src/HOL/ex/Set_Theory.thy
 author haftmann Thu Aug 18 13:10:24 2011 +0200 (2011-08-18) changeset 44276 fe769a0fcc96 parent 41460 src/HOL/ex/set.thy@ea56b98aee83 child 45966 03ce2b2a29a2 permissions -rw-r--r--
avoid case-sensitive name for example theory
     1 (*  Title:      HOL/ex/Set_Theory.thy

     2     Author:     Tobias Nipkow and Lawrence C Paulson

     3     Copyright   1991  University of Cambridge

     4 *)

     5

     6 header {* Set Theory examples: Cantor's Theorem, Schröder-Bernstein Theorem, etc. *}

     7

     8 theory Set_Theory

     9 imports Main

    10 begin

    11

    12 text{*

    13   These two are cited in Benzmueller and Kohlhase's system description

    14   of LEO, CADE-15, 1998 (pages 139-143) as theorems LEO could not

    15   prove.

    16 *}

    17

    18 lemma "(X = Y \<union> Z) =

    19     (Y \<subseteq> X \<and> Z \<subseteq> X \<and> (\<forall>V. Y \<subseteq> V \<and> Z \<subseteq> V \<longrightarrow> X \<subseteq> V))"

    20   by blast

    21

    22 lemma "(X = Y \<inter> Z) =

    23     (X \<subseteq> Y \<and> X \<subseteq> Z \<and> (\<forall>V. V \<subseteq> Y \<and> V \<subseteq> Z \<longrightarrow> V \<subseteq> X))"

    24   by blast

    25

    26 text {*

    27   Trivial example of term synthesis: apparently hard for some provers!

    28 *}

    29

    30 schematic_lemma "a \<noteq> b \<Longrightarrow> a \<in> ?X \<and> b \<notin> ?X"

    31   by blast

    32

    33

    34 subsection {* Examples for the @{text blast} paper *}

    35

    36 lemma "(\<Union>x \<in> C. f x \<union> g x) = \<Union>(f  C)  \<union>  \<Union>(g  C)"

    37   -- {* Union-image, called @{text Un_Union_image} in Main HOL *}

    38   by blast

    39

    40 lemma "(\<Inter>x \<in> C. f x \<inter> g x) = \<Inter>(f  C) \<inter> \<Inter>(g  C)"

    41   -- {* Inter-image, called @{text Int_Inter_image} in Main HOL *}

    42   by blast

    43

    44 lemma singleton_example_1:

    45      "\<And>S::'a set set. \<forall>x \<in> S. \<forall>y \<in> S. x \<subseteq> y \<Longrightarrow> \<exists>z. S \<subseteq> {z}"

    46   by blast

    47

    48 lemma singleton_example_2:

    49      "\<forall>x \<in> S. \<Union>S \<subseteq> x \<Longrightarrow> \<exists>z. S \<subseteq> {z}"

    50   -- {*Variant of the problem above. *}

    51   by blast

    52

    53 lemma "\<exists>!x. f (g x) = x \<Longrightarrow> \<exists>!y. g (f y) = y"

    54   -- {* A unique fixpoint theorem --- @{text fast}/@{text best}/@{text meson} all fail. *}

    55   by metis

    56

    57

    58 subsection {* Cantor's Theorem: There is no surjection from a set to its powerset *}

    59

    60 lemma cantor1: "\<not> (\<exists>f:: 'a \<Rightarrow> 'a set. \<forall>S. \<exists>x. f x = S)"

    61   -- {* Requires best-first search because it is undirectional. *}

    62   by best

    63

    64 schematic_lemma "\<forall>f:: 'a \<Rightarrow> 'a set. \<forall>x. f x \<noteq> ?S f"

    65   -- {*This form displays the diagonal term. *}

    66   by best

    67

    68 schematic_lemma "?S \<notin> range (f :: 'a \<Rightarrow> 'a set)"

    69   -- {* This form exploits the set constructs. *}

    70   by (rule notI, erule rangeE, best)

    71

    72 schematic_lemma "?S \<notin> range (f :: 'a \<Rightarrow> 'a set)"

    73   -- {* Or just this! *}

    74   by best

    75

    76

    77 subsection {* The Schröder-Berstein Theorem *}

    78

    79 lemma disj_lemma: "- (f  X) = g  (-X) \<Longrightarrow> f a = g b \<Longrightarrow> a \<in> X \<Longrightarrow> b \<in> X"

    80   by blast

    81

    82 lemma surj_if_then_else:

    83   "-(f  X) = g  (-X) \<Longrightarrow> surj (\<lambda>z. if z \<in> X then f z else g z)"

    84   by (simp add: surj_def) blast

    85

    86 lemma bij_if_then_else:

    87   "inj_on f X \<Longrightarrow> inj_on g (-X) \<Longrightarrow> -(f  X) = g  (-X) \<Longrightarrow>

    88     h = (\<lambda>z. if z \<in> X then f z else g z) \<Longrightarrow> inj h \<and> surj h"

    89   apply (unfold inj_on_def)

    90   apply (simp add: surj_if_then_else)

    91   apply (blast dest: disj_lemma sym)

    92   done

    93

    94 lemma decomposition: "\<exists>X. X = - (g  (- (f  X)))"

    95   apply (rule exI)

    96   apply (rule lfp_unfold)

    97   apply (rule monoI, blast)

    98   done

    99

   100 theorem Schroeder_Bernstein:

   101   "inj (f :: 'a \<Rightarrow> 'b) \<Longrightarrow> inj (g :: 'b \<Rightarrow> 'a)

   102     \<Longrightarrow> \<exists>h:: 'a \<Rightarrow> 'b. inj h \<and> surj h"

   103   apply (rule decomposition [where f=f and g=g, THEN exE])

   104   apply (rule_tac x = "(\<lambda>z. if z \<in> x then f z else inv g z)" in exI)

   105     --{*The term above can be synthesized by a sufficiently detailed proof.*}

   106   apply (rule bij_if_then_else)

   107      apply (rule_tac  refl)

   108     apply (rule_tac  inj_on_inv_into)

   109     apply (erule subset_inj_on [OF _ subset_UNIV])

   110    apply blast

   111   apply (erule ssubst, subst double_complement, erule inv_image_comp [symmetric])

   112   done

   113

   114

   115 subsection {* A simple party theorem *}

   116

   117 text{* \emph{At any party there are two people who know the same

   118 number of people}. Provided the party consists of at least two people

   119 and the knows relation is symmetric. Knowing yourself does not count

   120 --- otherwise knows needs to be reflexive. (From Freek Wiedijk's talk

   121 at TPHOLs 2007.) *}

   122

   123 lemma equal_number_of_acquaintances:

   124 assumes "Domain R <= A" and "sym R" and "card A \<ge> 2"

   125 shows "\<not> inj_on (%a. card(R  {a} - {a})) A"

   126 proof -

   127   let ?N = "%a. card(R  {a} - {a})"

   128   let ?n = "card A"

   129   have "finite A" using card A \<ge> 2 by(auto intro:ccontr)

   130   have 0: "R  A <= A" using sym R Domain R <= A

   131     unfolding Domain_def sym_def by blast

   132   have h: "ALL a:A. R  {a} <= A" using 0 by blast

   133   hence 1: "ALL a:A. finite(R  {a})" using finite A

   134     by(blast intro: finite_subset)

   135   have sub: "?N  A <= {0..<?n}"

   136   proof -

   137     have "ALL a:A. R  {a} - {a} < A" using h by blast

   138     thus ?thesis using psubset_card_mono[OF finite A] by auto

   139   qed

   140   show "~ inj_on ?N A" (is "~ ?I")

   141   proof

   142     assume ?I

   143     hence "?n = card(?N  A)" by(rule card_image[symmetric])

   144     with sub finite A have 2[simp]: "?N  A = {0..<?n}"

   145       using subset_card_intvl_is_intvl[of _ 0] by(auto)

   146     have "0 : ?N  A" and "?n - 1 : ?N  A"  using card A \<ge> 2 by simp+

   147     then obtain a b where ab: "a:A" "b:A" and Na: "?N a = 0" and Nb: "?N b = ?n - 1"

   148       by (auto simp del: 2)

   149     have "a \<noteq> b" using Na Nb card A \<ge> 2 by auto

   150     have "R  {a} - {a} = {}" by (metis 1 Na ab card_eq_0_iff finite_Diff)

   151     hence "b \<notin> R  {a}" using a\<noteq>b by blast

   152     hence "a \<notin> R  {b}" by (metis Image_singleton_iff assms(2) sym_def)

   153     hence 3: "R  {b} - {b} <= A - {a,b}" using 0 ab by blast

   154     have 4: "finite (A - {a,b})" using finite A by simp

   155     have "?N b <= ?n - 2" using ab a\<noteq>b finite A card_mono[OF 4 3] by simp

   156     then show False using Nb card A \<ge>  2 by arith

   157   qed

   158 qed

   159

   160 text {*

   161   From W. W. Bledsoe and Guohui Feng, SET-VAR. JAR 11 (3), 1993, pages

   162   293-314.

   163

   164   Isabelle can prove the easy examples without any special mechanisms,

   165   but it can't prove the hard ones.

   166 *}

   167

   168 lemma "\<exists>A. (\<forall>x \<in> A. x \<le> (0::int))"

   169   -- {* Example 1, page 295. *}

   170   by force

   171

   172 lemma "D \<in> F \<Longrightarrow> \<exists>G. \<forall>A \<in> G. \<exists>B \<in> F. A \<subseteq> B"

   173   -- {* Example 2. *}

   174   by force

   175

   176 lemma "P a \<Longrightarrow> \<exists>A. (\<forall>x \<in> A. P x) \<and> (\<exists>y. y \<in> A)"

   177   -- {* Example 3. *}

   178   by force

   179

   180 lemma "a < b \<and> b < (c::int) \<Longrightarrow> \<exists>A. a \<notin> A \<and> b \<in> A \<and> c \<notin> A"

   181   -- {* Example 4. *}

   182   by force

   183

   184 lemma "P (f b) \<Longrightarrow> \<exists>s A. (\<forall>x \<in> A. P x) \<and> f s \<in> A"

   185   -- {*Example 5, page 298. *}

   186   by force

   187

   188 lemma "P (f b) \<Longrightarrow> \<exists>s A. (\<forall>x \<in> A. P x) \<and> f s \<in> A"

   189   -- {* Example 6. *}

   190   by force

   191

   192 lemma "\<exists>A. a \<notin> A"

   193   -- {* Example 7. *}

   194   by force

   195

   196 lemma "(\<forall>u v. u < (0::int) \<longrightarrow> u \<noteq> abs v)

   197     \<longrightarrow> (\<exists>A::int set. (\<forall>y. abs y \<notin> A) \<and> -2 \<in> A)"

   198   -- {* Example 8 now needs a small hint. *}

   199   by (simp add: abs_if, force)

   200     -- {* not @{text blast}, which can't simplify @{text "-2 < 0"} *}

   201

   202 text {* Example 9 omitted (requires the reals). *}

   203

   204 text {* The paper has no Example 10! *}

   205

   206 lemma "(\<forall>A. 0 \<in> A \<and> (\<forall>x \<in> A. Suc x \<in> A) \<longrightarrow> n \<in> A) \<and>

   207   P 0 \<and> (\<forall>x. P x \<longrightarrow> P (Suc x)) \<longrightarrow> P n"

   208   -- {* Example 11: needs a hint. *}

   209 by(metis nat.induct)

   210

   211 lemma

   212   "(\<forall>A. (0, 0) \<in> A \<and> (\<forall>x y. (x, y) \<in> A \<longrightarrow> (Suc x, Suc y) \<in> A) \<longrightarrow> (n, m) \<in> A)

   213     \<and> P n \<longrightarrow> P m"

   214   -- {* Example 12. *}

   215   by auto

   216

   217 lemma

   218   "(\<forall>x. (\<exists>u. x = 2 * u) = (\<not> (\<exists>v. Suc x = 2 * v))) \<longrightarrow>

   219     (\<exists>A. \<forall>x. (x \<in> A) = (Suc x \<notin> A))"

   220   -- {* Example EO1: typo in article, and with the obvious fix it seems

   221       to require arithmetic reasoning. *}

   222   apply clarify

   223   apply (rule_tac x = "{x. \<exists>u. x = 2 * u}" in exI, auto)

   224    apply metis+

   225   done

   226

   227 end
`