1 (* Title: HOL/Multivariate_Analysis/Path_Connected.thy
2 Author: Robert Himmelmann, TU Muenchen
5 header {* Continuous paths and path-connected sets *}
8 imports Convex_Euclidean_Space
11 subsection {* Paths. *}
14 path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
15 where "path g \<longleftrightarrow> continuous_on {0 .. 1} g"
18 pathstart :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"
19 where "pathstart g = g 0"
22 pathfinish :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a"
23 where "pathfinish g = g 1"
26 path_image :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> 'a set"
27 where "path_image g = g ` {0 .. 1}"
30 reversepath :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> (real \<Rightarrow> 'a)"
31 where "reversepath g = (\<lambda>x. g(1 - x))"
34 joinpaths :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> (real \<Rightarrow> 'a) \<Rightarrow> (real \<Rightarrow> 'a)"
36 where "g1 +++ g2 = (\<lambda>x. if x \<le> 1/2 then g1 (2 * x) else g2 (2 * x - 1))"
39 simple_path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
40 where "simple_path g \<longleftrightarrow>
41 (\<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)"
44 injective_path :: "(real \<Rightarrow> 'a::topological_space) \<Rightarrow> bool"
45 where "injective_path g \<longleftrightarrow> (\<forall>x\<in>{0..1}. \<forall>y\<in>{0..1}. g x = g y \<longrightarrow> x = y)"
47 subsection {* Some lemmas about these concepts. *}
49 lemma injective_imp_simple_path:
50 "injective_path g \<Longrightarrow> simple_path g"
51 unfolding injective_path_def simple_path_def by auto
53 lemma path_image_nonempty: "path_image g \<noteq> {}"
54 unfolding path_image_def image_is_empty interval_eq_empty by auto
56 lemma pathstart_in_path_image[intro]: "(pathstart g) \<in> path_image g"
57 unfolding pathstart_def path_image_def by auto
59 lemma pathfinish_in_path_image[intro]: "(pathfinish g) \<in> path_image g"
60 unfolding pathfinish_def path_image_def by auto
62 lemma connected_path_image[intro]: "path g \<Longrightarrow> connected(path_image g)"
63 unfolding path_def path_image_def
64 apply (erule connected_continuous_image)
65 by(rule convex_connected, rule convex_real_interval)
67 lemma compact_path_image[intro]: "path g \<Longrightarrow> compact(path_image g)"
68 unfolding path_def path_image_def
69 by (erule compact_continuous_image, rule compact_interval)
71 lemma reversepath_reversepath[simp]: "reversepath(reversepath g) = g"
72 unfolding reversepath_def by auto
74 lemma pathstart_reversepath[simp]: "pathstart(reversepath g) = pathfinish g"
75 unfolding pathstart_def reversepath_def pathfinish_def by auto
77 lemma pathfinish_reversepath[simp]: "pathfinish(reversepath g) = pathstart g"
78 unfolding pathstart_def reversepath_def pathfinish_def by auto
80 lemma pathstart_join[simp]: "pathstart(g1 +++ g2) = pathstart g1"
81 unfolding pathstart_def joinpaths_def pathfinish_def by auto
83 lemma pathfinish_join[simp]:"pathfinish(g1 +++ g2) = pathfinish g2"
84 unfolding pathstart_def joinpaths_def pathfinish_def by auto
86 lemma path_image_reversepath[simp]: "path_image(reversepath g) = path_image g" proof-
87 have *:"\<And>g. path_image(reversepath g) \<subseteq> path_image g"
88 unfolding path_image_def subset_eq reversepath_def Ball_def image_iff apply(rule,rule,erule bexE)
89 apply(rule_tac x="1 - xa" in bexI) by auto
90 show ?thesis using *[of g] *[of "reversepath g"] unfolding reversepath_reversepath by auto qed
92 lemma path_reversepath[simp]: "path(reversepath g) \<longleftrightarrow> path g" proof-
93 have *:"\<And>g. path g \<Longrightarrow> path(reversepath g)" unfolding path_def reversepath_def
94 apply(rule continuous_on_compose[unfolded o_def, of _ "\<lambda>x. 1 - x"])
95 apply(intro continuous_on_intros)
96 apply(rule continuous_on_subset[of "{0..1}"], assumption) by auto
97 show ?thesis using *[of "reversepath g"] *[of g] unfolding reversepath_reversepath by (rule iffI) qed
99 lemmas reversepath_simps = path_reversepath path_image_reversepath pathstart_reversepath pathfinish_reversepath
101 lemma path_join[simp]: assumes "pathfinish g1 = pathstart g2" shows "path (g1 +++ g2) \<longleftrightarrow> path g1 \<and> path g2"
102 unfolding path_def pathfinish_def pathstart_def apply rule defer apply(erule conjE) proof-
103 assume as:"continuous_on {0..1} (g1 +++ g2)"
104 have *:"g1 = (\<lambda>x. g1 (2 *\<^sub>R x)) \<circ> (\<lambda>x. (1/2) *\<^sub>R x)"
105 "g2 = (\<lambda>x. g2 (2 *\<^sub>R x - 1)) \<circ> (\<lambda>x. (1/2) *\<^sub>R (x + 1))"
106 unfolding o_def by (auto simp add: add_divide_distrib)
107 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}"
109 thus "continuous_on {0..1} g1 \<and> continuous_on {0..1} g2" apply -apply rule
110 apply(subst *) defer apply(subst *) apply (rule_tac[!] continuous_on_compose)
111 apply (intro continuous_on_intros) defer
112 apply (intro continuous_on_intros)
113 apply(rule_tac[!] continuous_on_eq[of _ "g1 +++ g2"]) defer prefer 3
114 apply(rule_tac[1-2] continuous_on_subset[of "{0 .. 1}"]) apply(rule as, assumption, rule as, assumption)
115 apply(rule) defer apply rule proof-
116 fix x assume "x \<in> op *\<^sub>R (1 / 2) ` {0::real..1}"
117 hence "x \<le> 1 / 2" unfolding image_iff by auto
118 thus "(g1 +++ g2) x = g1 (2 *\<^sub>R x)" unfolding joinpaths_def by auto next
119 fix x assume "x \<in> (\<lambda>x. (1 / 2) *\<^sub>R (x + 1)) ` {0::real..1}"
120 hence "x \<ge> 1 / 2" unfolding image_iff by auto
121 thus "(g1 +++ g2) x = g2 (2 *\<^sub>R x - 1)" proof(cases "x = 1 / 2")
122 case True hence "x = (1/2) *\<^sub>R 1" by auto
123 thus ?thesis unfolding joinpaths_def using assms[unfolded pathstart_def pathfinish_def] by (auto simp add: mult_ac)
124 qed (auto simp add:le_less joinpaths_def) qed
125 next assume as:"continuous_on {0..1} g1" "continuous_on {0..1} g2"
126 have *:"{0 .. 1::real} = {0.. (1/2)*\<^sub>R 1} \<union> {(1/2) *\<^sub>R 1 .. 1}" by auto
127 have **:"op *\<^sub>R 2 ` {0..(1 / 2) *\<^sub>R 1} = {0..1::real}" apply(rule set_eqI, rule) unfolding image_iff
128 defer apply(rule_tac x="(1/2)*\<^sub>R x" in bexI) by auto
129 have ***:"(\<lambda>x. 2 *\<^sub>R x - 1) ` {(1 / 2) *\<^sub>R 1..1} = {0..1::real}"
130 apply (auto simp add: image_def)
131 apply (rule_tac x="(x + 1) / 2" in bexI)
132 apply (auto simp add: add_divide_distrib)
134 show "continuous_on {0..1} (g1 +++ g2)" unfolding * apply(rule continuous_on_union) apply (rule closed_real_atLeastAtMost)+ proof-
135 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
136 unfolding o_def[THEN sym] apply(rule continuous_on_compose) apply (intro continuous_on_intros)
137 unfolding ** apply(rule as(1)) unfolding joinpaths_def by auto next
138 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
139 apply(rule continuous_on_compose) apply (intro continuous_on_intros)
140 unfolding *** o_def joinpaths_def apply(rule as(2)) using assms[unfolded pathstart_def pathfinish_def]
141 by (auto simp add: mult_ac) qed qed
143 lemma path_image_join_subset: "path_image(g1 +++ g2) \<subseteq> (path_image g1 \<union> path_image g2)" proof
144 fix x assume "x \<in> path_image (g1 +++ g2)"
145 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))"
146 unfolding path_image_def image_iff joinpaths_def by auto
147 thus "x \<in> path_image g1 \<union> path_image g2" apply(cases "y \<le> 1/2")
148 apply(rule_tac UnI1) defer apply(rule_tac UnI2) unfolding y(2) path_image_def using y(1)
149 by(auto intro!: imageI) qed
151 lemma subset_path_image_join:
152 assumes "path_image g1 \<subseteq> s" "path_image g2 \<subseteq> s" shows "path_image(g1 +++ g2) \<subseteq> s"
153 using path_image_join_subset[of g1 g2] and assms by auto
155 lemma path_image_join:
156 assumes "path g1" "path g2" "pathfinish g1 = pathstart g2"
157 shows "path_image(g1 +++ g2) = (path_image g1) \<union> (path_image g2)"
158 apply(rule, rule path_image_join_subset, rule) unfolding Un_iff proof(erule disjE)
159 fix x assume "x \<in> path_image g1"
160 then obtain y where y:"y\<in>{0..1}" "x = g1 y" unfolding path_image_def image_iff by auto
161 thus "x \<in> path_image (g1 +++ g2)" unfolding joinpaths_def path_image_def image_iff
162 apply(rule_tac x="(1/2) *\<^sub>R y" in bexI) by auto next
163 fix x assume "x \<in> path_image g2"
164 then obtain y where y:"y\<in>{0..1}" "x = g2 y" unfolding path_image_def image_iff by auto
165 then show "x \<in> path_image (g1 +++ g2)" unfolding joinpaths_def path_image_def image_iff
166 apply(rule_tac x="(1/2) *\<^sub>R (y + 1)" in bexI) using assms(3)[unfolded pathfinish_def pathstart_def]
167 by (auto simp add: add_divide_distrib) qed
169 lemma not_in_path_image_join:
170 assumes "x \<notin> path_image g1" "x \<notin> path_image g2" shows "x \<notin> path_image(g1 +++ g2)"
171 using assms and path_image_join_subset[of g1 g2] by auto
173 lemma simple_path_reversepath: assumes "simple_path g" shows "simple_path (reversepath g)"
174 using assms unfolding simple_path_def reversepath_def apply- apply(rule ballI)+
175 apply(erule_tac x="1-x" in ballE, erule_tac x="1-y" in ballE)
178 lemma simple_path_join_loop:
179 assumes "injective_path g1" "injective_path g2" "pathfinish g2 = pathstart g1"
180 "(path_image g1 \<inter> path_image g2) \<subseteq> {pathstart g1,pathstart g2}"
181 shows "simple_path(g1 +++ g2)"
182 unfolding simple_path_def proof((rule ballI)+, rule impI) let ?g = "g1 +++ g2"
183 note inj = assms(1,2)[unfolded injective_path_def, rule_format]
184 fix x y::"real" assume xy:"x \<in> {0..1}" "y \<in> {0..1}" "?g x = ?g y"
185 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)
186 assume as:"x \<le> 1 / 2" "y \<le> 1 / 2"
187 hence "g1 (2 *\<^sub>R x) = g1 (2 *\<^sub>R y)" using xy(3) unfolding joinpaths_def by auto
188 moreover have "2 *\<^sub>R x \<in> {0..1}" "2 *\<^sub>R y \<in> {0..1}" using xy(1,2) as
190 ultimately show ?thesis using inj(1)[of "2*\<^sub>R x" "2*\<^sub>R y"] by auto
191 next assume as:"x > 1 / 2" "y > 1 / 2"
192 hence "g2 (2 *\<^sub>R x - 1) = g2 (2 *\<^sub>R y - 1)" using xy(3) unfolding joinpaths_def by auto
193 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
194 ultimately show ?thesis using inj(2)[of "2*\<^sub>R x - 1" "2*\<^sub>R y - 1"] by auto
195 next assume as:"x \<le> 1 / 2" "y > 1 / 2"
196 hence "?g x \<in> path_image g1" "?g y \<in> path_image g2" unfolding path_image_def joinpaths_def
197 using xy(1,2) by auto
198 moreover have "?g y \<noteq> pathstart g2" using as(2) unfolding pathstart_def joinpaths_def
199 using inj(2)[of "2 *\<^sub>R y - 1" 0] and xy(2)
200 by (auto simp add: field_simps)
201 ultimately have *:"?g x = pathstart g1" using assms(4) unfolding xy(3) by auto
202 hence "x = 0" unfolding pathstart_def joinpaths_def using as(1) and xy(1)
203 using inj(1)[of "2 *\<^sub>R x" 0] by auto
204 moreover have "y = 1" using * unfolding xy(3) assms(3)[THEN sym]
205 unfolding joinpaths_def pathfinish_def using as(2) and xy(2)
206 using inj(2)[of "2 *\<^sub>R y - 1" 1] by auto
207 ultimately show ?thesis by auto
208 next assume as:"x > 1 / 2" "y \<le> 1 / 2"
209 hence "?g x \<in> path_image g2" "?g y \<in> path_image g1" unfolding path_image_def joinpaths_def
210 using xy(1,2) by auto
211 moreover have "?g x \<noteq> pathstart g2" using as(1) unfolding pathstart_def joinpaths_def
212 using inj(2)[of "2 *\<^sub>R x - 1" 0] and xy(1)
213 by (auto simp add: field_simps)
214 ultimately have *:"?g y = pathstart g1" using assms(4) unfolding xy(3) by auto
215 hence "y = 0" unfolding pathstart_def joinpaths_def using as(2) and xy(2)
216 using inj(1)[of "2 *\<^sub>R y" 0] by auto
217 moreover have "x = 1" using * unfolding xy(3)[THEN sym] assms(3)[THEN sym]
218 unfolding joinpaths_def pathfinish_def using as(1) and xy(1)
219 using inj(2)[of "2 *\<^sub>R x - 1" 1] by auto
220 ultimately show ?thesis by auto qed qed
222 lemma injective_path_join:
223 assumes "injective_path g1" "injective_path g2" "pathfinish g1 = pathstart g2"
224 "(path_image g1 \<inter> path_image g2) \<subseteq> {pathstart g2}"
225 shows "injective_path(g1 +++ g2)"
226 unfolding injective_path_def proof(rule,rule,rule) let ?g = "g1 +++ g2"
227 note inj = assms(1,2)[unfolded injective_path_def, rule_format]
228 fix x y assume xy:"x \<in> {0..1}" "y \<in> {0..1}" "(g1 +++ g2) x = (g1 +++ g2) y"
229 show "x = y" proof(cases "x \<le> 1/2", case_tac[!] "y \<le> 1/2", unfold not_le)
230 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
231 unfolding joinpaths_def by auto
232 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
233 unfolding joinpaths_def by auto
234 next assume as:"x \<le> 1 / 2" "y > 1 / 2"
235 hence "?g x \<in> path_image g1" "?g y \<in> path_image g2" unfolding path_image_def joinpaths_def
236 using xy(1,2) by auto
237 hence "?g x = pathfinish g1" "?g y = pathstart g2" using assms(4) unfolding assms(3) xy(3) by auto
238 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)
239 unfolding pathstart_def pathfinish_def joinpaths_def
241 next assume as:"x > 1 / 2" "y \<le> 1 / 2"
242 hence "?g x \<in> path_image g2" "?g y \<in> path_image g1" unfolding path_image_def joinpaths_def
243 using xy(1,2) by auto
244 hence "?g x = pathstart g2" "?g y = pathfinish g1" using assms(4) unfolding assms(3) xy(3) by auto
245 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)
246 unfolding pathstart_def pathfinish_def joinpaths_def
249 lemmas join_paths_simps = path_join path_image_join pathstart_join pathfinish_join
251 subsection {* Reparametrizing a closed curve to start at some chosen point. *}
253 definition "shiftpath a (f::real \<Rightarrow> 'a::topological_space) =
254 (\<lambda>x. if (a + x) \<le> 1 then f(a + x) else f(a + x - 1))"
256 lemma pathstart_shiftpath: "a \<le> 1 \<Longrightarrow> pathstart(shiftpath a g) = g a"
257 unfolding pathstart_def shiftpath_def by auto
259 lemma pathfinish_shiftpath: assumes "0 \<le> a" "pathfinish g = pathstart g"
260 shows "pathfinish(shiftpath a g) = g a"
261 using assms unfolding pathstart_def pathfinish_def shiftpath_def
264 lemma endpoints_shiftpath:
265 assumes "pathfinish g = pathstart g" "a \<in> {0 .. 1}"
266 shows "pathfinish(shiftpath a g) = g a" "pathstart(shiftpath a g) = g a"
267 using assms by(auto intro!:pathfinish_shiftpath pathstart_shiftpath)
269 lemma closed_shiftpath:
270 assumes "pathfinish g = pathstart g" "a \<in> {0..1}"
271 shows "pathfinish(shiftpath a g) = pathstart(shiftpath a g)"
272 using endpoints_shiftpath[OF assms] by auto
274 lemma path_shiftpath:
275 assumes "path g" "pathfinish g = pathstart g" "a \<in> {0..1}"
276 shows "path(shiftpath a g)" proof-
277 have *:"{0 .. 1} = {0 .. 1-a} \<union> {1-a .. 1}" using assms(3) by auto
278 have **:"\<And>x. x + a = 1 \<Longrightarrow> g (x + a - 1) = g (x + a)"
279 using assms(2)[unfolded pathfinish_def pathstart_def] by auto
280 show ?thesis unfolding path_def shiftpath_def * apply(rule continuous_on_union)
281 apply(rule closed_real_atLeastAtMost)+ apply(rule continuous_on_eq[of _ "g \<circ> (\<lambda>x. a + x)"]) prefer 3
282 apply(rule continuous_on_eq[of _ "g \<circ> (\<lambda>x. a - 1 + x)"]) defer prefer 3
283 apply(rule continuous_on_intros)+ prefer 2 apply(rule continuous_on_intros)+
284 apply(rule_tac[1-2] continuous_on_subset[OF assms(1)[unfolded path_def]])
285 using assms(3) and ** by(auto, auto simp add: field_simps) qed
287 lemma shiftpath_shiftpath: assumes "pathfinish g = pathstart g" "a \<in> {0..1}" "x \<in> {0..1}"
288 shows "shiftpath (1 - a) (shiftpath a g) x = g x"
289 using assms unfolding pathfinish_def pathstart_def shiftpath_def by auto
291 lemma path_image_shiftpath:
292 assumes "a \<in> {0..1}" "pathfinish g = pathstart g"
293 shows "path_image(shiftpath a g) = path_image g" proof-
294 { 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)"
295 hence "\<exists>y\<in>{0..1} \<inter> {x. a + x \<le> 1}. g x = g (a + y)" proof(cases "a \<le> x")
296 case False thus ?thesis apply(rule_tac x="1 + x - a" in bexI)
297 using as(1,2) and as(3)[THEN bspec[where x="1 + x - a"]] and assms(1)
298 by(auto simp add: field_simps atomize_not) next
299 case True thus ?thesis using as(1-2) and assms(1) apply(rule_tac x="x - a" in bexI)
300 by(auto simp add: field_simps) qed }
301 thus ?thesis using assms unfolding shiftpath_def path_image_def pathfinish_def pathstart_def
302 by(auto simp add: image_iff) qed
304 subsection {* Special case of straight-line paths. *}
307 linepath :: "'a::real_normed_vector \<Rightarrow> 'a \<Rightarrow> real \<Rightarrow> 'a" where
308 "linepath a b = (\<lambda>x. (1 - x) *\<^sub>R a + x *\<^sub>R b)"
310 lemma pathstart_linepath[simp]: "pathstart(linepath a b) = a"
311 unfolding pathstart_def linepath_def by auto
313 lemma pathfinish_linepath[simp]: "pathfinish(linepath a b) = b"
314 unfolding pathfinish_def linepath_def by auto
316 lemma continuous_linepath_at[intro]: "continuous (at x) (linepath a b)"
317 unfolding linepath_def by (intro continuous_intros)
319 lemma continuous_on_linepath[intro]: "continuous_on s (linepath a b)"
320 using continuous_linepath_at by(auto intro!: continuous_at_imp_continuous_on)
322 lemma path_linepath[intro]: "path(linepath a b)"
323 unfolding path_def by(rule continuous_on_linepath)
325 lemma path_image_linepath[simp]: "path_image(linepath a b) = (closed_segment a b)"
326 unfolding path_image_def segment linepath_def apply (rule set_eqI, rule) defer
327 unfolding mem_Collect_eq image_iff apply(erule exE) apply(rule_tac x="u *\<^sub>R 1" in bexI)
330 lemma reversepath_linepath[simp]: "reversepath(linepath a b) = linepath b a"
331 unfolding reversepath_def linepath_def by(rule ext, auto)
333 lemma injective_path_linepath:
334 assumes "a \<noteq> b" shows "injective_path(linepath a b)"
337 assume "x *\<^sub>R b + y *\<^sub>R a = x *\<^sub>R a + y *\<^sub>R b"
338 hence "(x - y) *\<^sub>R a = (x - y) *\<^sub>R b" by (simp add: algebra_simps)
339 with assms have "x = y" by simp }
340 thus ?thesis unfolding injective_path_def linepath_def by(auto simp add: algebra_simps) qed
342 lemma simple_path_linepath[intro]: "a \<noteq> b \<Longrightarrow> simple_path(linepath a b)" by(auto intro!: injective_imp_simple_path injective_path_linepath)
344 subsection {* Bounding a point away from a path. *}
346 lemma not_on_path_ball:
347 fixes g :: "real \<Rightarrow> 'a::heine_borel"
348 assumes "path g" "z \<notin> path_image g"
349 shows "\<exists>e>0. ball z e \<inter> (path_image g) = {}" proof-
350 obtain a where "a\<in>path_image g" "\<forall>y\<in>path_image g. dist z a \<le> dist z y"
351 using distance_attains_inf[OF _ path_image_nonempty, of g z]
352 using compact_path_image[THEN compact_imp_closed, OF assms(1)] by auto
353 thus ?thesis apply(rule_tac x="dist z a" in exI) using assms(2) by(auto intro!: dist_pos_lt) qed
355 lemma not_on_path_cball:
356 fixes g :: "real \<Rightarrow> 'a::heine_borel"
357 assumes "path g" "z \<notin> path_image g"
358 shows "\<exists>e>0. cball z e \<inter> (path_image g) = {}" proof-
359 obtain e where "ball z e \<inter> path_image g = {}" "e>0" using not_on_path_ball[OF assms] by auto
360 moreover have "cball z (e/2) \<subseteq> ball z e" using `e>0` by auto
361 ultimately show ?thesis apply(rule_tac x="e/2" in exI) by auto qed
363 subsection {* Path component, considered as a "joinability" relation (from Tom Hales). *}
365 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)"
367 lemmas path_defs = path_def pathstart_def pathfinish_def path_image_def path_component_def
369 lemma path_component_mem: assumes "path_component s x y" shows "x \<in> s" "y \<in> s"
370 using assms unfolding path_defs by auto
372 lemma path_component_refl: assumes "x \<in> s" shows "path_component s x x"
373 unfolding path_defs apply(rule_tac x="\<lambda>u. x" in exI) using assms
374 by(auto intro!:continuous_on_intros)
376 lemma path_component_refl_eq: "path_component s x x \<longleftrightarrow> x \<in> s"
377 by(auto intro!: path_component_mem path_component_refl)
379 lemma path_component_sym: "path_component s x y \<Longrightarrow> path_component s y x"
380 using assms unfolding path_component_def apply(erule exE) apply(rule_tac x="reversepath g" in exI)
383 lemma path_component_trans: assumes "path_component s x y" "path_component s y z" shows "path_component s x z"
384 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)
386 lemma path_component_of_subset: "s \<subseteq> t \<Longrightarrow> path_component s x y \<Longrightarrow> path_component t x y"
387 unfolding path_component_def by auto
389 subsection {* Can also consider it as a set, as the name suggests. *}
391 lemma path_component_set: "{y. path_component s x y} = { y. (\<exists>g. path g \<and> path_image g \<subseteq> s \<and> pathstart g = x \<and> pathfinish g = y )}"
392 apply(rule set_eqI) unfolding mem_Collect_eq unfolding path_component_def by auto
394 lemma path_component_subset: "{y. path_component s x y} \<subseteq> s"
395 apply(rule, rule path_component_mem(2)) by auto
397 lemma path_component_eq_empty: "{y. path_component s x y} = {} \<longleftrightarrow> x \<notin> s"
398 apply rule apply(drule equals0D[of _ x]) defer apply(rule equals0I) unfolding mem_Collect_eq
399 apply(drule path_component_mem(1)) using path_component_refl by auto
401 subsection {* Path connectedness of a space. *}
403 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 lemma path_connected_component: "path_connected s \<longleftrightarrow> (\<forall>x\<in>s. \<forall>y\<in>s. path_component s x y)"
406 unfolding path_connected_def path_component_def by auto
408 lemma path_connected_component_set: "path_connected s \<longleftrightarrow> (\<forall>x\<in>s. {y. path_component s x y} = s)"
409 unfolding path_connected_component apply(rule, rule, rule, rule path_component_subset)
410 unfolding subset_eq mem_Collect_eq Ball_def by auto
412 subsection {* Some useful lemmas about path-connectedness. *}
414 lemma convex_imp_path_connected:
415 fixes s :: "'a::real_normed_vector set"
416 assumes "convex s" shows "path_connected s"
417 unfolding path_connected_def apply(rule,rule,rule_tac x="linepath x y" in exI)
418 unfolding path_image_linepath using assms[unfolded convex_contains_segment] by auto
420 lemma path_connected_imp_connected: assumes "path_connected s" shows "connected s"
421 unfolding connected_def not_ex apply(rule,rule,rule ccontr) unfolding not_not apply(erule conjE)+ proof-
422 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> {}"
423 then obtain x1 x2 where obt:"x1\<in>e1\<inter>s" "x2\<in>e2\<inter>s" by auto
424 then obtain g where g:"path g" "path_image g \<subseteq> s" "pathstart g = x1" "pathfinish g = x2"
425 using assms[unfolded path_connected_def,rule_format,of x1 x2] by auto
426 have *:"connected {0..1::real}" by(auto intro!: convex_connected convex_real_interval)
427 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
428 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
429 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
430 by (simp add: ex_in_conv [symmetric], metis zero_le_one order_refl)
431 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}"]
432 using continuous_open_in_preimage[OF g(1)[unfolded path_def] as(1)]
433 using continuous_open_in_preimage[OF g(1)[unfolded path_def] as(2)] by auto qed
435 lemma open_path_component:
436 fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)
437 assumes "open s" shows "open {y. path_component s x y}"
438 unfolding open_contains_ball proof
439 fix y assume as:"y \<in> {y. path_component s x y}"
440 hence "y\<in>s" apply- apply(rule path_component_mem(2)) unfolding mem_Collect_eq by auto
441 then obtain e where e:"e>0" "ball y e \<subseteq> s" using assms[unfolded open_contains_ball] by auto
442 show "\<exists>e>0. ball y e \<subseteq> {y. path_component s x y}" apply(rule_tac x=e in exI) apply(rule,rule `e>0`,rule) unfolding mem_ball mem_Collect_eq proof-
443 fix z assume "dist y z < e" thus "path_component s x z" apply(rule_tac path_component_trans[of _ _ y]) defer
444 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`
445 using as by auto qed qed
447 lemma open_non_path_component:
448 fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)
449 assumes "open s" shows "open(s - {y. path_component s x y})"
450 unfolding open_contains_ball proof
451 fix y assume as:"y\<in>s - {y. path_component s x y}"
452 then obtain e where e:"e>0" "ball y e \<subseteq> s" using assms[unfolded open_contains_ball] by auto
453 show "\<exists>e>0. ball y e \<subseteq> s - {y. path_component s x y}" apply(rule_tac x=e in exI) apply(rule,rule `e>0`,rule,rule) defer proof(rule ccontr)
454 fix z assume "z\<in>ball y e" "\<not> z \<notin> {y. path_component s x y}"
455 hence "y \<in> {y. path_component s x y}" unfolding not_not mem_Collect_eq using `e>0`
456 apply- apply(rule path_component_trans,assumption) apply(rule path_component_of_subset[OF e(2)])
457 apply(rule convex_imp_path_connected[OF convex_ball, unfolded path_connected_component, rule_format]) by auto
458 thus False using as by auto qed(insert e(2), auto) qed
460 lemma connected_open_path_connected:
461 fixes s :: "'a::real_normed_vector set" (*TODO: generalize to metric_space*)
462 assumes "open s" "connected s" shows "path_connected s"
463 unfolding path_connected_component_set proof(rule,rule,rule path_component_subset, rule)
464 fix x y assume "x \<in> s" "y \<in> s" show "y \<in> {y. path_component s x y}" proof(rule ccontr)
465 assume "y \<notin> {y. path_component s x y}" moreover
466 have "{y. path_component s x y} \<inter> s \<noteq> {}" using `x\<in>s` path_component_eq_empty path_component_subset[of s x] by auto
467 ultimately show False using `y\<in>s` open_non_path_component[OF assms(1)] open_path_component[OF assms(1)]
468 using assms(2)[unfolded connected_def not_ex, rule_format, of"{y. path_component s x y}" "s - {y. path_component s x y}"] by auto
471 lemma path_connected_continuous_image:
472 assumes "continuous_on s f" "path_connected s" shows "path_connected (f ` s)"
473 unfolding path_connected_def proof(rule,rule)
474 fix x' y' assume "x' \<in> f ` s" "y' \<in> f ` s"
475 then obtain x y where xy:"x\<in>s" "y\<in>s" "x' = f x" "y' = f y" by auto
476 guess g using assms(2)[unfolded path_connected_def,rule_format,OF xy(1,2)] ..
477 thus "\<exists>g. path g \<and> path_image g \<subseteq> f ` s \<and> pathstart g = x' \<and> pathfinish g = y'"
478 unfolding xy apply(rule_tac x="f \<circ> g" in exI) unfolding path_defs
479 using assms(1) by(auto intro!: continuous_on_compose continuous_on_subset[of _ _ "g ` {0..1}"]) qed
481 lemma homeomorphic_path_connectedness:
482 "s homeomorphic t \<Longrightarrow> (path_connected s \<longleftrightarrow> path_connected t)"
483 unfolding homeomorphic_def homeomorphism_def apply(erule exE|erule conjE)+ apply rule
484 apply(drule_tac f=f in path_connected_continuous_image) prefer 3
485 apply(drule_tac f=g in path_connected_continuous_image) by auto
487 lemma path_connected_empty: "path_connected {}"
488 unfolding path_connected_def by auto
490 lemma path_connected_singleton: "path_connected {a}"
491 unfolding path_connected_def pathstart_def pathfinish_def path_image_def
492 apply (clarify, rule_tac x="\<lambda>x. a" in exI, simp add: image_constant_conv)
493 apply (simp add: path_def continuous_on_const)
496 lemma path_connected_Un: assumes "path_connected s" "path_connected t" "s \<inter> t \<noteq> {}"
497 shows "path_connected (s \<union> t)" unfolding path_connected_component proof(rule,rule)
498 fix x y assume as:"x \<in> s \<union> t" "y \<in> s \<union> t"
499 from assms(3) obtain z where "z \<in> s \<inter> t" by auto
500 thus "path_component (s \<union> t) x y" using as using assms(1-2)[unfolded path_connected_component] apply-
501 apply(erule_tac[!] UnE)+ apply(rule_tac[2-3] path_component_trans[of _ _ z])
502 by(auto simp add:path_component_of_subset [OF Un_upper1] path_component_of_subset[OF Un_upper2]) qed
504 lemma path_connected_UNION:
505 assumes "\<And>i. i \<in> A \<Longrightarrow> path_connected (S i)"
506 assumes "\<And>i. i \<in> A \<Longrightarrow> z \<in> S i"
507 shows "path_connected (\<Union>i\<in>A. S i)"
508 unfolding path_connected_component proof(clarify)
510 assume *: "i \<in> A" "x \<in> S i" "j \<in> A" "y \<in> S j"
511 hence "path_component (S i) x z" and "path_component (S j) z y"
512 using assms by (simp_all add: path_connected_component)
513 hence "path_component (\<Union>i\<in>A. S i) x z" and "path_component (\<Union>i\<in>A. S i) z y"
514 using *(1,3) by (auto elim!: path_component_of_subset [rotated])
515 thus "path_component (\<Union>i\<in>A. S i) x y"
516 by (rule path_component_trans)
519 subsection {* sphere is path-connected. *}
521 lemma path_connected_punctured_universe:
522 assumes "2 \<le> DIM('a::euclidean_space)"
523 shows "path_connected((UNIV::'a::euclidean_space set) - {a})"
525 let ?A = "{x::'a. \<exists>i\<in>{..<DIM('a)}. x $$ i < a $$ i}"
526 let ?B = "{x::'a. \<exists>i\<in>{..<DIM('a)}. a $$ i < x $$ i}"
528 have A: "path_connected ?A" unfolding Collect_bex_eq
529 proof (rule path_connected_UNION)
530 fix i assume "i \<in> {..<DIM('a)}"
531 thus "(\<chi>\<chi> i. a $$ i - 1) \<in> {x::'a. x $$ i < a $$ i}" by simp
532 show "path_connected {x. x $$ i < a $$ i}" unfolding euclidean_component_def
533 by (rule convex_imp_path_connected [OF convex_halfspace_lt])
535 have B: "path_connected ?B" unfolding Collect_bex_eq
536 proof (rule path_connected_UNION)
537 fix i assume "i \<in> {..<DIM('a)}"
538 thus "(\<chi>\<chi> i. a $$ i + 1) \<in> {x::'a. a $$ i < x $$ i}" by simp
539 show "path_connected {x. a $$ i < x $$ i}" unfolding euclidean_component_def
540 by (rule convex_imp_path_connected [OF convex_halfspace_gt])
542 from assms have "1 < DIM('a)" by auto
543 hence "a + basis 0 - basis 1 \<in> ?A \<inter> ?B" by auto
544 hence "?A \<inter> ?B \<noteq> {}" by fast
545 with A B have "path_connected (?A \<union> ?B)"
546 by (rule path_connected_Un)
547 also have "?A \<union> ?B = {x. \<exists>i\<in>{..<DIM('a)}. x $$ i \<noteq> a $$ i}"
548 unfolding neq_iff bex_disj_distrib Collect_disj_eq ..
549 also have "\<dots> = {x. x \<noteq> a}"
550 unfolding Bex_def euclidean_eq [where 'a='a] by simp
551 also have "\<dots> = UNIV - {a}" by auto
552 finally show ?thesis .
555 lemma path_connected_sphere:
556 assumes "2 \<le> DIM('a::euclidean_space)"
557 shows "path_connected {x::'a::euclidean_space. norm(x - a) = r}"
558 proof (rule linorder_cases [of r 0])
559 assume "r < 0" hence "{x::'a. norm(x - a) = r} = {}" by auto
560 thus ?thesis using path_connected_empty by simp
563 thus ?thesis using path_connected_singleton by simp
566 hence *:"{x::'a. norm(x - a) = r} = (\<lambda>x. a + r *\<^sub>R x) ` {x. norm x = 1}" apply -apply(rule set_eqI,rule)
567 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)
568 have **:"{x::'a. norm x = 1} = (\<lambda>x. (1/norm x) *\<^sub>R x) ` (UNIV - {0})" apply(rule set_eqI,rule)
569 unfolding image_iff apply(rule_tac x=x in bexI) unfolding mem_Collect_eq by(auto split:split_if_asm)
570 have "continuous_on (UNIV - {0}) (\<lambda>x::'a. 1 / norm x)"
571 unfolding field_divide_inverse by (simp add: continuous_on_intros)
572 thus ?thesis unfolding * ** using path_connected_punctured_universe[OF assms]
573 by(auto intro!: path_connected_continuous_image continuous_on_intros)
576 lemma connected_sphere: "2 \<le> DIM('a::euclidean_space) \<Longrightarrow> connected {x::'a. norm(x - a) = r}"
577 using path_connected_sphere path_connected_imp_connected by auto