src/HOL/Isar_Examples/Mutilated_Checkerboard.thy
author wenzelm
Thu Jul 01 18:31:46 2010 +0200 (2010-07-01)
changeset 37671 fa53d267dab3
parent 35416 d8d7d1b785af
child 40880 be44a567ed28
permissions -rw-r--r--
misc tuning and modernization;
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(*  Title:      HOL/Isar_Examples/Mutilated_Checkerboard.thy
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    Author:     Markus Wenzel, TU Muenchen (Isar document)
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    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory (original scripts)
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*)
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header {* The Mutilated Checker Board Problem *}
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theory Mutilated_Checkerboard
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imports Main
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begin
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text {* The Mutilated Checker Board Problem, formalized inductively.
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  See \cite{paulson-mutilated-board} and
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  \url{http://isabelle.in.tum.de/library/HOL/Induct/Mutil.html} for
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  the original tactic script version. *}
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subsection {* Tilings *}
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inductive_set tiling :: "'a set set => 'a set set"
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  for A :: "'a set set"
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where
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  empty: "{} : tiling A"
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| Un: "a : A ==> t : tiling A ==> a <= - t ==> a Un t : tiling A"
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text "The union of two disjoint tilings is a tiling."
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lemma tiling_Un:
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  assumes "t : tiling A"
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    and "u : tiling A"
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    and "t Int u = {}"
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  shows "t Un u : tiling A"
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proof -
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  let ?T = "tiling A"
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  from `t : ?T` and `t Int u = {}`
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  show "t Un u : ?T"
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  proof (induct t)
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    case empty
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    with `u : ?T` show "{} Un u : ?T" by simp
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  next
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    case (Un a t)
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    show "(a Un t) Un u : ?T"
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    proof -
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      have "a Un (t Un u) : ?T"
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        using `a : A`
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      proof (rule tiling.Un)
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        from `(a Un t) Int u = {}` have "t Int u = {}" by blast
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        then show "t Un u: ?T" by (rule Un)
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        from `a <= - t` and `(a Un t) Int u = {}`
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        show "a <= - (t Un u)" by blast
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      qed
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      also have "a Un (t Un u) = (a Un t) Un u"
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        by (simp only: Un_assoc)
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      finally show ?thesis .
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    qed
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  qed
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qed
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subsection {* Basic properties of ``below'' *}
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definition below :: "nat => nat set"
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  where "below n = {i. i < n}"
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lemma below_less_iff [iff]: "(i: below k) = (i < k)"
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  by (simp add: below_def)
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lemma below_0: "below 0 = {}"
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  by (simp add: below_def)
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lemma Sigma_Suc1:
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    "m = n + 1 ==> below m <*> B = ({n} <*> B) Un (below n <*> B)"
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  by (simp add: below_def less_Suc_eq) blast
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lemma Sigma_Suc2:
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  "m = n + 2 ==> A <*> below m =
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    (A <*> {n}) Un (A <*> {n + 1}) Un (A <*> below n)"
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  by (auto simp add: below_def)
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lemmas Sigma_Suc = Sigma_Suc1 Sigma_Suc2
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subsection {* Basic properties of ``evnodd'' *}
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definition evnodd :: "(nat * nat) set => nat => (nat * nat) set"
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  where "evnodd A b = A Int {(i, j). (i + j) mod 2 = b}"
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lemma evnodd_iff: "(i, j): evnodd A b = ((i, j): A  & (i + j) mod 2 = b)"
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  by (simp add: evnodd_def)
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lemma evnodd_subset: "evnodd A b <= A"
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  unfolding evnodd_def by (rule Int_lower1)
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lemma evnoddD: "x : evnodd A b ==> x : A"
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  by (rule subsetD) (rule evnodd_subset)
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lemma evnodd_finite: "finite A ==> finite (evnodd A b)"
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  by (rule finite_subset) (rule evnodd_subset)
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lemma evnodd_Un: "evnodd (A Un B) b = evnodd A b Un evnodd B b"
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  unfolding evnodd_def by blast
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lemma evnodd_Diff: "evnodd (A - B) b = evnodd A b - evnodd B b"
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  unfolding evnodd_def by blast
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lemma evnodd_empty: "evnodd {} b = {}"
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  by (simp add: evnodd_def)
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lemma evnodd_insert: "evnodd (insert (i, j) C) b =
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    (if (i + j) mod 2 = b
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      then insert (i, j) (evnodd C b) else evnodd C b)"
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  by (simp add: evnodd_def)
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subsection {* Dominoes *}
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inductive_set domino :: "(nat * nat) set set"
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where
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  horiz: "{(i, j), (i, j + 1)} : domino"
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| vertl: "{(i, j), (i + 1, j)} : domino"
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lemma dominoes_tile_row:
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  "{i} <*> below (2 * n) : tiling domino"
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  (is "?B n : ?T")
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proof (induct n)
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  case 0
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  show ?case by (simp add: below_0 tiling.empty)
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next
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  case (Suc n)
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  let ?a = "{i} <*> {2 * n + 1} Un {i} <*> {2 * n}"
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  have "?B (Suc n) = ?a Un ?B n"
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    by (auto simp add: Sigma_Suc Un_assoc)
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  moreover have "... : ?T"
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  proof (rule tiling.Un)
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    have "{(i, 2 * n), (i, 2 * n + 1)} : domino"
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      by (rule domino.horiz)
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    also have "{(i, 2 * n), (i, 2 * n + 1)} = ?a" by blast
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    finally show "... : domino" .
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    show "?B n : ?T" by (rule Suc)
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    show "?a <= - ?B n" by blast
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  qed
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  ultimately show ?case by simp
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qed
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lemma dominoes_tile_matrix:
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  "below m <*> below (2 * n) : tiling domino"
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  (is "?B m : ?T")
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proof (induct m)
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  case 0
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  show ?case by (simp add: below_0 tiling.empty)
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next
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  case (Suc m)
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  let ?t = "{m} <*> below (2 * n)"
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  have "?B (Suc m) = ?t Un ?B m" by (simp add: Sigma_Suc)
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  moreover have "... : ?T"
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  proof (rule tiling_Un)
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    show "?t : ?T" by (rule dominoes_tile_row)
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    show "?B m : ?T" by (rule Suc)
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    show "?t Int ?B m = {}" by blast
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  qed
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  ultimately show ?case by simp
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qed
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lemma domino_singleton:
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  assumes "d : domino"
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    and "b < 2"
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  shows "EX i j. evnodd d b = {(i, j)}"  (is "?P d")
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  using assms
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proof induct
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  from `b < 2` have b_cases: "b = 0 | b = 1" by arith
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  fix i j
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  note [simp] = evnodd_empty evnodd_insert mod_Suc
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  from b_cases show "?P {(i, j), (i, j + 1)}" by rule auto
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  from b_cases show "?P {(i, j), (i + 1, j)}" by rule auto
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qed
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lemma domino_finite:
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  assumes "d: domino"
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  shows "finite d"
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  using assms
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proof induct
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  fix i j :: nat
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  show "finite {(i, j), (i, j + 1)}" by (intro finite.intros)
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  show "finite {(i, j), (i + 1, j)}" by (intro finite.intros)
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qed
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subsection {* Tilings of dominoes *}
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lemma tiling_domino_finite:
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  assumes t: "t : tiling domino"  (is "t : ?T")
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  shows "finite t"  (is "?F t")
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  using t
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proof induct
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  show "?F {}" by (rule finite.emptyI)
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  fix a t assume "?F t"
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  assume "a : domino" then have "?F a" by (rule domino_finite)
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  from this and `?F t` show "?F (a Un t)" by (rule finite_UnI)
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qed
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lemma tiling_domino_01:
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  assumes t: "t : tiling domino"  (is "t : ?T")
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  shows "card (evnodd t 0) = card (evnodd t 1)"
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  using t
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proof induct
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  case empty
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  show ?case by (simp add: evnodd_def)
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next
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  case (Un a t)
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  let ?e = evnodd
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  note hyp = `card (?e t 0) = card (?e t 1)`
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    and at = `a <= - t`
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  have card_suc:
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    "!!b. b < 2 ==> card (?e (a Un t) b) = Suc (card (?e t b))"
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  proof -
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    fix b :: nat assume "b < 2"
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    have "?e (a Un t) b = ?e a b Un ?e t b" by (rule evnodd_Un)
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    also obtain i j where e: "?e a b = {(i, j)}"
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    proof -
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      from `a \<in> domino` and `b < 2`
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      have "EX i j. ?e a b = {(i, j)}" by (rule domino_singleton)
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      then show ?thesis by (blast intro: that)
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    qed
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    moreover have "... Un ?e t b = insert (i, j) (?e t b)" by simp
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    moreover have "card ... = Suc (card (?e t b))"
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    proof (rule card_insert_disjoint)
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      from `t \<in> tiling domino` have "finite t"
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        by (rule tiling_domino_finite)
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      then show "finite (?e t b)"
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        by (rule evnodd_finite)
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      from e have "(i, j) : ?e a b" by simp
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      with at show "(i, j) ~: ?e t b" by (blast dest: evnoddD)
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    qed
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    ultimately show "?thesis b" by simp
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  qed
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  then have "card (?e (a Un t) 0) = Suc (card (?e t 0))" by simp
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  also from hyp have "card (?e t 0) = card (?e t 1)" .
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  also from card_suc have "Suc ... = card (?e (a Un t) 1)"
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    by simp
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  finally show ?case .
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qed
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subsection {* Main theorem *}
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definition mutilated_board :: "nat => nat => (nat * nat) set"
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where
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  "mutilated_board m n =
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    below (2 * (m + 1)) <*> below (2 * (n + 1))
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      - {(0, 0)} - {(2 * m + 1, 2 * n + 1)}"
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theorem mutil_not_tiling: "mutilated_board m n ~: tiling domino"
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proof (unfold mutilated_board_def)
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  let ?T = "tiling domino"
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  let ?t = "below (2 * (m + 1)) <*> below (2 * (n + 1))"
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  let ?t' = "?t - {(0, 0)}"
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  let ?t'' = "?t' - {(2 * m + 1, 2 * n + 1)}"
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  show "?t'' ~: ?T"
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  proof
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    have t: "?t : ?T" by (rule dominoes_tile_matrix)
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    assume t'': "?t'' : ?T"
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    let ?e = evnodd
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    have fin: "finite (?e ?t 0)"
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      by (rule evnodd_finite, rule tiling_domino_finite, rule t)
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    note [simp] = evnodd_iff evnodd_empty evnodd_insert evnodd_Diff
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    have "card (?e ?t'' 0) < card (?e ?t' 0)"
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    proof -
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      have "card (?e ?t' 0 - {(2 * m + 1, 2 * n + 1)})
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        < card (?e ?t' 0)"
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      proof (rule card_Diff1_less)
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        from _ fin show "finite (?e ?t' 0)"
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          by (rule finite_subset) auto
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        show "(2 * m + 1, 2 * n + 1) : ?e ?t' 0" by simp
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      qed
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      then show ?thesis by simp
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    qed
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    also have "... < card (?e ?t 0)"
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    proof -
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      have "(0, 0) : ?e ?t 0" by simp
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      with fin have "card (?e ?t 0 - {(0, 0)}) < card (?e ?t 0)"
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        by (rule card_Diff1_less)
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      then show ?thesis by simp
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    qed
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    also from t have "... = card (?e ?t 1)"
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      by (rule tiling_domino_01)
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    also have "?e ?t 1 = ?e ?t'' 1" by simp
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    also from t'' have "card ... = card (?e ?t'' 0)"
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      by (rule tiling_domino_01 [symmetric])
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    finally have "... < ..." . then show False ..
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  qed
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qed
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end