "reversepath g = (\<lambda>x. g(1  x))"


32 


33 
definition


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joinpaths :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> (real \<Rightarrow> 'a) \<Rightarrow> (real \<Rightarrow> 'a)"


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(infixr "+++" 75)


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where "g1 +++ g2 = (\<lambda>x. if x \<le> 1/2 then g1 (2 * x) else g2 (2 * x  1))"


37 


38 
definition


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simple_path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"


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where "simple_path g \<longleftrightarrow>


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(\<forall>x\<in>{0..1}. \<forall>y\<in>{0..1}. g x = g y \<longrightarrow> x = y \<or> x = 0 \<and> y = 1 \<or> x = 1 \<and> y = 0)"


42 


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definition


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injective_path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"


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where "injective_path g \<longleftrightarrow> (\<forall>x\<in>{0..1}. \<forall>y\<in>{0..1}. g x = g y \<longrightarrow> x = y)"


46 


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subsection {* Some lemmas about these concepts. *}


48 


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lemma injective_imp_simple_path:


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"injective_path g \<Longrightarrow> simple_path g"


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unfolding injective_path_def simple_path_def by auto


52 


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lemma path_image_nonempty: "path_image g \<noteq> {}"


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unfolding path_image_def image_is_empty interval_eq_empty by auto


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lemma pathstart_in_path_image[intro]: "(pathstart g) \<in> path_image g"


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unfolding pathstart_def path_image_def by auto


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lemma pathfinish_in_path_image[intro]: "(pathfinish g) \<in> path_image g"


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unfolding pathfinish_def path_image_def by auto


61 


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lemma connected_path_image[intro]: "path g \<Longrightarrow> connected(path_image g)"


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unfolding path_def path_image_def


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apply (erule connected_continuous_image)


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by(rule convex_connected, rule convex_real_interval)


66 


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lemma compact_path_image[intro]: "path g \<Longrightarrow> compact(path_image g)"


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unfolding path_def path_image_def


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by (erule compact_continuous_image, rule compact_real_interval)


70 


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lemma reversepath_reversepath[simp]: "reversepath(reversepath g) = g"


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unfolding reversepath_def by auto


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lemma pathstart_reversepath[simp]: "pathstart(reversepath g) = pathfinish g"


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unfolding pathstart_def reversepath_def pathfinish_def by auto


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lemma pathfinish_reversepath[simp]: "pathfinish(reversepath g) = pathstart g"


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unfolding pathstart_def reversepath_def pathfinish_def by auto


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lemma pathstart_join[simp]: "pathstart(g1 +++ g2) = pathstart g1"


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unfolding pathstart_def joinpaths_def pathfinish_def by auto


82 


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lemma pathfinish_join[simp]:"pathfinish(g1 +++ g2) = pathfinish g2"


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unfolding pathstart_def joinpaths_def pathfinish_def by auto


85 


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lemma path_image_reversepath[simp]: "path_image(reversepath g) = path_image g" proof


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have *:"\<And>g. path_image(reversepath g) \<subseteq> path_image g"


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unfolding path_image_def subset_eq reversepath_def Ball_def image_iff apply(rule,rule,erule bexE)


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apply(rule_tac x="1  xa" in bexI) by auto


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show ?thesis using *[of g] *[of "reversepath g"] unfolding reversepath_reversepath by auto qed


91 


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lemma path_reversepath[simp]: "path(reversepath g) \<longleftrightarrow> path g" proof


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have *:"\<And>g. path g \<Longrightarrow> path(reversepath g)" unfolding path_def reversepath_def


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apply(rule continuous_on_compose[unfolded o_def, of _ "\<lambda>x. 1  x"])


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apply(rule continuous_on_sub, rule continuous_on_const, rule continuous_on_id)


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apply(rule continuous_on_subset[of "{0..1}"], assumption) by auto


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show ?thesis using *[of "reversepath g"] *[of g] unfolding reversepath_reversepath by (rule iffI) qed


98 


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lemmas reversepath_simps = path_reversepath path_image_reversepath pathstart_reversepath pathfinish_reversepath


100 


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lemma path_join[simp]: assumes "pathfinish g1 = pathstart g2" shows "path (g1 +++ g2) \<longleftrightarrow> path g1 \<and> path g2"


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unfolding path_def pathfinish_def pathstart_def apply rule defer apply(erule conjE) proof


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assume as:"continuous_on {0..1} (g1 +++ g2)"


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have *:"g1 = (\<lambda>x. g1 (2 *\<^sub>R x)) \<circ> (\<lambda>x. (1/2) *\<^sub>R x)"


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"g2 = (\<lambda>x. g2 (2 *\<^sub>R x  1)) \<circ> (\<lambda>x. (1/2) *\<^sub>R (x + 1))"


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unfolding o_def by (auto simp add: add_divide_distrib)


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have "op *\<^sub>R (1 / 2) ` {0::real..1} \<subseteq> {0..1}" "(\<lambda>x. (1 / 2) *\<^sub>R (x + 1)) ` {(0::real)..1} \<subseteq> {0..1}"


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by auto


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thus "continuous_on {0..1} g1 \<and> continuous_on {0..1} g2" apply apply rule


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apply(subst *) defer apply(subst *) apply (rule_tac[!] continuous_on_compose)


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apply (rule continuous_on_cmul, rule continuous_on_add, rule continuous_on_id, rule continuous_on_const) defer


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apply (rule continuous_on_cmul, rule continuous_on_id) apply(rule_tac[!] continuous_on_eq[of _ "g1 +++ g2"]) defer prefer 3


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apply(rule_tac[12] continuous_on_subset[of "{0 .. 1}"]) apply(rule as, assumption, rule as, assumption)


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apply(rule) defer apply rule proof


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fix x assume "x \<in> op *\<^sub>R (1 / 2) ` {0::real..1}"


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hence "x \<le> 1 / 2" unfolding image_iff by auto


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thus "(g1 +++ g2) x = g1 (2 *\<^sub>R x)" unfolding joinpaths_def by auto next


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fix x assume "x \<in> (\<lambda>x. (1 / 2) *\<^sub>R (x + 1)) ` {0::real..1}"


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hence "x \<ge> 1 / 2" unfolding image_iff by auto


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thus "(g1 +++ g2) x = g2 (2 *\<^sub>R x  1)" proof(cases "x = 1 / 2")


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case True hence "x = (1/2) *\<^sub>R 1" unfolding Cart_eq by auto


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thus ?thesis unfolding joinpaths_def using assms[unfolded pathstart_def pathfinish_def] by (auto simp add: mult_ac)


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qed (auto simp add:le_less joinpaths_def) qed


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next assume as:"continuous_on {0..1} g1" "continuous_on {0..1} g2"


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have *:"{0 .. 1::real} = {0.. (1/2)*\<^sub>R 1} \<union> {(1/2) *\<^sub>R 1 .. 1}" by auto


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have **:"op *\<^sub>R 2 ` {0..(1 / 2) *\<^sub>R 1} = {0..1::real}" apply(rule set_ext, rule) unfolding image_iff


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defer apply(rule_tac x="(1/2)*\<^sub>R x" in bexI) by auto


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have ***:"(\<lambda>x. 2 *\<^sub>R x  1) ` {(1 / 2) *\<^sub>R 1..1} = {0..1::real}"


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apply (auto simp add: image_def)


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apply (rule_tac x="(x + 1) / 2" in bexI)


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apply (auto simp add: add_divide_distrib)


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done


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show "continuous_on {0..1} (g1 +++ g2)" unfolding * apply(rule continuous_on_union) apply (rule closed_real_atLeastAtMost)+ proof


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show "continuous_on {0..(1 / 2) *\<^sub>R 1} (g1 +++ g2)" apply(rule continuous_on_eq[of _ "\<lambda>x. g1 (2 *\<^sub>R x)"]) defer


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unfolding o_def[THEN sym] apply(rule continuous_on_compose) apply(rule continuous_on_cmul, rule continuous_on_id)


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unfolding ** apply(rule as(1)) unfolding joinpaths_def by auto next


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show "continuous_on {(1/2)*\<^sub>R1..1} (g1 +++ g2)" apply(rule continuous_on_eq[of _ "g2 \<circ> (\<lambda>x. 2 *\<^sub>R x  1)"]) defer


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apply(rule continuous_on_compose) apply(rule continuous_on_sub, rule continuous_on_cmul, rule continuous_on_id, rule continuous_on_const)


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unfolding *** o_def joinpaths_def apply(rule as(2)) using assms[unfolded pathstart_def pathfinish_def]


140 
by (auto simp add: mult_ac) qed qed


141 


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lemma path_image_join_subset: "path_image(g1 +++ g2) \<subseteq> (path_image g1 \<union> path_image g2)" proof


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fix x assume "x \<in> path_image (g1 +++ g2)"


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then obtain y where y:"y\<in>{0..1}" "x = (if y \<le> 1 / 2 then g1 (2 *\<^sub>R y) else g2 (2 *\<^sub>R y  1))"


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unfolding path_image_def image_iff joinpaths_def by auto


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thus "x \<in> path_image g1 \<union> path_image g2" apply(cases "y \<le> 1/2")


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apply(rule_tac UnI1) defer apply(rule_tac UnI2) unfolding y(2) path_image_def using y(1)


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by(auto intro!: imageI) qed


149 


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lemma subset_path_image_join:


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assumes "path_image g1 \<subseteq> s" "path_image g2 \<subseteq> s" shows "path_image(g1 +++ g2) \<subseteq> s"


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using path_image_join_subset[of g1 g2] and assms by auto


153 


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lemma path_image_join:


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assumes "path g1" "path g2" "pathfinish g1 = pathstart g2"


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shows "path_image(g1 +++ g2) = (path_image g1) \<union> (path_image g2)"


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apply(rule, rule path_image_join_subset, rule) unfolding Un_iff proof(erule disjE)


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fix x assume "x \<in> path_image g1"


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then obtain y where y:"y\<in>{0..1}" "x = g1 y" unfolding path_image_def image_iff by auto


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thus "x \<in> path_image (g1 +++ g2)" unfolding joinpaths_def path_image_def image_iff


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apply(rule_tac x="(1/2) *\<^sub>R y" in bexI) by auto next


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fix x assume "x \<in> path_image g2"


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then obtain y where y:"y\<in>{0..1}" "x = g2 y" unfolding path_image_def image_iff by auto


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then show "x \<in> path_image (g1 +++ g2)" unfolding joinpaths_def path_image_def image_iff


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apply(rule_tac x="(1/2) *\<^sub>R (y + 1)" in bexI) using assms(3)[unfolded pathfinish_def pathstart_def]


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by (auto simp add: add_divide_distrib) qed


167 


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lemma not_in_path_image_join:


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assumes "x \<notin> path_image g1" "x \<notin> path_image g2" shows "x \<notin> path_image(g1 +++ g2)"


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using assms and path_image_join_subset[of g1 g2] by auto


171 


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lemma simple_path_reversepath: assumes "simple_path g" shows "simple_path (reversepath g)"


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using assms unfolding simple_path_def reversepath_def apply apply(rule ballI)+


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apply(erule_tac x="1x" in ballE, erule_tac x="1y" in ballE)


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by auto


176 


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lemma simple_path_join_loop:


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assumes "injective_path g1" "injective_path g2" "pathfinish g2 = pathstart g1"


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"(path_image g1 \<inter> path_image g2) \<subseteq> {pathstart g1,pathstart g2}"


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shows "simple_path(g1 +++ g2)"


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unfolding simple_path_def proof((rule ballI)+, rule impI) let ?g = "g1 +++ g2"


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note inj = assms(1,2)[unfolded injective_path_def, rule_format]


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fix x y::"real" assume xy:"x \<in> {0..1}" "y \<in> {0..1}" "?g x = ?g y"


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show "x = y \<or> x = 0 \<and> y = 1 \<or> x = 1 \<and> y = 0" proof(case_tac "x \<le> 1/2",case_tac[!] "y \<le> 1/2", unfold not_le)


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assume as:"x \<le> 1 / 2" "y \<le> 1 / 2"


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hence "g1 (2 *\<^sub>R x) = g1 (2 *\<^sub>R y)" using xy(3) unfolding joinpaths_def by auto


187 
moreover have "2 *\<^sub>R x \<in> {0..1}" "2 *\<^sub>R y \<in> {0..1}" using xy(1,2) as


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by auto


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ultimately show ?thesis using inj(1)[of "2*\<^sub>R x" "2*\<^sub>R y"] by auto


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next assume as:"x > 1 / 2" "y > 1 / 2"


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hence "g2 (2 *\<^sub>R x  1) = g2 (2 *\<^sub>R y  1)" using xy(3) unfolding joinpaths_def by auto


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moreover have "2 *\<^sub>R x  1 \<in> {0..1}" "2 *\<^sub>R y  1 \<in> {0..1}" using xy(1,2) as by auto


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ultimately show ?thesis using inj(2)[of "2*\<^sub>R x  1" "2*\<^sub>R y  1"] by auto


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next assume as:"x \<le> 1 / 2" "y > 1 / 2"


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hence "?g x \<in> path_image g1" "?g y \<in> path_image g2" unfolding path_image_def joinpaths_def


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using xy(1,2) by auto


197 
moreover have "?g y \<noteq> pathstart g2" using as(2) unfolding pathstart_def joinpaths_def


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using inj(2)[of "2 *\<^sub>R y  1" 0] and xy(2)


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by (auto simp add: field_simps)


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ultimately have *:"?g x = pathstart g1" using assms(4) unfolding xy(3) by auto


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hence "x = 0" unfolding pathstart_def joinpaths_def using as(1) and xy(1)


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using inj(1)[of "2 *\<^sub>R x" 0] by auto


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moreover have "y = 1" using * unfolding xy(3) assms(3)[THEN sym]


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unfolding joinpaths_def pathfinish_def using as(2) and xy(2)


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using inj(2)[of "2 *\<^sub>R y  1" 1] by auto


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ultimately show ?thesis by auto


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next assume as:"x > 1 / 2" "y \<le> 1 / 2"


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hence "?g x \<in> path_image g2" "?g y \<in> path_image g1" unfolding path_image_def joinpaths_def


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using xy(1,2) by auto


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moreover have "?g x \<noteq> pathstart g2" using as(1) unfolding pathstart_def joinpaths_def


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using inj(2)[of "2 *\<^sub>R x  1" 0] and xy(1)


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by (auto simp add: field_simps)


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ultimately have *:"?g y = pathstart g1" using assms(4) unfolding xy(3) by auto


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hence "y = 0" unfolding pathstart_def joinpaths_def using as(2) and xy(2)


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using inj(1)[of "2 *\<^sub>R y" 0] by auto


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moreover have "x = 1" using * unfolding xy(3)[THEN sym] assms(3)[THEN sym]


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unfolding joinpaths_def pathfinish_def using as(1) and xy(1)


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using inj(2)[of "2 *\<^sub>R x  1" 1] by auto


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ultimately show ?thesis by auto qed qed


220 


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lemma injective_path_join:


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assumes "injective_path g1" "injective_path g2" "pathfinish g1 = pathstart g2"


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"(path_image g1 \<inter> path_image g2) \<subseteq> {pathstart g2}"


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shows "injective_path(g1 +++ g2)"


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unfolding injective_path_def proof(rule,rule,rule) let ?g = "g1 +++ g2"


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note inj = assms(1,2)[unfolded injective_path_def, rule_format]


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fix x y assume xy:"x \<in> {0..1}" "y \<in> {0..1}" "(g1 +++ g2) x = (g1 +++ g2) y"


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show "x = y" proof(cases "x \<le> 1/2", case_tac[!] "y \<le> 1/2", unfold not_le)


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assume "x \<le> 1 / 2" "y \<le> 1 / 2" thus ?thesis using inj(1)[of "2*\<^sub>R x" "2*\<^sub>R y"] and xy


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unfolding joinpaths_def by auto


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next assume "x > 1 / 2" "y > 1 / 2" thus ?thesis using inj(2)[of "2*\<^sub>R x  1" "2*\<^sub>R y  1"] and xy


232 
unfolding joinpaths_def by auto


233 
next assume as:"x \<le> 1 / 2" "y > 1 / 2"


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hence "?g x \<in> path_image g1" "?g y \<in> path_image g2" unfolding path_image_def joinpaths_def


235 
using xy(1,2) by auto


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hence "?g x = pathfinish g1" "?g y = pathstart g2" using assms(4) unfolding assms(3) xy(3) by auto


237 
thus ?thesis using as and inj(1)[of "2 *\<^sub>R x" 1] inj(2)[of "2 *\<^sub>R y  1" 0] and xy(1,2)


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unfolding pathstart_def pathfinish_def joinpaths_def


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by auto


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next assume as:"x > 1 / 2" "y \<le> 1 / 2"


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hence "?g x \<in> path_image g2" "?g y \<in> path_image g1" unfolding path_image_def joinpaths_def


242 
using xy(1,2) by auto


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hence "?g x = pathstart g2" "?g y = pathfinish g1" using assms(4) unfolding assms(3) xy(3) by auto


244 
thus ?thesis using as and inj(2)[of "2 *\<^sub>R x  1" 0] inj(1)[of "2 *\<^sub>R y" 1] and xy(1,2)


245 
unfolding pathstart_def pathfinish_def joinpaths_def


246 
by auto qed qed


247 


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lemmas join_paths_simps = path_join path_image_join pathstart_join pathfinish_join


249 


250 
subsection {* Reparametrizing a closed curve to start at some chosen point. *}


251 


252 
definition "shiftpath a (f::real \<Rightarrow> 'a::topological_space) =


253 
(\<lambda>x. if (a + x) \<le> 1 then f(a + x) else f(a + x  1))"


254 


255 
lemma pathstart_shiftpath: "a \<le> 1 \<Longrightarrow> pathstart(shiftpath a g) = g a"


256 
unfolding pathstart_def shiftpath_def by auto


257 


258 
lemma pathfinish_shiftpath: assumes "0 \<le> a" "pathfinish g = pathstart g"


259 
shows "pathfinish(shiftpath a g) = g a"


260 
using assms unfolding pathstart_def pathfinish_def shiftpath_def


261 
by auto


262 


263 
lemma endpoints_shiftpath:


264 
assumes "pathfinish g = pathstart g" "a \<in> {0 .. 1}"


265 
shows "pathfinish(shiftpath a g) = g a" "pathstart(shiftpath a g) = g a"


266 
using assms by(auto intro!:pathfinish_shiftpath pathstart_shiftpath)


267 


268 
lemma closed_shiftpath:


269 
assumes "pathfinish g = pathstart g" "a \<in> {0..1}"


270 
shows "pathfinish(shiftpath a g) = pathstart(shiftpath a g)"


271 
using endpoints_shiftpath[OF assms] by auto


272 


273 
lemma path_shiftpath:


274 
assumes "path g" "pathfinish g = pathstart g" "a \<in> {0..1}"


275 
shows "path(shiftpath a g)" proof


276 
have *:"{0 .. 1} = {0 .. 1a} \<union> {1a .. 1}" using assms(3) by auto


277 
have **:"\<And>x. x + a = 1 \<Longrightarrow> g (x + a  1) = g (x + a)"


278 
using assms(2)[unfolded pathfinish_def pathstart_def] by auto


279 
show ?thesis unfolding path_def shiftpath_def * apply(rule continuous_on_union)


280 
apply(rule closed_real_atLeastAtMost)+ apply(rule continuous_on_eq[of _ "g \<circ> (\<lambda>x. a + x)"]) prefer 3


281 
apply(rule continuous_on_eq[of _ "g \<circ> (\<lambda>x. a  1 + x)"]) defer prefer 3


282 
apply(rule continuous_on_intros)+ prefer 2 apply(rule continuous_on_intros)+


283 
apply(rule_tac[12] continuous_on_subset[OF assms(1)[unfolded path_def]])


284 
using assms(3) and ** by(auto, auto simp add: field_simps) qed


285 


286 
lemma shiftpath_shiftpath: assumes "pathfinish g = pathstart g" "a \<in> {0..1}" "x \<in> {0..1}"


287 
shows "shiftpath (1  a) (shiftpath a g) x = g x"


288 
using assms unfolding pathfinish_def pathstart_def shiftpath_def by auto


289 


290 
lemma path_image_shiftpath:


291 
assumes "a \<in> {0..1}" "pathfinish g = pathstart g"


292 
shows "path_image(shiftpath a g) = path_image g" proof


293 
{ fix x assume as:"g 1 = g 0" "x \<in> {0..1::real}" " \<forall>y\<in>{0..1} \<inter> {x. \<not> a + x \<le> 1}. g x \<noteq> g (a + y  1)"


294 
hence "\<exists>y\<in>{0..1} \<inter> {x. a + x \<le> 1}. g x = g (a + y)" proof(cases "a \<le> x")


295 
case False thus ?thesis apply(rule_tac x="1 + x  a" in bexI)


296 
using as(1,2) and as(3)[THEN bspec[where x="1 + x  a"]] and assms(1)


297 
by(auto simp add: field_simps atomize_not) next


298 
case True thus ?thesis using as(12) and assms(1) apply(rule_tac x="x  a" in bexI)


299 
by(auto simp add: field_simps) qed }


300 
thus ?thesis using assms unfolding shiftpath_def path_image_def pathfinish_def pathstart_def


301 
by(auto simp add: image_iff) qed


302 


303 
subsection {* Special case of straightline paths. *}


304 


305 
definition


306 
linepath :: "'a::real_normed_vector \<Rightarrow> 'a \<Rightarrow> real \<Rightarrow> 'a" where


307 
"linepath a b = (\<lambda>x. (1  x) *\<^sub>R a + x *\<^sub>R b)"


308 


309 
lemma pathstart_linepath[simp]: "pathstart(linepath a b) = a"


310 
unfolding pathstart_def linepath_def by auto


311 


312 
lemma pathfinish_linepath[simp]: "pathfinish(linepath a b) = b"


313 
unfolding pathfinish_def linepath_def by auto


314 


315 
lemma continuous_linepath_at[intro]: "continuous (at x) (linepath a b)"


316 
unfolding linepath_def by (intro continuous_intros)


317 


318 
lemma continuous_on_linepath[intro]: "continuous_on s (linepath a b)"


319 
using continuous_linepath_at by(auto intro!: continuous_at_imp_continuous_on)


320 


321 
lemma path_linepath[intro]: "path(linepath a b)"


322 
unfolding path_def by(rule continuous_on_linepath)


323 


324 
lemma path_image_linepath[simp]: "path_image(linepath a b) = (closed_segment a b)"


325 
unfolding path_image_def segment linepath_def apply (rule set_ext, rule) defer


326 
unfolding mem_Collect_eq image_iff apply(erule exE) apply(rule_tac x="u *\<^sub>R 1" in bexI)


327 
by auto


328 


329 
lemma reversepath_linepath[simp]: "reversepath(linepath a b) = linepath b a"


330 
unfolding reversepath_def linepath_def by(rule ext, auto)


331 


332 
lemma injective_path_linepath:


333 
assumes "a \<noteq> b" shows "injective_path(linepath a b)"


334 
proof 


335 
{ fix x y :: "real"


336 
assume "x *\<^sub>R b + y *\<^sub>R a = x *\<^sub>R a + y *\<^sub>R b"


337 
hence "(x  y) *\<^sub>R a = (x  y) *\<^sub>R b" by (simp add: algebra_simps)


338 
with assms have "x = y" by simp }


339 
thus ?thesis unfolding injective_path_def linepath_def by(auto simp add: algebra_simps) qed


340 


341 
lemma simple_path_linepath[intro]: "a \<noteq> b \<Longrightarrow> simple_path(linepath a b)" by(auto intro!: injective_imp_simple_path injective_path_linepath)


342 


343 
subsection {* Bounding a point away from a path. *}


344 


345 
lemma not_on_path_ball:


346 
fixes g :: "real \<Rightarrow> 'a::heine_borel"


347 
assumes "path g" "z \<notin> path_image g"


348 
shows "\<exists>e>0. ball z e \<inter> (path_image g) = {}" proof


349 
obtain a where "a\<in>path_image g" "\<forall>y\<in>path_image g. dist z a \<le> dist z y"


350 
using distance_attains_inf[OF _ path_image_nonempty, of g z]


351 
using compact_path_image[THEN compact_imp_closed, OF assms(1)] by auto


352 
thus ?thesis apply(rule_tac x="dist z a" in exI) using assms(2) by(auto intro!: dist_pos_lt) qed


353 


354 
lemma not_on_path_cball:


355 
fixes g :: "real \<Rightarrow> 'a::heine_borel"


356 
assumes "path g" "z \<notin> path_image g"


357 
shows "\<exists>e>0. cball z e \<inter> (path_image g) = {}" proof


358 
obtain e where "ball z e \<inter> path_image g = {}" "e>0" using not_on_path_ball[OF assms] by auto


359 
moreover have "cball z (e/2) \<subseteq> ball z e" using `e>0` by auto


360 
ultimately show ?thesis apply(rule_tac x="e/2" in exI) by auto qed


361 


362 
subsection {* Path component, considered as a "joinability" relation (from Tom Hales). *}


363 


364 
definition "path_component s x y \<longleftrightarrow> (\<exists>g. path g \<and> path_image g \<subseteq> s \<and> pathstart g = x \<and> pathfinish g = y)"


365 


366 
lemmas path_defs = path_def pathstart_def pathfinish_def path_image_def path_component_def


367 


368 
lemma path_component_mem: assumes "path_component s x y" shows "x \<in> s" "y \<in> s"


369 
using assms unfolding path_defs by auto


370 


371 
lemma path_component_refl: assumes "x \<in> s" shows "path_component s x x"


372 
unfolding path_defs apply(rule_tac x="\<lambda>u. x" in exI) using assms


373 
by(auto intro!:continuous_on_intros)


374 


375 
lemma path_component_refl_eq: "path_component s x x \<longleftrightarrow> x \<in> s"


376 
by(auto intro!: path_component_mem path_component_refl)


377 


378 
lemma path_component_sym: "path_component s x y \<Longrightarrow> path_component s y x"


379 
using assms unfolding path_component_def apply(erule exE) apply(rule_tac x="reversepath g" in exI)


380 
by auto


381 


382 
lemma path_component_trans: assumes "path_component s x y" "path_component s y z" shows "path_component s x z"


383 
using assms unfolding path_component_def apply apply(erule exE)+ apply(rule_tac x="g +++ ga" in exI) by(auto simp add: path_image_join)


384 


385 
lemma path_component_of_subset: "s \<subseteq> t \<Longrightarrow> path_component s x y \<Longrightarrow> path_component t x y"


386 
unfolding path_component_def by auto


387 


388 
subsection {* Can also consider it as a set, as the name suggests. *}


389 


390 
lemma path_component_set: "path_component s x = { y. (\<exists>g. path g \<and> path_image g \<subseteq> s \<and> pathstart g = x \<and> pathfinish g = y )}"


391 
apply(rule set_ext) unfolding mem_Collect_eq unfolding mem_def path_component_def by auto


392 


393 
lemma mem_path_component_set:"x \<in> path_component s y \<longleftrightarrow> path_component s y x" unfolding mem_def by auto


394 


395 
lemma path_component_subset: "(path_component s x) \<subseteq> s"


396 
apply(rule, rule path_component_mem(2)) by(auto simp add:mem_def)


397 


398 
lemma path_component_eq_empty: "path_component s x = {} \<longleftrightarrow> x \<notin> s"


399 
apply rule apply(drule equals0D[of _ x]) defer apply(rule equals0I) unfolding mem_path_component_set


400 
apply(drule path_component_mem(1)) using path_component_refl by auto


401 


402 
subsection {* Path connectedness of a space. *}


403 


404 
definition "path_connected s \<longleftrightarrow> (\<forall>x\<in>s. \<forall>y\<in>s. \<exists>g. path g \<and> (path_image g) \<subseteq> s \<and> pathstart g = x \<and> pathfinish g = y)"


405 


406 
lemma path_connected_component: "path_connected s \<longleftrightarrow> (\<forall>x\<in>s. \<forall>y\<in>s. path_component s x y)"


407 
unfolding path_connected_def path_component_def by auto


408 


409 
lemma path_connected_component_set: "path_connected s \<longleftrightarrow> (\<forall>x\<in>s. path_component s x = s)"


410 
unfolding path_connected_component apply(rule, rule, rule, rule path_component_subset)


411 
unfolding subset_eq mem_path_component_set Ball_def mem_def by auto


412 


413 
subsection {* Some useful lemmas about pathconnectedness. *}


414 


415 
lemma convex_imp_path_connected:


416 
fixes s :: "'a::real_normed_vector set"


417 
assumes "convex s" shows "path_connected s"


418 
unfolding path_connected_def apply(rule,rule,rule_tac x="linepath x y" in exI)


419 
unfolding path_image_linepath using assms[unfolded convex_contains_segment] by auto


420 


421 
lemma path_connected_imp_connected: assumes "path_connected s" shows "connected s"


422 
unfolding connected_def not_ex apply(rule,rule,rule ccontr) unfolding not_not apply(erule conjE)+ proof


423 
fix e1 e2 assume as:"open e1" "open e2" "s \<subseteq> e1 \<union> e2" "e1 \<inter> e2 \<inter> s = {}" "e1 \<inter> s \<noteq> {}" "e2 \<inter> s \<noteq> {}"


424 
then obtain x1 x2 where obt:"x1\<in>e1\<inter>s" "x2\<in>e2\<inter>s" by auto


425 
then obtain g where g:"path g" "path_image g \<subseteq> s" "pathstart g = x1" "pathfinish g = x2"


426 
using assms[unfolded path_connected_def,rule_format,of x1 x2] by auto


427 
have *:"connected {0..1::real}" by(auto intro!: convex_connected convex_real_interval)


428 
have "{0..1} \<subseteq> {x \<in> {0..1}. g x \<in> e1} \<union> {x \<in> {0..1}. g x \<in> e2}" using as(3) g(2)[unfolded path_defs] by blast


429 
moreover have "{x \<in> {0..1}. g x \<in> e1} \<inter> {x \<in> {0..1}. g x \<in> e2} = {}" using as(4) g(2)[unfolded path_defs] unfolding subset_eq by auto


430 
moreover have "{x \<in> {0..1}. g x \<in> e1} \<noteq> {} \<and> {x \<in> {0..1}. g x \<in> e2} \<noteq> {}" using g(3,4)[unfolded path_defs] using obt


431 
by (simp add: ex_in_conv [symmetric], metis zero_le_one order_refl)


432 
ultimately show False using *[unfolded connected_local not_ex,rule_format, of "{x\<in>{0..1}. g x \<in> e1}" "{x\<in>{0..1}. g x \<in> e2}"]


433 
using continuous_open_in_preimage[OF g(1)[unfolded path_def] as(1)]


434 
using continuous_open_in_preimage[OF g(1)[unfolded path_def] as(2)] by auto qed


435 


436 
lemma open_path_component:


437 
fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)


438 
assumes "open s" shows "open(path_component s x)"


439 
unfolding open_contains_ball proof


440 
fix y assume as:"y \<in> path_component s x"


441 
hence "y\<in>s" apply apply(rule path_component_mem(2)) unfolding mem_def by auto


442 
then obtain e where e:"e>0" "ball y e \<subseteq> s" using assms[unfolded open_contains_ball] by auto


443 
show "\<exists>e>0. ball y e \<subseteq> path_component s x" apply(rule_tac x=e in exI) apply(rule,rule `e>0`,rule) unfolding mem_ball mem_path_component_set proof


444 
fix z assume "dist y z < e" thus "path_component s x z" apply(rule_tac path_component_trans[of _ _ y]) defer


445 
apply(rule path_component_of_subset[OF e(2)]) apply(rule convex_imp_path_connected[OF convex_ball, unfolded path_connected_component, rule_format]) using `e>0`


446 
using as[unfolded mem_def] by auto qed qed


447 


448 
lemma open_non_path_component:


449 
fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)


450 
assumes "open s" shows "open(s  path_component s x)"


451 
unfolding open_contains_ball proof


452 
fix y assume as:"y\<in>s  path_component s x"


453 
then obtain e where e:"e>0" "ball y e \<subseteq> s" using assms[unfolded open_contains_ball] by auto


454 
show "\<exists>e>0. ball y e \<subseteq> s  path_component s x" apply(rule_tac x=e in exI) apply(rule,rule `e>0`,rule,rule) defer proof(rule ccontr)


455 
fix z assume "z\<in>ball y e" "\<not> z \<notin> path_component s x"


456 
hence "y \<in> path_component s x" unfolding not_not mem_path_component_set using `e>0`


457 
apply apply(rule path_component_trans,assumption) apply(rule path_component_of_subset[OF e(2)])


458 
apply(rule convex_imp_path_connected[OF convex_ball, unfolded path_connected_component, rule_format]) by auto


459 
thus False using as by auto qed(insert e(2), auto) qed


460 


461 
lemma connected_open_path_connected:


462 
fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)


463 
assumes "open s" "connected s" shows "path_connected s"


464 
unfolding path_connected_component_set proof(rule,rule,rule path_component_subset, rule)


465 
fix x y assume "x \<in> s" "y \<in> s" show "y \<in> path_component s x" proof(rule ccontr)


466 
assume "y \<notin> path_component s x" moreover


467 
have "path_component s x \<inter> s \<noteq> {}" using `x\<in>s` path_component_eq_empty path_component_subset[of s x] by auto


468 
ultimately show False using `y\<in>s` open_non_path_component[OF assms(1)] open_path_component[OF assms(1)]


469 
using assms(2)[unfolded connected_def not_ex, rule_format, of"path_component s x" "s  path_component s x"] by auto


470 
qed qed


471 


472 
lemma path_connected_continuous_image:


473 
assumes "continuous_on s f" "path_connected s" shows "path_connected (f ` s)"


474 
unfolding path_connected_def proof(rule,rule)


475 
fix x' y' assume "x' \<in> f ` s" "y' \<in> f ` s"


476 
then obtain x y where xy:"x\<in>s" "y\<in>s" "x' = f x" "y' = f y" by auto


477 
guess g using assms(2)[unfolded path_connected_def,rule_format,OF xy(1,2)] ..


478 
thus "\<exists>g. path g \<and> path_image g \<subseteq> f ` s \<and> pathstart g = x' \<and> pathfinish g = y'"


479 
unfolding xy apply(rule_tac x="f \<circ> g" in exI) unfolding path_defs


480 
using assms(1) by(auto intro!: continuous_on_compose continuous_on_subset[of _ _ "g ` {0..1}"]) qed


481 


482 
lemma homeomorphic_path_connectedness:


483 
"s homeomorphic t \<Longrightarrow> (path_connected s \<longleftrightarrow> path_connected t)"


484 
unfolding homeomorphic_def homeomorphism_def apply(erule exEerule conjE)+ apply rule


485 
apply(drule_tac f=f in path_connected_continuous_image) prefer 3


486 
apply(drule_tac f=g in path_connected_continuous_image) by auto


487 


488 
lemma path_connected_empty: "path_connected {}"


489 
unfolding path_connected_def by auto


490 


491 
lemma path_connected_singleton: "path_connected {a}"


492 
unfolding path_connected_def pathstart_def pathfinish_def path_image_def


493 
apply (clarify, rule_tac x="\<lambda>x. a" in exI, simp add: image_constant_conv)


494 
apply (simp add: path_def continuous_on_const)


495 
done


496 


497 
lemma path_connected_Un: assumes "path_connected s" "path_connected t" "s \<inter> t \<noteq> {}"


498 
shows "path_connected (s \<union> t)" unfolding path_connected_component proof(rule,rule)


499 
fix x y assume as:"x \<in> s \<union> t" "y \<in> s \<union> t"


500 
from assms(3) obtain z where "z \<in> s \<inter> t" by auto


501 
thus "path_component (s \<union> t) x y" using as using assms(12)[unfolded path_connected_component] apply


502 
apply(erule_tac[!] UnE)+ apply(rule_tac[23] path_component_trans[of _ _ z])


503 
by(auto simp add:path_component_of_subset [OF Un_upper1] path_component_of_subset[OF Un_upper2]) qed


504 


505 
subsection {* sphere is pathconnected. *}


506 


507 
lemma path_connected_punctured_universe:


508 
assumes "2 \<le> CARD('n::finite)" shows "path_connected((UNIV::(real^'n) set)  {a})" proof


509 
obtain \<psi> where \<psi>:"bij_betw \<psi> {1..CARD('n)} (UNIV::'n set)" using ex_bij_betw_nat_finite_1[OF finite_UNIV] by auto


510 
let ?U = "UNIV::(real^'n) set" let ?u = "?U  {0}"


511 
let ?basis = "\<lambda>k. basis (\<psi> k)"


512 
let ?A = "\<lambda>k. {x::real^'n. \<exists>i\<in>{1..k}. inner (basis (\<psi> i)) x \<noteq> 0}"


513 
have "\<forall>k\<in>{2..CARD('n)}. path_connected (?A k)" proof


514 
have *:"\<And>k. ?A (Suc k) = {x. inner (?basis (Suc k)) x < 0} \<union> {x. inner (?basis (Suc k)) x > 0} \<union> ?A k" apply(rule set_ext,rule) defer


515 
apply(erule UnE)+ unfolding mem_Collect_eq apply(rule_tac[12] x="Suc k" in bexI)


516 
by(auto elim!: ballE simp add: not_less le_Suc_eq)


517 
fix k assume "k \<in> {2..CARD('n)}" thus "path_connected (?A k)" proof(induct k)


518 
case (Suc k) show ?case proof(cases "k = 1")


519 
case False from Suc have d:"k \<in> {1..CARD('n)}" "Suc k \<in> {1..CARD('n)}" by auto


520 
hence "\<psi> k \<noteq> \<psi> (Suc k)" using \<psi>[unfolded bij_betw_def inj_on_def, THEN conjunct1, THEN bspec[where x=k]] by auto


521 
hence **:"?basis k + ?basis (Suc k) \<in> {x. 0 < inner (?basis (Suc k)) x} \<inter> (?A k)"


522 
"?basis k  ?basis (Suc k) \<in> {x. 0 > inner (?basis (Suc k)) x} \<inter> ({x. 0 < inner (?basis (Suc k)) x} \<union> (?A k))" using d


523 
by(auto simp add: inner_basis intro!:bexI[where x=k])


524 
show ?thesis unfolding * Un_assoc apply(rule path_connected_Un) defer apply(rule path_connected_Un)


525 
prefer 5 apply(rule_tac[12] convex_imp_path_connected, rule convex_halfspace_lt, rule convex_halfspace_gt)


526 
apply(rule Suc(1)) using d ** False by auto


527 
next case True hence d:"1\<in>{1..CARD('n)}" "2\<in>{1..CARD('n)}" using Suc(2) by auto


528 
have ***:"Suc 1 = 2" by auto


529 
have **:"\<And>s t P Q. s \<union> t \<union> {x. P x \<or> Q x} = (s \<union> {x. P x}) \<union> (t \<union> {x. Q x})" by auto


530 
have nequals0I:"\<And>x A. x\<in>A \<Longrightarrow> A \<noteq> {}" by auto


531 
have "\<psi> 2 \<noteq> \<psi> (Suc 0)" using \<psi>[unfolded bij_betw_def inj_on_def, THEN conjunct1, THEN bspec[where x=2]] using assms by auto


532 
thus ?thesis unfolding * True unfolding ** neq_iff bex_disj_distrib apply 


533 
apply(rule path_connected_Un, rule_tac[12] path_connected_Un) defer 3 apply(rule_tac[14] convex_imp_path_connected)


534 
apply(rule_tac[5] x=" ?basis 1 + ?basis 2" in nequals0I)


535 
apply(rule_tac[6] x="?basis 1 + ?basis 2" in nequals0I)


536 
apply(rule_tac[7] x="?basis 1  ?basis 2" in nequals0I)


537 
using d unfolding *** by(auto intro!: convex_halfspace_gt convex_halfspace_lt, auto simp add: inner_basis)


538 
qed qed auto qed note lem = this


539 


540 
have ***:"\<And>x::real^'n. (\<exists>i\<in>{1..CARD('n)}. inner (basis (\<psi> i)) x \<noteq> 0) \<longleftrightarrow> (\<exists>i. inner (basis i) x \<noteq> 0)"


541 
apply rule apply(erule bexE) apply(rule_tac x="\<psi> i" in exI) defer apply(erule exE) proof


542 
fix x::"real^'n" and i assume as:"inner (basis i) x \<noteq> 0"


543 
have "i\<in>\<psi> ` {1..CARD('n)}" using \<psi>[unfolded bij_betw_def, THEN conjunct2] by auto


544 
then obtain j where "j\<in>{1..CARD('n)}" "\<psi> j = i" by auto


545 
thus "\<exists>i\<in>{1..CARD('n)}. inner (basis (\<psi> i)) x \<noteq> 0" apply(rule_tac x=j in bexI) using as by auto qed auto


546 
have *:"?U  {a} = (\<lambda>x. x + a) ` {x. x \<noteq> 0}" apply(rule set_ext) unfolding image_iff


547 
apply rule apply(rule_tac x="x  a" in bexI) by auto


548 
have **:"\<And>x::real^'n. x\<noteq>0 \<longleftrightarrow> (\<exists>i. inner (basis i) x \<noteq> 0)" unfolding Cart_eq by(auto simp add: inner_basis)


549 
show ?thesis unfolding * apply(rule path_connected_continuous_image) apply(rule continuous_on_intros)+


550 
unfolding ** apply(rule lem[THEN bspec[where x="CARD('n)"], unfolded ***]) using assms by auto qed


551 


552 
lemma path_connected_sphere: assumes "2 \<le> CARD('n::finite)" shows "path_connected {x::real^'n. norm(x  a) = r}" proof(cases "r\<le>0")


553 
case True thus ?thesis proof(cases "r=0")


554 
case False hence "{x::real^'n. norm(x  a) = r} = {}" using True by auto


555 
thus ?thesis using path_connected_empty by auto


556 
qed(auto intro!:path_connected_singleton) next


557 
case False hence *:"{x::real^'n. norm(x  a) = r} = (\<lambda>x. a + r *\<^sub>R x) ` {x. norm x = 1}" unfolding not_le apply apply(rule set_ext,rule)


558 
unfolding image_iff apply(rule_tac x="(1/r) *\<^sub>R (x  a)" in bexI) unfolding mem_Collect_eq norm_scaleR by (auto simp add: scaleR_right_diff_distrib)


559 
have **:"{x::real^'n. norm x = 1} = (\<lambda>x. (1/norm x) *\<^sub>R x) ` (UNIV  {0})" apply(rule set_ext,rule)


560 
unfolding image_iff apply(rule_tac x=x in bexI) unfolding mem_Collect_eq by(auto split:split_if_asm)


561 
have "continuous_on (UNIV  {0}) (\<lambda>x::real^'n. 1 / norm x)" unfolding o_def continuous_on_eq_continuous_within


562 
apply(rule, rule continuous_at_within_inv[unfolded o_def inverse_eq_divide]) apply(rule continuous_at_within)


563 
apply(rule continuous_at_norm[unfolded o_def]) by auto


564 
thus ?thesis unfolding * ** using path_connected_punctured_universe[OF assms]


565 
by(auto intro!: path_connected_continuous_image continuous_on_intros) qed


566 


567 
lemma connected_sphere: "2 \<le> CARD('n) \<Longrightarrow> connected {x::real^'n. norm(x  a) = r}"


568 
using path_connected_sphere path_connected_imp_connected by auto


569 


570 
end
