1(* Title: HOL/Zorn.thy 2 Author: Jacques D. Fleuriot 3 Author: Tobias Nipkow, TUM 4 Author: Christian Sternagel, JAIST 5 6Zorn's Lemma (ported from Larry Paulson's Zorn.thy in ZF). 7The well-ordering theorem. 8*) 9 10section \<open>Zorn's Lemma\<close> 11 12theory Zorn 13 imports Order_Relation Hilbert_Choice 14begin 15 16subsection \<open>Zorn's Lemma for the Subset Relation\<close> 17 18subsubsection \<open>Results that do not require an order\<close> 19 20text \<open>Let \<open>P\<close> be a binary predicate on the set \<open>A\<close>.\<close> 21locale pred_on = 22 fixes A :: "'a set" 23 and P :: "'a \<Rightarrow> 'a \<Rightarrow> bool" (infix "\<sqsubset>" 50) 24begin 25 26abbreviation Peq :: "'a \<Rightarrow> 'a \<Rightarrow> bool" (infix "\<sqsubseteq>" 50) 27 where "x \<sqsubseteq> y \<equiv> P\<^sup>=\<^sup>= x y" 28 29text \<open>A chain is a totally ordered subset of \<open>A\<close>.\<close> 30definition chain :: "'a set \<Rightarrow> bool" 31 where "chain C \<longleftrightarrow> C \<subseteq> A \<and> (\<forall>x\<in>C. \<forall>y\<in>C. x \<sqsubseteq> y \<or> y \<sqsubseteq> x)" 32 33text \<open> 34 We call a chain that is a proper superset of some set \<open>X\<close>, 35 but not necessarily a chain itself, a superchain of \<open>X\<close>. 36\<close> 37abbreviation superchain :: "'a set \<Rightarrow> 'a set \<Rightarrow> bool" (infix "<c" 50) 38 where "X <c C \<equiv> chain C \<and> X \<subset> C" 39 40text \<open>A maximal chain is a chain that does not have a superchain.\<close> 41definition maxchain :: "'a set \<Rightarrow> bool" 42 where "maxchain C \<longleftrightarrow> chain C \<and> (\<nexists>S. C <c S)" 43 44text \<open> 45 We define the successor of a set to be an arbitrary 46 superchain, if such exists, or the set itself, otherwise. 47\<close> 48definition suc :: "'a set \<Rightarrow> 'a set" 49 where "suc C = (if \<not> chain C \<or> maxchain C then C else (SOME D. C <c D))" 50 51lemma chainI [Pure.intro?]: "C \<subseteq> A \<Longrightarrow> (\<And>x y. x \<in> C \<Longrightarrow> y \<in> C \<Longrightarrow> x \<sqsubseteq> y \<or> y \<sqsubseteq> x) \<Longrightarrow> chain C" 52 unfolding chain_def by blast 53 54lemma chain_total: "chain C \<Longrightarrow> x \<in> C \<Longrightarrow> y \<in> C \<Longrightarrow> x \<sqsubseteq> y \<or> y \<sqsubseteq> x" 55 by (simp add: chain_def) 56 57lemma not_chain_suc [simp]: "\<not> chain X \<Longrightarrow> suc X = X" 58 by (simp add: suc_def) 59 60lemma maxchain_suc [simp]: "maxchain X \<Longrightarrow> suc X = X" 61 by (simp add: suc_def) 62 63lemma suc_subset: "X \<subseteq> suc X" 64 by (auto simp: suc_def maxchain_def intro: someI2) 65 66lemma chain_empty [simp]: "chain {}" 67 by (auto simp: chain_def) 68 69lemma not_maxchain_Some: "chain C \<Longrightarrow> \<not> maxchain C \<Longrightarrow> C <c (SOME D. C <c D)" 70 by (rule someI_ex) (auto simp: maxchain_def) 71 72lemma suc_not_equals: "chain C \<Longrightarrow> \<not> maxchain C \<Longrightarrow> suc C \<noteq> C" 73 using not_maxchain_Some by (auto simp: suc_def) 74 75lemma subset_suc: 76 assumes "X \<subseteq> Y" 77 shows "X \<subseteq> suc Y" 78 using assms by (rule subset_trans) (rule suc_subset) 79 80text \<open> 81 We build a set @{term \<C>} that is closed under applications 82 of @{term suc} and contains the union of all its subsets. 83\<close> 84inductive_set suc_Union_closed ("\<C>") 85 where 86 suc: "X \<in> \<C> \<Longrightarrow> suc X \<in> \<C>" 87 | Union [unfolded Pow_iff]: "X \<in> Pow \<C> \<Longrightarrow> \<Union>X \<in> \<C>" 88 89text \<open> 90 Since the empty set as well as the set itself is a subset of 91 every set, @{term \<C>} contains at least @{term "{} \<in> \<C>"} and 92 @{term "\<Union>\<C> \<in> \<C>"}. 93\<close> 94lemma suc_Union_closed_empty: "{} \<in> \<C>" 95 and suc_Union_closed_Union: "\<Union>\<C> \<in> \<C>" 96 using Union [of "{}"] and Union [of "\<C>"] by simp_all 97 98text \<open>Thus closure under @{term suc} will hit a maximal chain 99 eventually, as is shown below.\<close> 100 101lemma suc_Union_closed_induct [consumes 1, case_names suc Union, induct pred: suc_Union_closed]: 102 assumes "X \<in> \<C>" 103 and "\<And>X. X \<in> \<C> \<Longrightarrow> Q X \<Longrightarrow> Q (suc X)" 104 and "\<And>X. X \<subseteq> \<C> \<Longrightarrow> \<forall>x\<in>X. Q x \<Longrightarrow> Q (\<Union>X)" 105 shows "Q X" 106 using assms by induct blast+ 107 108lemma suc_Union_closed_cases [consumes 1, case_names suc Union, cases pred: suc_Union_closed]: 109 assumes "X \<in> \<C>" 110 and "\<And>Y. X = suc Y \<Longrightarrow> Y \<in> \<C> \<Longrightarrow> Q" 111 and "\<And>Y. X = \<Union>Y \<Longrightarrow> Y \<subseteq> \<C> \<Longrightarrow> Q" 112 shows "Q" 113 using assms by cases simp_all 114 115text \<open>On chains, @{term suc} yields a chain.\<close> 116lemma chain_suc: 117 assumes "chain X" 118 shows "chain (suc X)" 119 using assms 120 by (cases "\<not> chain X \<or> maxchain X") (force simp: suc_def dest: not_maxchain_Some)+ 121 122lemma chain_sucD: 123 assumes "chain X" 124 shows "suc X \<subseteq> A \<and> chain (suc X)" 125proof - 126 from \<open>chain X\<close> have *: "chain (suc X)" 127 by (rule chain_suc) 128 then have "suc X \<subseteq> A" 129 unfolding chain_def by blast 130 with * show ?thesis by blast 131qed 132 133lemma suc_Union_closed_total': 134 assumes "X \<in> \<C>" and "Y \<in> \<C>" 135 and *: "\<And>Z. Z \<in> \<C> \<Longrightarrow> Z \<subseteq> Y \<Longrightarrow> Z = Y \<or> suc Z \<subseteq> Y" 136 shows "X \<subseteq> Y \<or> suc Y \<subseteq> X" 137 using \<open>X \<in> \<C>\<close> 138proof induct 139 case (suc X) 140 with * show ?case by (blast del: subsetI intro: subset_suc) 141next 142 case Union 143 then show ?case by blast 144qed 145 146lemma suc_Union_closed_subsetD: 147 assumes "Y \<subseteq> X" and "X \<in> \<C>" and "Y \<in> \<C>" 148 shows "X = Y \<or> suc Y \<subseteq> X" 149 using assms(2,3,1) 150proof (induct arbitrary: Y) 151 case (suc X) 152 note * = \<open>\<And>Y. Y \<in> \<C> \<Longrightarrow> Y \<subseteq> X \<Longrightarrow> X = Y \<or> suc Y \<subseteq> X\<close> 153 with suc_Union_closed_total' [OF \<open>Y \<in> \<C>\<close> \<open>X \<in> \<C>\<close>] 154 have "Y \<subseteq> X \<or> suc X \<subseteq> Y" by blast 155 then show ?case 156 proof 157 assume "Y \<subseteq> X" 158 with * and \<open>Y \<in> \<C>\<close> have "X = Y \<or> suc Y \<subseteq> X" by blast 159 then show ?thesis 160 proof 161 assume "X = Y" 162 then show ?thesis by simp 163 next 164 assume "suc Y \<subseteq> X" 165 then have "suc Y \<subseteq> suc X" by (rule subset_suc) 166 then show ?thesis by simp 167 qed 168 next 169 assume "suc X \<subseteq> Y" 170 with \<open>Y \<subseteq> suc X\<close> show ?thesis by blast 171 qed 172next 173 case (Union X) 174 show ?case 175 proof (rule ccontr) 176 assume "\<not> ?thesis" 177 with \<open>Y \<subseteq> \<Union>X\<close> obtain x y z 178 where "\<not> suc Y \<subseteq> \<Union>X" 179 and "x \<in> X" and "y \<in> x" and "y \<notin> Y" 180 and "z \<in> suc Y" and "\<forall>x\<in>X. z \<notin> x" by blast 181 with \<open>X \<subseteq> \<C>\<close> have "x \<in> \<C>" by blast 182 from Union and \<open>x \<in> X\<close> have *: "\<And>y. y \<in> \<C> \<Longrightarrow> y \<subseteq> x \<Longrightarrow> x = y \<or> suc y \<subseteq> x" 183 by blast 184 with suc_Union_closed_total' [OF \<open>Y \<in> \<C>\<close> \<open>x \<in> \<C>\<close>] have "Y \<subseteq> x \<or> suc x \<subseteq> Y" 185 by blast 186 then show False 187 proof 188 assume "Y \<subseteq> x" 189 with * [OF \<open>Y \<in> \<C>\<close>] have "x = Y \<or> suc Y \<subseteq> x" by blast 190 then show False 191 proof 192 assume "x = Y" 193 with \<open>y \<in> x\<close> and \<open>y \<notin> Y\<close> show False by blast 194 next 195 assume "suc Y \<subseteq> x" 196 with \<open>x \<in> X\<close> have "suc Y \<subseteq> \<Union>X" by blast 197 with \<open>\<not> suc Y \<subseteq> \<Union>X\<close> show False by contradiction 198 qed 199 next 200 assume "suc x \<subseteq> Y" 201 moreover from suc_subset and \<open>y \<in> x\<close> have "y \<in> suc x" by blast 202 ultimately show False using \<open>y \<notin> Y\<close> by blast 203 qed 204 qed 205qed 206 207text \<open>The elements of @{term \<C>} are totally ordered by the subset relation.\<close> 208lemma suc_Union_closed_total: 209 assumes "X \<in> \<C>" and "Y \<in> \<C>" 210 shows "X \<subseteq> Y \<or> Y \<subseteq> X" 211proof (cases "\<forall>Z\<in>\<C>. Z \<subseteq> Y \<longrightarrow> Z = Y \<or> suc Z \<subseteq> Y") 212 case True 213 with suc_Union_closed_total' [OF assms] 214 have "X \<subseteq> Y \<or> suc Y \<subseteq> X" by blast 215 with suc_subset [of Y] show ?thesis by blast 216next 217 case False 218 then obtain Z where "Z \<in> \<C>" and "Z \<subseteq> Y" and "Z \<noteq> Y" and "\<not> suc Z \<subseteq> Y" 219 by blast 220 with suc_Union_closed_subsetD and \<open>Y \<in> \<C>\<close> show ?thesis 221 by blast 222qed 223 224text \<open>Once we hit a fixed point w.r.t. @{term suc}, all other elements 225 of @{term \<C>} are subsets of this fixed point.\<close> 226lemma suc_Union_closed_suc: 227 assumes "X \<in> \<C>" and "Y \<in> \<C>" and "suc Y = Y" 228 shows "X \<subseteq> Y" 229 using \<open>X \<in> \<C>\<close> 230proof induct 231 case (suc X) 232 with \<open>Y \<in> \<C>\<close> and suc_Union_closed_subsetD have "X = Y \<or> suc X \<subseteq> Y" 233 by blast 234 then show ?case 235 by (auto simp: \<open>suc Y = Y\<close>) 236next 237 case Union 238 then show ?case by blast 239qed 240 241lemma eq_suc_Union: 242 assumes "X \<in> \<C>" 243 shows "suc X = X \<longleftrightarrow> X = \<Union>\<C>" 244 (is "?lhs \<longleftrightarrow> ?rhs") 245proof 246 assume ?lhs 247 then have "\<Union>\<C> \<subseteq> X" 248 by (rule suc_Union_closed_suc [OF suc_Union_closed_Union \<open>X \<in> \<C>\<close>]) 249 with \<open>X \<in> \<C>\<close> show ?rhs 250 by blast 251next 252 from \<open>X \<in> \<C>\<close> have "suc X \<in> \<C>" by (rule suc) 253 then have "suc X \<subseteq> \<Union>\<C>" by blast 254 moreover assume ?rhs 255 ultimately have "suc X \<subseteq> X" by simp 256 moreover have "X \<subseteq> suc X" by (rule suc_subset) 257 ultimately show ?lhs .. 258qed 259 260lemma suc_in_carrier: 261 assumes "X \<subseteq> A" 262 shows "suc X \<subseteq> A" 263 using assms 264 by (cases "\<not> chain X \<or> maxchain X") (auto dest: chain_sucD) 265 266lemma suc_Union_closed_in_carrier: 267 assumes "X \<in> \<C>" 268 shows "X \<subseteq> A" 269 using assms 270 by induct (auto dest: suc_in_carrier) 271 272text \<open>All elements of @{term \<C>} are chains.\<close> 273lemma suc_Union_closed_chain: 274 assumes "X \<in> \<C>" 275 shows "chain X" 276 using assms 277proof induct 278 case (suc X) 279 then show ?case 280 using not_maxchain_Some by (simp add: suc_def) 281next 282 case (Union X) 283 then have "\<Union>X \<subseteq> A" 284 by (auto dest: suc_Union_closed_in_carrier) 285 moreover have "\<forall>x\<in>\<Union>X. \<forall>y\<in>\<Union>X. x \<sqsubseteq> y \<or> y \<sqsubseteq> x" 286 proof (intro ballI) 287 fix x y 288 assume "x \<in> \<Union>X" and "y \<in> \<Union>X" 289 then obtain u v where "x \<in> u" and "u \<in> X" and "y \<in> v" and "v \<in> X" 290 by blast 291 with Union have "u \<in> \<C>" and "v \<in> \<C>" and "chain u" and "chain v" 292 by blast+ 293 with suc_Union_closed_total have "u \<subseteq> v \<or> v \<subseteq> u" 294 by blast 295 then show "x \<sqsubseteq> y \<or> y \<sqsubseteq> x" 296 proof 297 assume "u \<subseteq> v" 298 from \<open>chain v\<close> show ?thesis 299 proof (rule chain_total) 300 show "y \<in> v" by fact 301 show "x \<in> v" using \<open>u \<subseteq> v\<close> and \<open>x \<in> u\<close> by blast 302 qed 303 next 304 assume "v \<subseteq> u" 305 from \<open>chain u\<close> show ?thesis 306 proof (rule chain_total) 307 show "x \<in> u" by fact 308 show "y \<in> u" using \<open>v \<subseteq> u\<close> and \<open>y \<in> v\<close> by blast 309 qed 310 qed 311 qed 312 ultimately show ?case unfolding chain_def .. 313qed 314 315subsubsection \<open>Hausdorff's Maximum Principle\<close> 316 317text \<open>There exists a maximal totally ordered subset of \<open>A\<close>. (Note that we do not 318 require \<open>A\<close> to be partially ordered.)\<close> 319 320theorem Hausdorff: "\<exists>C. maxchain C" 321proof - 322 let ?M = "\<Union>\<C>" 323 have "maxchain ?M" 324 proof (rule ccontr) 325 assume "\<not> ?thesis" 326 then have "suc ?M \<noteq> ?M" 327 using suc_not_equals and suc_Union_closed_chain [OF suc_Union_closed_Union] by simp 328 moreover have "suc ?M = ?M" 329 using eq_suc_Union [OF suc_Union_closed_Union] by simp 330 ultimately show False by contradiction 331 qed 332 then show ?thesis by blast 333qed 334 335text \<open>Make notation @{term \<C>} available again.\<close> 336no_notation suc_Union_closed ("\<C>") 337 338lemma chain_extend: "chain C \<Longrightarrow> z \<in> A \<Longrightarrow> \<forall>x\<in>C. x \<sqsubseteq> z \<Longrightarrow> chain ({z} \<union> C)" 339 unfolding chain_def by blast 340 341lemma maxchain_imp_chain: "maxchain C \<Longrightarrow> chain C" 342 by (simp add: maxchain_def) 343 344end 345 346text \<open>Hide constant @{const pred_on.suc_Union_closed}, which was just needed 347 for the proof of Hausforff's maximum principle.\<close> 348hide_const pred_on.suc_Union_closed 349 350lemma chain_mono: 351 assumes "\<And>x y. x \<in> A \<Longrightarrow> y \<in> A \<Longrightarrow> P x y \<Longrightarrow> Q x y" 352 and "pred_on.chain A P C" 353 shows "pred_on.chain A Q C" 354 using assms unfolding pred_on.chain_def by blast 355 356 357subsubsection \<open>Results for the proper subset relation\<close> 358 359interpretation subset: pred_on "A" "(\<subset>)" for A . 360 361lemma subset_maxchain_max: 362 assumes "subset.maxchain A C" 363 and "X \<in> A" 364 and "\<Union>C \<subseteq> X" 365 shows "\<Union>C = X" 366proof (rule ccontr) 367 let ?C = "{X} \<union> C" 368 from \<open>subset.maxchain A C\<close> have "subset.chain A C" 369 and *: "\<And>S. subset.chain A S \<Longrightarrow> \<not> C \<subset> S" 370 by (auto simp: subset.maxchain_def) 371 moreover have "\<forall>x\<in>C. x \<subseteq> X" using \<open>\<Union>C \<subseteq> X\<close> by auto 372 ultimately have "subset.chain A ?C" 373 using subset.chain_extend [of A C X] and \<open>X \<in> A\<close> by auto 374 moreover assume **: "\<Union>C \<noteq> X" 375 moreover from ** have "C \<subset> ?C" using \<open>\<Union>C \<subseteq> X\<close> by auto 376 ultimately show False using * by blast 377qed 378 379 380subsubsection \<open>Zorn's lemma\<close> 381 382text \<open>If every chain has an upper bound, then there is a maximal set.\<close> 383lemma subset_Zorn: 384 assumes "\<And>C. subset.chain A C \<Longrightarrow> \<exists>U\<in>A. \<forall>X\<in>C. X \<subseteq> U" 385 shows "\<exists>M\<in>A. \<forall>X\<in>A. M \<subseteq> X \<longrightarrow> X = M" 386proof - 387 from subset.Hausdorff [of A] obtain M where "subset.maxchain A M" .. 388 then have "subset.chain A M" 389 by (rule subset.maxchain_imp_chain) 390 with assms obtain Y where "Y \<in> A" and "\<forall>X\<in>M. X \<subseteq> Y" 391 by blast 392 moreover have "\<forall>X\<in>A. Y \<subseteq> X \<longrightarrow> Y = X" 393 proof (intro ballI impI) 394 fix X 395 assume "X \<in> A" and "Y \<subseteq> X" 396 show "Y = X" 397 proof (rule ccontr) 398 assume "\<not> ?thesis" 399 with \<open>Y \<subseteq> X\<close> have "\<not> X \<subseteq> Y" by blast 400 from subset.chain_extend [OF \<open>subset.chain A M\<close> \<open>X \<in> A\<close>] and \<open>\<forall>X\<in>M. X \<subseteq> Y\<close> 401 have "subset.chain A ({X} \<union> M)" 402 using \<open>Y \<subseteq> X\<close> by auto 403 moreover have "M \<subset> {X} \<union> M" 404 using \<open>\<forall>X\<in>M. X \<subseteq> Y\<close> and \<open>\<not> X \<subseteq> Y\<close> by auto 405 ultimately show False 406 using \<open>subset.maxchain A M\<close> by (auto simp: subset.maxchain_def) 407 qed 408 qed 409 ultimately show ?thesis by blast 410qed 411 412text \<open>Alternative version of Zorn's lemma for the subset relation.\<close> 413lemma subset_Zorn': 414 assumes "\<And>C. subset.chain A C \<Longrightarrow> \<Union>C \<in> A" 415 shows "\<exists>M\<in>A. \<forall>X\<in>A. M \<subseteq> X \<longrightarrow> X = M" 416proof - 417 from subset.Hausdorff [of A] obtain M where "subset.maxchain A M" .. 418 then have "subset.chain A M" 419 by (rule subset.maxchain_imp_chain) 420 with assms have "\<Union>M \<in> A" . 421 moreover have "\<forall>Z\<in>A. \<Union>M \<subseteq> Z \<longrightarrow> \<Union>M = Z" 422 proof (intro ballI impI) 423 fix Z 424 assume "Z \<in> A" and "\<Union>M \<subseteq> Z" 425 with subset_maxchain_max [OF \<open>subset.maxchain A M\<close>] 426 show "\<Union>M = Z" . 427 qed 428 ultimately show ?thesis by blast 429qed 430 431 432subsection \<open>Zorn's Lemma for Partial Orders\<close> 433 434text \<open>Relate old to new definitions.\<close> 435 436definition chain_subset :: "'a set set \<Rightarrow> bool" ("chain\<^sub>\<subseteq>") (* Define globally? In Set.thy? *) 437 where "chain\<^sub>\<subseteq> C \<longleftrightarrow> (\<forall>A\<in>C. \<forall>B\<in>C. A \<subseteq> B \<or> B \<subseteq> A)" 438 439definition chains :: "'a set set \<Rightarrow> 'a set set set" 440 where "chains A = {C. C \<subseteq> A \<and> chain\<^sub>\<subseteq> C}" 441 442definition Chains :: "('a \<times> 'a) set \<Rightarrow> 'a set set" (* Define globally? In Relation.thy? *) 443 where "Chains r = {C. \<forall>a\<in>C. \<forall>b\<in>C. (a, b) \<in> r \<or> (b, a) \<in> r}" 444 445lemma chains_extend: "c \<in> chains S \<Longrightarrow> z \<in> S \<Longrightarrow> \<forall>x \<in> c. x \<subseteq> z \<Longrightarrow> {z} \<union> c \<in> chains S" 446 for z :: "'a set" 447 unfolding chains_def chain_subset_def by blast 448 449lemma mono_Chains: "r \<subseteq> s \<Longrightarrow> Chains r \<subseteq> Chains s" 450 unfolding Chains_def by blast 451 452lemma chain_subset_alt_def: "chain\<^sub>\<subseteq> C = subset.chain UNIV C" 453 unfolding chain_subset_def subset.chain_def by fast 454 455lemma chains_alt_def: "chains A = {C. subset.chain A C}" 456 by (simp add: chains_def chain_subset_alt_def subset.chain_def) 457 458lemma Chains_subset: "Chains r \<subseteq> {C. pred_on.chain UNIV (\<lambda>x y. (x, y) \<in> r) C}" 459 by (force simp add: Chains_def pred_on.chain_def) 460 461lemma Chains_subset': 462 assumes "refl r" 463 shows "{C. pred_on.chain UNIV (\<lambda>x y. (x, y) \<in> r) C} \<subseteq> Chains r" 464 using assms 465 by (auto simp add: Chains_def pred_on.chain_def refl_on_def) 466 467lemma Chains_alt_def: 468 assumes "refl r" 469 shows "Chains r = {C. pred_on.chain UNIV (\<lambda>x y. (x, y) \<in> r) C}" 470 using assms Chains_subset Chains_subset' by blast 471 472lemma pairwise_chain_Union: 473 assumes P: "\<And>S. S \<in> \<C> \<Longrightarrow> pairwise R S" and "chain\<^sub>\<subseteq> \<C>" 474 shows "pairwise R (\<Union>\<C>)" 475 using \<open>chain\<^sub>\<subseteq> \<C>\<close> unfolding pairwise_def chain_subset_def 476 by (blast intro: P [unfolded pairwise_def, rule_format]) 477 478lemma Zorn_Lemma: "\<forall>C\<in>chains A. \<Union>C \<in> A \<Longrightarrow> \<exists>M\<in>A. \<forall>X\<in>A. M \<subseteq> X \<longrightarrow> X = M" 479 using subset_Zorn' [of A] by (force simp: chains_alt_def) 480 481lemma Zorn_Lemma2: "\<forall>C\<in>chains A. \<exists>U\<in>A. \<forall>X\<in>C. X \<subseteq> U \<Longrightarrow> \<exists>M\<in>A. \<forall>X\<in>A. M \<subseteq> X \<longrightarrow> X = M" 482 using subset_Zorn [of A] by (auto simp: chains_alt_def) 483 484text \<open>Various other lemmas\<close> 485 486lemma chainsD: "c \<in> chains S \<Longrightarrow> x \<in> c \<Longrightarrow> y \<in> c \<Longrightarrow> x \<subseteq> y \<or> y \<subseteq> x" 487 unfolding chains_def chain_subset_def by blast 488 489lemma chainsD2: "c \<in> chains S \<Longrightarrow> c \<subseteq> S" 490 unfolding chains_def by blast 491 492lemma Zorns_po_lemma: 493 assumes po: "Partial_order r" 494 and u: "\<forall>C\<in>Chains r. \<exists>u\<in>Field r. \<forall>a\<in>C. (a, u) \<in> r" 495 shows "\<exists>m\<in>Field r. \<forall>a\<in>Field r. (m, a) \<in> r \<longrightarrow> a = m" 496proof - 497 have "Preorder r" 498 using po by (simp add: partial_order_on_def) 499 txt \<open>Mirror \<open>r\<close> in the set of subsets below (wrt \<open>r\<close>) elements of \<open>A\<close>.\<close> 500 let ?B = "\<lambda>x. r\<inverse> `` {x}" 501 let ?S = "?B ` Field r" 502 have "\<exists>u\<in>Field r. \<forall>A\<in>C. A \<subseteq> r\<inverse> `` {u}" (is "\<exists>u\<in>Field r. ?P u") 503 if 1: "C \<subseteq> ?S" and 2: "\<forall>A\<in>C. \<forall>B\<in>C. A \<subseteq> B \<or> B \<subseteq> A" for C 504 proof - 505 let ?A = "{x\<in>Field r. \<exists>M\<in>C. M = ?B x}" 506 from 1 have "C = ?B ` ?A" by (auto simp: image_def) 507 have "?A \<in> Chains r" 508 proof (simp add: Chains_def, intro allI impI, elim conjE) 509 fix a b 510 assume "a \<in> Field r" and "?B a \<in> C" and "b \<in> Field r" and "?B b \<in> C" 511 with 2 have "?B a \<subseteq> ?B b \<or> ?B b \<subseteq> ?B a" by auto 512 then show "(a, b) \<in> r \<or> (b, a) \<in> r" 513 using \<open>Preorder r\<close> and \<open>a \<in> Field r\<close> and \<open>b \<in> Field r\<close> 514 by (simp add:subset_Image1_Image1_iff) 515 qed 516 with u obtain u where uA: "u \<in> Field r" "\<forall>a\<in>?A. (a, u) \<in> r" by auto 517 have "?P u" 518 proof auto 519 fix a B assume aB: "B \<in> C" "a \<in> B" 520 with 1 obtain x where "x \<in> Field r" and "B = r\<inverse> `` {x}" by auto 521 then show "(a, u) \<in> r" 522 using uA and aB and \<open>Preorder r\<close> 523 unfolding preorder_on_def refl_on_def by simp (fast dest: transD) 524 qed 525 then show ?thesis 526 using \<open>u \<in> Field r\<close> by blast 527 qed 528 then have "\<forall>C\<in>chains ?S. \<exists>U\<in>?S. \<forall>A\<in>C. A \<subseteq> U" 529 by (auto simp: chains_def chain_subset_def) 530 from Zorn_Lemma2 [OF this] obtain m B 531 where "m \<in> Field r" 532 and "B = r\<inverse> `` {m}" 533 and "\<forall>x\<in>Field r. B \<subseteq> r\<inverse> `` {x} \<longrightarrow> r\<inverse> `` {x} = B" 534 by auto 535 then have "\<forall>a\<in>Field r. (m, a) \<in> r \<longrightarrow> a = m" 536 using po and \<open>Preorder r\<close> and \<open>m \<in> Field r\<close> 537 by (auto simp: subset_Image1_Image1_iff Partial_order_eq_Image1_Image1_iff) 538 then show ?thesis 539 using \<open>m \<in> Field r\<close> by blast 540qed 541 542 543subsection \<open>The Well Ordering Theorem\<close> 544 545(* The initial segment of a relation appears generally useful. 546 Move to Relation.thy? 547 Definition correct/most general? 548 Naming? 549*) 550definition init_seg_of :: "(('a \<times> 'a) set \<times> ('a \<times> 'a) set) set" 551 where "init_seg_of = {(r, s). r \<subseteq> s \<and> (\<forall>a b c. (a, b) \<in> s \<and> (b, c) \<in> r \<longrightarrow> (a, b) \<in> r)}" 552 553abbreviation initial_segment_of_syntax :: "('a \<times> 'a) set \<Rightarrow> ('a \<times> 'a) set \<Rightarrow> bool" 554 (infix "initial'_segment'_of" 55) 555 where "r initial_segment_of s \<equiv> (r, s) \<in> init_seg_of" 556 557lemma refl_on_init_seg_of [simp]: "r initial_segment_of r" 558 by (simp add: init_seg_of_def) 559 560lemma trans_init_seg_of: 561 "r initial_segment_of s \<Longrightarrow> s initial_segment_of t \<Longrightarrow> r initial_segment_of t" 562 by (simp (no_asm_use) add: init_seg_of_def) blast 563 564lemma antisym_init_seg_of: "r initial_segment_of s \<Longrightarrow> s initial_segment_of r \<Longrightarrow> r = s" 565 unfolding init_seg_of_def by safe 566 567lemma Chains_init_seg_of_Union: "R \<in> Chains init_seg_of \<Longrightarrow> r\<in>R \<Longrightarrow> r initial_segment_of \<Union>R" 568 by (auto simp: init_seg_of_def Ball_def Chains_def) blast 569 570lemma chain_subset_trans_Union: 571 assumes "chain\<^sub>\<subseteq> R" "\<forall>r\<in>R. trans r" 572 shows "trans (\<Union>R)" 573proof (intro transI, elim UnionE) 574 fix S1 S2 :: "'a rel" and x y z :: 'a 575 assume "S1 \<in> R" "S2 \<in> R" 576 with assms(1) have "S1 \<subseteq> S2 \<or> S2 \<subseteq> S1" 577 unfolding chain_subset_def by blast 578 moreover assume "(x, y) \<in> S1" "(y, z) \<in> S2" 579 ultimately have "((x, y) \<in> S1 \<and> (y, z) \<in> S1) \<or> ((x, y) \<in> S2 \<and> (y, z) \<in> S2)" 580 by blast 581 with \<open>S1 \<in> R\<close> \<open>S2 \<in> R\<close> assms(2) show "(x, z) \<in> \<Union>R" 582 by (auto elim: transE) 583qed 584 585lemma chain_subset_antisym_Union: 586 assumes "chain\<^sub>\<subseteq> R" "\<forall>r\<in>R. antisym r" 587 shows "antisym (\<Union>R)" 588proof (intro antisymI, elim UnionE) 589 fix S1 S2 :: "'a rel" and x y :: 'a 590 assume "S1 \<in> R" "S2 \<in> R" 591 with assms(1) have "S1 \<subseteq> S2 \<or> S2 \<subseteq> S1" 592 unfolding chain_subset_def by blast 593 moreover assume "(x, y) \<in> S1" "(y, x) \<in> S2" 594 ultimately have "((x, y) \<in> S1 \<and> (y, x) \<in> S1) \<or> ((x, y) \<in> S2 \<and> (y, x) \<in> S2)" 595 by blast 596 with \<open>S1 \<in> R\<close> \<open>S2 \<in> R\<close> assms(2) show "x = y" 597 unfolding antisym_def by auto 598qed 599 600lemma chain_subset_Total_Union: 601 assumes "chain\<^sub>\<subseteq> R" and "\<forall>r\<in>R. Total r" 602 shows "Total (\<Union>R)" 603proof (simp add: total_on_def Ball_def, auto del: disjCI) 604 fix r s a b 605 assume A: "r \<in> R" "s \<in> R" "a \<in> Field r" "b \<in> Field s" "a \<noteq> b" 606 from \<open>chain\<^sub>\<subseteq> R\<close> and \<open>r \<in> R\<close> and \<open>s \<in> R\<close> have "r \<subseteq> s \<or> s \<subseteq> r" 607 by (auto simp add: chain_subset_def) 608 then show "(\<exists>r\<in>R. (a, b) \<in> r) \<or> (\<exists>r\<in>R. (b, a) \<in> r)" 609 proof 610 assume "r \<subseteq> s" 611 then have "(a, b) \<in> s \<or> (b, a) \<in> s" 612 using assms(2) A mono_Field[of r s] 613 by (auto simp add: total_on_def) 614 then show ?thesis 615 using \<open>s \<in> R\<close> by blast 616 next 617 assume "s \<subseteq> r" 618 then have "(a, b) \<in> r \<or> (b, a) \<in> r" 619 using assms(2) A mono_Field[of s r] 620 by (fastforce simp add: total_on_def) 621 then show ?thesis 622 using \<open>r \<in> R\<close> by blast 623 qed 624qed 625 626lemma wf_Union_wf_init_segs: 627 assumes "R \<in> Chains init_seg_of" 628 and "\<forall>r\<in>R. wf r" 629 shows "wf (\<Union>R)" 630proof (simp add: wf_iff_no_infinite_down_chain, rule ccontr, auto) 631 fix f 632 assume 1: "\<forall>i. \<exists>r\<in>R. (f (Suc i), f i) \<in> r" 633 then obtain r where "r \<in> R" and "(f (Suc 0), f 0) \<in> r" by auto 634 have "(f (Suc i), f i) \<in> r" for i 635 proof (induct i) 636 case 0 637 show ?case by fact 638 next 639 case (Suc i) 640 then obtain s where s: "s \<in> R" "(f (Suc (Suc i)), f(Suc i)) \<in> s" 641 using 1 by auto 642 then have "s initial_segment_of r \<or> r initial_segment_of s" 643 using assms(1) \<open>r \<in> R\<close> by (simp add: Chains_def) 644 with Suc s show ?case by (simp add: init_seg_of_def) blast 645 qed 646 then show False 647 using assms(2) and \<open>r \<in> R\<close> 648 by (simp add: wf_iff_no_infinite_down_chain) blast 649qed 650 651lemma initial_segment_of_Diff: "p initial_segment_of q \<Longrightarrow> p - s initial_segment_of q - s" 652 unfolding init_seg_of_def by blast 653 654lemma Chains_inits_DiffI: "R \<in> Chains init_seg_of \<Longrightarrow> {r - s |r. r \<in> R} \<in> Chains init_seg_of" 655 unfolding Chains_def by (blast intro: initial_segment_of_Diff) 656 657theorem well_ordering: "\<exists>r::'a rel. Well_order r \<and> Field r = UNIV" 658proof - 659\<comment> \<open>The initial segment relation on well-orders:\<close> 660 let ?WO = "{r::'a rel. Well_order r}" 661 define I where "I = init_seg_of \<inter> ?WO \<times> ?WO" 662 then have I_init: "I \<subseteq> init_seg_of" by simp 663 then have subch: "\<And>R. R \<in> Chains I \<Longrightarrow> chain\<^sub>\<subseteq> R" 664 unfolding init_seg_of_def chain_subset_def Chains_def by blast 665 have Chains_wo: "\<And>R r. R \<in> Chains I \<Longrightarrow> r \<in> R \<Longrightarrow> Well_order r" 666 by (simp add: Chains_def I_def) blast 667 have FI: "Field I = ?WO" 668 by (auto simp add: I_def init_seg_of_def Field_def) 669 then have 0: "Partial_order I" 670 by (auto simp: partial_order_on_def preorder_on_def antisym_def antisym_init_seg_of refl_on_def 671 trans_def I_def elim!: trans_init_seg_of) 672\<comment> \<open>\<open>I\<close>-chains have upper bounds in \<open>?WO\<close> wrt \<open>I\<close>: their Union\<close> 673 have "\<Union>R \<in> ?WO \<and> (\<forall>r\<in>R. (r, \<Union>R) \<in> I)" if "R \<in> Chains I" for R 674 proof - 675 from that have Ris: "R \<in> Chains init_seg_of" 676 using mono_Chains [OF I_init] by blast 677 have subch: "chain\<^sub>\<subseteq> R" 678 using \<open>R \<in> Chains I\<close> I_init by (auto simp: init_seg_of_def chain_subset_def Chains_def) 679 have "\<forall>r\<in>R. Refl r" and "\<forall>r\<in>R. trans r" and "\<forall>r\<in>R. antisym r" 680 and "\<forall>r\<in>R. Total r" and "\<forall>r\<in>R. wf (r - Id)" 681 using Chains_wo [OF \<open>R \<in> Chains I\<close>] by (simp_all add: order_on_defs) 682 have "Refl (\<Union>R)" 683 using \<open>\<forall>r\<in>R. Refl r\<close> unfolding refl_on_def by fastforce 684 moreover have "trans (\<Union>R)" 685 by (rule chain_subset_trans_Union [OF subch \<open>\<forall>r\<in>R. trans r\<close>]) 686 moreover have "antisym (\<Union>R)" 687 by (rule chain_subset_antisym_Union [OF subch \<open>\<forall>r\<in>R. antisym r\<close>]) 688 moreover have "Total (\<Union>R)" 689 by (rule chain_subset_Total_Union [OF subch \<open>\<forall>r\<in>R. Total r\<close>]) 690 moreover have "wf ((\<Union>R) - Id)" 691 proof - 692 have "(\<Union>R) - Id = \<Union>{r - Id | r. r \<in> R}" by blast 693 with \<open>\<forall>r\<in>R. wf (r - Id)\<close> and wf_Union_wf_init_segs [OF Chains_inits_DiffI [OF Ris]] 694 show ?thesis by fastforce 695 qed 696 ultimately have "Well_order (\<Union>R)" 697 by (simp add:order_on_defs) 698 moreover have "\<forall>r \<in> R. r initial_segment_of \<Union>R" 699 using Ris by (simp add: Chains_init_seg_of_Union) 700 ultimately show ?thesis 701 using mono_Chains [OF I_init] Chains_wo[of R] and \<open>R \<in> Chains I\<close> 702 unfolding I_def by blast 703 qed 704 then have 1: "\<forall>R \<in> Chains I. \<exists>u\<in>Field I. \<forall>r\<in>R. (r, u) \<in> I" 705 by (subst FI) blast 706\<comment> \<open>Zorn's Lemma yields a maximal well-order \<open>m\<close>:\<close> 707 then obtain m :: "'a rel" 708 where "Well_order m" 709 and max: "\<forall>r. Well_order r \<and> (m, r) \<in> I \<longrightarrow> r = m" 710 using Zorns_po_lemma[OF 0 1] unfolding FI by fastforce 711\<comment> \<open>Now show by contradiction that \<open>m\<close> covers the whole type:\<close> 712 have False if "x \<notin> Field m" for x :: 'a 713 proof - 714\<comment> \<open>Assuming that \<open>x\<close> is not covered and extend \<open>m\<close> at the top with \<open>x\<close>\<close> 715 have "m \<noteq> {}" 716 proof 717 assume "m = {}" 718 moreover have "Well_order {(x, x)}" 719 by (simp add: order_on_defs refl_on_def trans_def antisym_def total_on_def Field_def) 720 ultimately show False using max 721 by (auto simp: I_def init_seg_of_def simp del: Field_insert) 722 qed 723 then have "Field m \<noteq> {}" by (auto simp: Field_def) 724 moreover have "wf (m - Id)" 725 using \<open>Well_order m\<close> by (simp add: well_order_on_def) 726\<comment> \<open>The extension of \<open>m\<close> by \<open>x\<close>:\<close> 727 let ?s = "{(a, x) | a. a \<in> Field m}" 728 let ?m = "insert (x, x) m \<union> ?s" 729 have Fm: "Field ?m = insert x (Field m)" 730 by (auto simp: Field_def) 731 have "Refl m" and "trans m" and "antisym m" and "Total m" and "wf (m - Id)" 732 using \<open>Well_order m\<close> by (simp_all add: order_on_defs) 733\<comment> \<open>We show that the extension is a well-order\<close> 734 have "Refl ?m" 735 using \<open>Refl m\<close> Fm unfolding refl_on_def by blast 736 moreover have "trans ?m" using \<open>trans m\<close> and \<open>x \<notin> Field m\<close> 737 unfolding trans_def Field_def by blast 738 moreover have "antisym ?m" 739 using \<open>antisym m\<close> and \<open>x \<notin> Field m\<close> unfolding antisym_def Field_def by blast 740 moreover have "Total ?m" 741 using \<open>Total m\<close> and Fm by (auto simp: total_on_def) 742 moreover have "wf (?m - Id)" 743 proof - 744 have "wf ?s" 745 using \<open>x \<notin> Field m\<close> by (auto simp: wf_eq_minimal Field_def Bex_def) 746 then show ?thesis 747 using \<open>wf (m - Id)\<close> and \<open>x \<notin> Field m\<close> wf_subset [OF \<open>wf ?s\<close> Diff_subset] 748 by (auto simp: Un_Diff Field_def intro: wf_Un) 749 qed 750 ultimately have "Well_order ?m" 751 by (simp add: order_on_defs) 752\<comment> \<open>We show that the extension is above \<open>m\<close>\<close> 753 moreover have "(m, ?m) \<in> I" 754 using \<open>Well_order ?m\<close> and \<open>Well_order m\<close> and \<open>x \<notin> Field m\<close> 755 by (fastforce simp: I_def init_seg_of_def Field_def) 756 ultimately 757\<comment> \<open>This contradicts maximality of \<open>m\<close>:\<close> 758 show False 759 using max and \<open>x \<notin> Field m\<close> unfolding Field_def by blast 760 qed 761 then have "Field m = UNIV" by auto 762 with \<open>Well_order m\<close> show ?thesis by blast 763qed 764 765corollary well_order_on: "\<exists>r::'a rel. well_order_on A r" 766proof - 767 obtain r :: "'a rel" where wo: "Well_order r" and univ: "Field r = UNIV" 768 using well_ordering [where 'a = "'a"] by blast 769 let ?r = "{(x, y). x \<in> A \<and> y \<in> A \<and> (x, y) \<in> r}" 770 have 1: "Field ?r = A" 771 using wo univ by (fastforce simp: Field_def order_on_defs refl_on_def) 772 from \<open>Well_order r\<close> have "Refl r" "trans r" "antisym r" "Total r" "wf (r - Id)" 773 by (simp_all add: order_on_defs) 774 from \<open>Refl r\<close> have "Refl ?r" 775 by (auto simp: refl_on_def 1 univ) 776 moreover from \<open>trans r\<close> have "trans ?r" 777 unfolding trans_def by blast 778 moreover from \<open>antisym r\<close> have "antisym ?r" 779 unfolding antisym_def by blast 780 moreover from \<open>Total r\<close> have "Total ?r" 781 by (simp add:total_on_def 1 univ) 782 moreover have "wf (?r - Id)" 783 by (rule wf_subset [OF \<open>wf (r - Id)\<close>]) blast 784 ultimately have "Well_order ?r" 785 by (simp add: order_on_defs) 786 with 1 show ?thesis by auto 787qed 788 789(* Move this to Hilbert Choice and wfrec to Wellfounded*) 790 791lemma wfrec_def_adm: "f \<equiv> wfrec R F \<Longrightarrow> wf R \<Longrightarrow> adm_wf R F \<Longrightarrow> f = F f" 792 using wfrec_fixpoint by simp 793 794lemma dependent_wf_choice: 795 fixes P :: "('a \<Rightarrow> 'b) \<Rightarrow> 'a \<Rightarrow> 'b \<Rightarrow> bool" 796 assumes "wf R" 797 and adm: "\<And>f g x r. (\<And>z. (z, x) \<in> R \<Longrightarrow> f z = g z) \<Longrightarrow> P f x r = P g x r" 798 and P: "\<And>x f. (\<And>y. (y, x) \<in> R \<Longrightarrow> P f y (f y)) \<Longrightarrow> \<exists>r. P f x r" 799 shows "\<exists>f. \<forall>x. P f x (f x)" 800proof (intro exI allI) 801 fix x 802 define f where "f \<equiv> wfrec R (\<lambda>f x. SOME r. P f x r)" 803 from \<open>wf R\<close> show "P f x (f x)" 804 proof (induct x) 805 case (less x) 806 show "P f x (f x)" 807 proof (subst (2) wfrec_def_adm[OF f_def \<open>wf R\<close>]) 808 show "adm_wf R (\<lambda>f x. SOME r. P f x r)" 809 by (auto simp add: adm_wf_def intro!: arg_cong[where f=Eps] ext adm) 810 show "P f x (Eps (P f x))" 811 using P by (rule someI_ex) fact 812 qed 813 qed 814qed 815 816lemma (in wellorder) dependent_wellorder_choice: 817 assumes "\<And>r f g x. (\<And>y. y < x \<Longrightarrow> f y = g y) \<Longrightarrow> P f x r = P g x r" 818 and P: "\<And>x f. (\<And>y. y < x \<Longrightarrow> P f y (f y)) \<Longrightarrow> \<exists>r. P f x r" 819 shows "\<exists>f. \<forall>x. P f x (f x)" 820 using wf by (rule dependent_wf_choice) (auto intro!: assms) 821 822end 823