src/HOL/Library/Countable_Set.thy
 author Andreas Lochbihler Fri Sep 20 10:09:16 2013 +0200 (2013-09-20) changeset 53745 788730ab7da4 parent 53381 355a4cac5440 child 54410 0a578fb7fb73 permissions -rw-r--r--
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```     1 (*  Title:      HOL/Library/Countable_Set.thy
```
```     2     Author:     Johannes Hölzl
```
```     3     Author:     Andrei Popescu
```
```     4 *)
```
```     5
```
```     6 header {* Countable sets *}
```
```     7
```
```     8 theory Countable_Set
```
```     9 imports Countable Infinite_Set
```
```    10 begin
```
```    11
```
```    12 subsection {* Predicate for countable sets *}
```
```    13
```
```    14 definition countable :: "'a set \<Rightarrow> bool" where
```
```    15   "countable S \<longleftrightarrow> (\<exists>f::'a \<Rightarrow> nat. inj_on f S)"
```
```    16
```
```    17 lemma countableE:
```
```    18   assumes S: "countable S" obtains f :: "'a \<Rightarrow> nat" where "inj_on f S"
```
```    19   using S by (auto simp: countable_def)
```
```    20
```
```    21 lemma countableI: "inj_on (f::'a \<Rightarrow> nat) S \<Longrightarrow> countable S"
```
```    22   by (auto simp: countable_def)
```
```    23
```
```    24 lemma countableI': "inj_on (f::'a \<Rightarrow> 'b::countable) S \<Longrightarrow> countable S"
```
```    25   using comp_inj_on[of f S to_nat] by (auto intro: countableI)
```
```    26
```
```    27 lemma countableE_bij:
```
```    28   assumes S: "countable S" obtains f :: "nat \<Rightarrow> 'a" and C :: "nat set" where "bij_betw f C S"
```
```    29   using S by (blast elim: countableE dest: inj_on_imp_bij_betw bij_betw_inv)
```
```    30
```
```    31 lemma countableI_bij: "bij_betw f (C::nat set) S \<Longrightarrow> countable S"
```
```    32   by (blast intro: countableI bij_betw_inv_into bij_betw_imp_inj_on)
```
```    33
```
```    34 lemma countable_finite: "finite S \<Longrightarrow> countable S"
```
```    35   by (blast dest: finite_imp_inj_to_nat_seg countableI)
```
```    36
```
```    37 lemma countableI_bij1: "bij_betw f A B \<Longrightarrow> countable A \<Longrightarrow> countable B"
```
```    38   by (blast elim: countableE_bij intro: bij_betw_trans countableI_bij)
```
```    39
```
```    40 lemma countableI_bij2: "bij_betw f B A \<Longrightarrow> countable A \<Longrightarrow> countable B"
```
```    41   by (blast elim: countableE_bij intro: bij_betw_trans bij_betw_inv_into countableI_bij)
```
```    42
```
```    43 lemma countable_iff_bij[simp]: "bij_betw f A B \<Longrightarrow> countable A \<longleftrightarrow> countable B"
```
```    44   by (blast intro: countableI_bij1 countableI_bij2)
```
```    45
```
```    46 lemma countable_subset: "A \<subseteq> B \<Longrightarrow> countable B \<Longrightarrow> countable A"
```
```    47   by (auto simp: countable_def intro: subset_inj_on)
```
```    48
```
```    49 lemma countableI_type[intro, simp]: "countable (A:: 'a :: countable set)"
```
```    50   using countableI[of to_nat A] by auto
```
```    51
```
```    52 subsection {* Enumerate a countable set *}
```
```    53
```
```    54 lemma countableE_infinite:
```
```    55   assumes "countable S" "infinite S"
```
```    56   obtains e :: "'a \<Rightarrow> nat" where "bij_betw e S UNIV"
```
```    57 proof -
```
```    58   obtain f :: "'a \<Rightarrow> nat" where "inj_on f S"
```
```    59     using `countable S` by (rule countableE)
```
```    60   then have "bij_betw f S (f`S)"
```
```    61     unfolding bij_betw_def by simp
```
```    62   moreover
```
```    63   from `inj_on f S` `infinite S` have inf_fS: "infinite (f`S)"
```
```    64     by (auto dest: finite_imageD)
```
```    65   then have "bij_betw (the_inv_into UNIV (enumerate (f`S))) (f`S) UNIV"
```
```    66     by (intro bij_betw_the_inv_into bij_enumerate)
```
```    67   ultimately have "bij_betw (the_inv_into UNIV (enumerate (f`S)) \<circ> f) S UNIV"
```
```    68     by (rule bij_betw_trans)
```
```    69   then show thesis ..
```
```    70 qed
```
```    71
```
```    72 lemma countable_enum_cases:
```
```    73   assumes "countable S"
```
```    74   obtains (finite) f :: "'a \<Rightarrow> nat" where "finite S" "bij_betw f S {..<card S}"
```
```    75         | (infinite) f :: "'a \<Rightarrow> nat" where "infinite S" "bij_betw f S UNIV"
```
```    76   using ex_bij_betw_finite_nat[of S] countableE_infinite `countable S`
```
```    77   by (cases "finite S") (auto simp add: atLeast0LessThan)
```
```    78
```
```    79 definition to_nat_on :: "'a set \<Rightarrow> 'a \<Rightarrow> nat" where
```
```    80   "to_nat_on S = (SOME f. if finite S then bij_betw f S {..< card S} else bij_betw f S UNIV)"
```
```    81
```
```    82 definition from_nat_into :: "'a set \<Rightarrow> nat \<Rightarrow> 'a" where
```
```    83   "from_nat_into S n = (if n \<in> to_nat_on S ` S then inv_into S (to_nat_on S) n else SOME s. s\<in>S)"
```
```    84
```
```    85 lemma to_nat_on_finite: "finite S \<Longrightarrow> bij_betw (to_nat_on S) S {..< card S}"
```
```    86   using ex_bij_betw_finite_nat unfolding to_nat_on_def
```
```    87   by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S {..<card S}"]) (auto simp add: atLeast0LessThan)
```
```    88
```
```    89 lemma to_nat_on_infinite: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (to_nat_on S) S UNIV"
```
```    90   using countableE_infinite unfolding to_nat_on_def
```
```    91   by (intro someI2_ex[where Q="\<lambda>f. bij_betw f S UNIV"]) auto
```
```    92
```
```    93 lemma bij_betw_from_nat_into_finite: "finite S \<Longrightarrow> bij_betw (from_nat_into S) {..< card S} S"
```
```    94   unfolding from_nat_into_def[abs_def]
```
```    95   using to_nat_on_finite[of S]
```
```    96   apply (subst bij_betw_cong)
```
```    97   apply (split split_if)
```
```    98   apply (simp add: bij_betw_def)
```
```    99   apply (auto cong: bij_betw_cong
```
```   100               intro: bij_betw_inv_into to_nat_on_finite)
```
```   101   done
```
```   102
```
```   103 lemma bij_betw_from_nat_into: "countable S \<Longrightarrow> infinite S \<Longrightarrow> bij_betw (from_nat_into S) UNIV S"
```
```   104   unfolding from_nat_into_def[abs_def]
```
```   105   using to_nat_on_infinite[of S, unfolded bij_betw_def]
```
```   106   by (auto cong: bij_betw_cong intro: bij_betw_inv_into to_nat_on_infinite)
```
```   107
```
```   108 lemma inj_on_to_nat_on[intro]: "countable A \<Longrightarrow> inj_on (to_nat_on A) A"
```
```   109   using to_nat_on_infinite[of A] to_nat_on_finite[of A]
```
```   110   by (cases "finite A") (auto simp: bij_betw_def)
```
```   111
```
```   112 lemma to_nat_on_inj[simp]:
```
```   113   "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> b \<in> A \<Longrightarrow> to_nat_on A a = to_nat_on A b \<longleftrightarrow> a = b"
```
```   114   using inj_on_to_nat_on[of A] by (auto dest: inj_onD)
```
```   115
```
```   116 lemma from_nat_into_to_nat_on[simp]: "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> from_nat_into A (to_nat_on A a) = a"
```
```   117   by (auto simp: from_nat_into_def intro!: inv_into_f_f)
```
```   118
```
```   119 lemma subset_range_from_nat_into: "countable A \<Longrightarrow> A \<subseteq> range (from_nat_into A)"
```
```   120   by (auto intro: from_nat_into_to_nat_on[symmetric])
```
```   121
```
```   122 lemma from_nat_into: "A \<noteq> {} \<Longrightarrow> from_nat_into A n \<in> A"
```
```   123   unfolding from_nat_into_def by (metis equals0I inv_into_into someI_ex)
```
```   124
```
```   125 lemma range_from_nat_into_subset: "A \<noteq> {} \<Longrightarrow> range (from_nat_into A) \<subseteq> A"
```
```   126   using from_nat_into[of A] by auto
```
```   127
```
```   128 lemma range_from_nat_into[simp]: "A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> range (from_nat_into A) = A"
```
```   129   by (metis equalityI range_from_nat_into_subset subset_range_from_nat_into)
```
```   130
```
```   131 lemma image_to_nat_on: "countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A ` A = UNIV"
```
```   132   using to_nat_on_infinite[of A] by (simp add: bij_betw_def)
```
```   133
```
```   134 lemma to_nat_on_surj: "countable A \<Longrightarrow> infinite A \<Longrightarrow> \<exists>a\<in>A. to_nat_on A a = n"
```
```   135   by (metis (no_types) image_iff iso_tuple_UNIV_I image_to_nat_on)
```
```   136
```
```   137 lemma to_nat_on_from_nat_into[simp]: "n \<in> to_nat_on A ` A \<Longrightarrow> to_nat_on A (from_nat_into A n) = n"
```
```   138   by (simp add: f_inv_into_f from_nat_into_def)
```
```   139
```
```   140 lemma to_nat_on_from_nat_into_infinite[simp]:
```
```   141   "countable A \<Longrightarrow> infinite A \<Longrightarrow> to_nat_on A (from_nat_into A n) = n"
```
```   142   by (metis image_iff to_nat_on_surj to_nat_on_from_nat_into)
```
```   143
```
```   144 lemma from_nat_into_inj:
```
```   145   "countable A \<Longrightarrow> m \<in> to_nat_on A ` A \<Longrightarrow> n \<in> to_nat_on A ` A \<Longrightarrow>
```
```   146     from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n"
```
```   147   by (subst to_nat_on_inj[symmetric, of A]) auto
```
```   148
```
```   149 lemma from_nat_into_inj_infinite[simp]:
```
```   150   "countable A \<Longrightarrow> infinite A \<Longrightarrow> from_nat_into A m = from_nat_into A n \<longleftrightarrow> m = n"
```
```   151   using image_to_nat_on[of A] from_nat_into_inj[of A m n] by simp
```
```   152
```
```   153 lemma eq_from_nat_into_iff:
```
```   154   "countable A \<Longrightarrow> x \<in> A \<Longrightarrow> i \<in> to_nat_on A ` A \<Longrightarrow> x = from_nat_into A i \<longleftrightarrow> i = to_nat_on A x"
```
```   155   by auto
```
```   156
```
```   157 lemma from_nat_into_surj: "countable A \<Longrightarrow> a \<in> A \<Longrightarrow> \<exists>n. from_nat_into A n = a"
```
```   158   by (rule exI[of _ "to_nat_on A a"]) simp
```
```   159
```
```   160 lemma from_nat_into_inject[simp]:
```
```   161   "A \<noteq> {} \<Longrightarrow> countable A \<Longrightarrow> B \<noteq> {} \<Longrightarrow> countable B \<Longrightarrow> from_nat_into A = from_nat_into B \<longleftrightarrow> A = B"
```
```   162   by (metis range_from_nat_into)
```
```   163
```
```   164 lemma inj_on_from_nat_into: "inj_on from_nat_into ({A. A \<noteq> {} \<and> countable A})"
```
```   165   unfolding inj_on_def by auto
```
```   166
```
```   167 subsection {* Closure properties of countability *}
```
```   168
```
```   169 lemma countable_SIGMA[intro, simp]:
```
```   170   "countable I \<Longrightarrow> (\<And>i. i \<in> I \<Longrightarrow> countable (A i)) \<Longrightarrow> countable (SIGMA i : I. A i)"
```
```   171   by (intro countableI'[of "\<lambda>(i, a). (to_nat_on I i, to_nat_on (A i) a)"]) (auto simp: inj_on_def)
```
```   172
```
```   173 lemma countable_image[intro, simp]:
```
```   174   assumes "countable A"
```
```   175   shows "countable (f`A)"
```
```   176 proof -
```
```   177   obtain g :: "'a \<Rightarrow> nat" where "inj_on g A"
```
```   178     using assms by (rule countableE)
```
```   179   moreover have "inj_on (inv_into A f) (f`A)" "inv_into A f ` f ` A \<subseteq> A"
```
```   180     by (auto intro: inj_on_inv_into inv_into_into)
```
```   181   ultimately show ?thesis
```
```   182     by (blast dest: comp_inj_on subset_inj_on intro: countableI)
```
```   183 qed
```
```   184
```
```   185 lemma countable_UN[intro, simp]:
```
```   186   fixes I :: "'i set" and A :: "'i => 'a set"
```
```   187   assumes I: "countable I"
```
```   188   assumes A: "\<And>i. i \<in> I \<Longrightarrow> countable (A i)"
```
```   189   shows "countable (\<Union>i\<in>I. A i)"
```
```   190 proof -
```
```   191   have "(\<Union>i\<in>I. A i) = snd ` (SIGMA i : I. A i)" by (auto simp: image_iff)
```
```   192   then show ?thesis by (simp add: assms)
```
```   193 qed
```
```   194
```
```   195 lemma countable_Un[intro]: "countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A \<union> B)"
```
```   196   by (rule countable_UN[of "{True, False}" "\<lambda>True \<Rightarrow> A | False \<Rightarrow> B", simplified])
```
```   197      (simp split: bool.split)
```
```   198
```
```   199 lemma countable_Un_iff[simp]: "countable (A \<union> B) \<longleftrightarrow> countable A \<and> countable B"
```
```   200   by (metis countable_Un countable_subset inf_sup_ord(3,4))
```
```   201
```
```   202 lemma countable_Plus[intro, simp]:
```
```   203   "countable A \<Longrightarrow> countable B \<Longrightarrow> countable (A <+> B)"
```
```   204   by (simp add: Plus_def)
```
```   205
```
```   206 lemma countable_empty[intro, simp]: "countable {}"
```
```   207   by (blast intro: countable_finite)
```
```   208
```
```   209 lemma countable_insert[intro, simp]: "countable A \<Longrightarrow> countable (insert a A)"
```
```   210   using countable_Un[of "{a}" A] by (auto simp: countable_finite)
```
```   211
```
```   212 lemma countable_Int1[intro, simp]: "countable A \<Longrightarrow> countable (A \<inter> B)"
```
```   213   by (force intro: countable_subset)
```
```   214
```
```   215 lemma countable_Int2[intro, simp]: "countable B \<Longrightarrow> countable (A \<inter> B)"
```
```   216   by (blast intro: countable_subset)
```
```   217
```
```   218 lemma countable_INT[intro, simp]: "i \<in> I \<Longrightarrow> countable (A i) \<Longrightarrow> countable (\<Inter>i\<in>I. A i)"
```
```   219   by (blast intro: countable_subset)
```
```   220
```
```   221 lemma countable_Diff[intro, simp]: "countable A \<Longrightarrow> countable (A - B)"
```
```   222   by (blast intro: countable_subset)
```
```   223
```
```   224 lemma countable_vimage: "B \<subseteq> range f \<Longrightarrow> countable (f -` B) \<Longrightarrow> countable B"
```
```   225   by (metis Int_absorb2 assms countable_image image_vimage_eq)
```
```   226
```
```   227 lemma surj_countable_vimage: "surj f \<Longrightarrow> countable (f -` B) \<Longrightarrow> countable B"
```
```   228   by (metis countable_vimage top_greatest)
```
```   229
```
```   230 lemma countable_Collect[simp]: "countable A \<Longrightarrow> countable {a \<in> A. \<phi> a}"
```
```   231   by (metis Collect_conj_eq Int_absorb Int_commute Int_def countable_Int1)
```
```   232
```
```   233 lemma countable_lists[intro, simp]:
```
```   234   assumes A: "countable A" shows "countable (lists A)"
```
```   235 proof -
```
```   236   have "countable (lists (range (from_nat_into A)))"
```
```   237     by (auto simp: lists_image)
```
```   238   with A show ?thesis
```
```   239     by (auto dest: subset_range_from_nat_into countable_subset lists_mono)
```
```   240 qed
```
```   241
```
```   242 lemma Collect_finite_eq_lists: "Collect finite = set ` lists UNIV"
```
```   243   using finite_list by auto
```
```   244
```
```   245 lemma countable_Collect_finite: "countable (Collect (finite::'a::countable set\<Rightarrow>bool))"
```
```   246   by (simp add: Collect_finite_eq_lists)
```
```   247
```
```   248 lemma countable_rat: "countable \<rat>"
```
```   249   unfolding Rats_def by auto
```
```   250
```
```   251 lemma Collect_finite_subset_eq_lists: "{A. finite A \<and> A \<subseteq> T} = set ` lists T"
```
```   252   using finite_list by (auto simp: lists_eq_set)
```
```   253
```
```   254 lemma countable_Collect_finite_subset:
```
```   255   "countable T \<Longrightarrow> countable {A. finite A \<and> A \<subseteq> T}"
```
```   256   unfolding Collect_finite_subset_eq_lists by auto
```
```   257
```
```   258 subsection {* Misc lemmas *}
```
```   259
```
```   260 lemma countable_all:
```
```   261   assumes S: "countable S"
```
```   262   shows "(\<forall>s\<in>S. P s) \<longleftrightarrow> (\<forall>n::nat. from_nat_into S n \<in> S \<longrightarrow> P (from_nat_into S n))"
```
```   263   using S[THEN subset_range_from_nat_into] by auto
```
```   264
```
```   265 end
```