src/HOL/Isar_examples/MutilatedCheckerboard.thy

improved presentation;

(*  Title:      HOL/Isar_examples/MutilatedCheckerboard.thy
    ID:         $Id$
    Author:     Markus Wenzel, TU Muenchen (Isar document)
                Lawrence C Paulson, Cambridge University Computer Laboratory (original scripts)
*)

header {* The Mutilated Checker Board Problem *};

theory MutilatedCheckerboard = Main:;

text {*
 The Mutilated Checker Board Problem, formalized inductively.  See
 \cite{paulson-mutilated-board} and
 \url{http://isabelle.in.tum.de/library/HOL/Induct/Mutil.html} for the
 original tactic script version.
*};

subsection {* Tilings *};

consts
  tiling :: "'a set set => 'a set set";

inductive "tiling A"
  intrs
    empty: "{} : tiling A"
    Un:    "[| a : A;  t : tiling A;  a <= - t |]
              ==> a Un t : tiling A";


text "The union of two disjoint tilings is a tiling.";

lemma tiling_Un:
  "t : tiling A --> u : tiling A --> t Int u = {}
    --> t Un u : tiling A";
proof;
  assume "t : tiling A" (is "_ : ?T");
  thus "u : ?T --> t Int u = {} --> t Un u : ?T" (is "?P t");
  proof (induct t set: tiling);
    show "?P {}"; by simp;

    fix a t;
    assume "a : A" "t : ?T" "?P t" "a <= - t";
    show "?P (a Un t)";
    proof (intro impI);
      assume "u : ?T" "(a Un t) Int u = {}";
      have hyp: "t Un u: ?T"; by (blast!);
      have "a <= - (t Un u)"; by (blast!);
      with _ hyp; have "a Un (t Un u) : ?T"; by (rule tiling.Un);
      also; have "a Un (t Un u) = (a Un t) Un u";
        by (simp only: Un_assoc);
      finally; show "... : ?T"; .;
    qed;
  qed;
qed;


subsection {* Basic properties of ``below'' *};

constdefs
  below :: "nat => nat set"
  "below n == {i. i < n}";

lemma below_less_iff [iff]: "(i: below k) = (i < k)";
  by (simp add: below_def);

lemma below_0: "below 0 = {}";
  by (simp add: below_def);

lemma Sigma_Suc1:
    "below (Suc n) Times B = ({n} Times B) Un (below n Times B)";
  by (simp add: below_def less_Suc_eq) blast;

lemma Sigma_Suc2:
    "A Times below (Suc n) = (A Times {n}) Un (A Times (below n))";
  by (simp add: below_def less_Suc_eq) blast;

lemmas Sigma_Suc = Sigma_Suc1 Sigma_Suc2;


subsection {* Basic properties of ``evnodd'' *};

constdefs
  evnodd :: "(nat * nat) set => nat => (nat * nat) set"
  "evnodd A b == A Int {(i, j). (i + j) mod 2 = b}";

lemma evnodd_iff:
    "(i, j): evnodd A b = ((i, j): A  & (i + j) mod 2 = b)";
  by (simp add: evnodd_def);

lemma evnodd_subset: "evnodd A b <= A";
  by (unfold evnodd_def, rule Int_lower1);

lemma evnoddD: "x : evnodd A b ==> x : A";
  by (rule subsetD, rule evnodd_subset);

lemma evnodd_finite: "finite A ==> finite (evnodd A b)";
  by (rule finite_subset, rule evnodd_subset);

lemma evnodd_Un: "evnodd (A Un B) b = evnodd A b Un evnodd B b";
  by (unfold evnodd_def) blast;

lemma evnodd_Diff: "evnodd (A - B) b = evnodd A b - evnodd B b";
  by (unfold evnodd_def) blast;

lemma evnodd_empty: "evnodd {} b = {}";
  by (simp add: evnodd_def);

lemma evnodd_insert: "evnodd (insert (i, j) C) b =
    (if (i + j) mod 2 = b
      then insert (i, j) (evnodd C b) else evnodd C b)";
  by (simp add: evnodd_def) blast;


subsection {* Dominoes *};

consts 
  domino  :: "(nat * nat) set set";

inductive domino
  intrs
    horiz:  "{(i, j), (i, j + 1)} : domino"
    vertl:  "{(i, j), (i + 1, j)} : domino";

lemma dominoes_tile_row:
  "{i} Times below (2 * n) : tiling domino"
  (is "?P n" is "?B n : ?T");
proof (induct n);
  show "?P 0"; by (simp add: below_0 tiling.empty);

  fix n; assume hyp: "?P n";
  let ?a = "{i} Times {2 * n + 1} Un {i} Times {2 * n}";

  have "?B (Suc n) = ?a Un ?B n"; by (simp add: Sigma_Suc Un_assoc);
  also; have "... : ?T";
  proof (rule tiling.Un);
    have "{(i, 2 * n), (i, 2 * n + 1)} : domino";
      by (rule domino.horiz);
    also; have "{(i, 2 * n), (i, 2 * n + 1)} = ?a"; by blast;
    finally; show "... : domino"; .;
    from hyp; show "?B n : ?T"; .;
    show "?a <= - ?B n"; by force;
  qed;
  finally; show "?P (Suc n)"; .;
qed;

lemma dominoes_tile_matrix:
  "below m Times below (2 * n) : tiling domino"
  (is "?P m" is "?B m : ?T");
proof (induct m);
  show "?P 0"; by (simp add: below_0 tiling.empty);

  fix m; assume hyp: "?P m";
  let ?t = "{m} Times below (2 * n)";

  have "?B (Suc m) = ?t Un ?B m"; by (simp add: Sigma_Suc);
  also; have "... : ?T";
  proof (rule tiling_Un [rulify]);
    show "?t : ?T"; by (rule dominoes_tile_row);
    from hyp; show "?B m : ?T"; .;
    show "?t Int ?B m = {}"; by blast;
  qed;
  finally; show "?P (Suc m)"; .;
qed;

lemma domino_singleton:
  "[| d : domino; b < 2 |] ==> EX i j. evnodd d b = {(i, j)}";
proof -;
  assume b: "b < 2";
  assume "d : domino";
  thus ?thesis (is "?P d");
  proof (induct d set: domino);
    from b; have b_cases: "b = 0 | b = 1"; by arith;
    fix i j;
    note [simp] = evnodd_empty evnodd_insert mod_Suc;
    from b_cases; show "?P {(i, j), (i, j + 1)}"; by rule auto;
    from b_cases; show "?P {(i, j), (i + 1, j)}"; by rule auto;
  qed;
qed;

lemma domino_finite: "d: domino ==> finite d";
proof (induct set: domino);
  fix i j :: nat;
  show "finite {(i, j), (i, j + 1)}"; by (intro Finites.intrs);
  show "finite {(i, j), (i + 1, j)}"; by (intro Finites.intrs);
qed;


subsection {* Tilings of dominoes *};

lemma tiling_domino_finite:
  "t : tiling domino ==> finite t" (is "t : ?T ==> ?F t");
proof -;
  assume "t : ?T";
  thus "?F t";
  proof (induct t set: tiling);
    show "?F {}"; by (rule Finites.emptyI);
    fix a t; assume "?F t";
    assume "a : domino"; hence "?F a"; by (rule domino_finite);
    thus "?F (a Un t)"; by (rule finite_UnI);
  qed;
qed;

lemma tiling_domino_01:
  "t : tiling domino ==> card (evnodd t 0) = card (evnodd t 1)"
  (is "t : ?T ==> ?P t");
proof -;
  assume "t : ?T";
  thus "?P t";
  proof (induct t set: tiling);
    show "?P {}"; by (simp add: evnodd_def);

    fix a t;
    let ?e = evnodd;
    assume "a : domino" "t : ?T"
      and hyp: "card (?e t 0) = card (?e t 1)"
      and "a <= - t";

    have card_suc:
      "!!b. b < 2 ==> card (?e (a Un t) b) = Suc (card (?e t b))";
    proof -;
      fix b; assume "b < 2";
      have "EX i j. ?e a b = {(i, j)}"; by (rule domino_singleton);
      thus "?thesis b";
      proof (elim exE);
	have "?e (a Un t) b = ?e a b Un ?e t b"; by (rule evnodd_Un);
	also; fix i j; assume "?e a b = {(i, j)}";
	also; have "... Un ?e t b = insert (i, j) (?e t b)"; by simp;
	also; have "card ... = Suc (card (?e t b))";
	proof (rule card_insert_disjoint);
	  show "finite (?e t b)";
            by (rule evnodd_finite, rule tiling_domino_finite);
	  have "(i, j) : ?e a b"; by (simp!);
	  thus "(i, j) ~: ?e t b"; by (force! dest: evnoddD);
	qed;
	finally; show ?thesis; .;
      qed;
    qed;
    hence "card (?e (a Un t) 0) = Suc (card (?e t 0))"; by simp;
    also; from hyp; have "card (?e t 0) = card (?e t 1)"; .;
    also; from card_suc; have "Suc ... = card (?e (a Un t) 1)";
      by simp;
    finally; show "?P (a Un t)"; .;
  qed;
qed;


subsection {* Main theorem *};

constdefs
  mutilated_board :: "nat => nat => (nat * nat) set"
  "mutilated_board m n ==
    below (2 * (m + 1)) Times below (2 * (n + 1))
      - {(0, 0)} - {(2 * m + 1, 2 * n + 1)}";

theorem mutil_not_tiling: "mutilated_board m n ~: tiling domino";
proof (unfold mutilated_board_def);
  let ?T = "tiling domino";
  let ?t = "below (2 * (m + 1)) Times below (2 * (n + 1))";
  let ?t' = "?t - {(0, 0)}";
  let ?t'' = "?t' - {(2 * m + 1, 2 * n + 1)}";

  show "?t'' ~: ?T";
  proof;
    have t: "?t : ?T"; by (rule dominoes_tile_matrix);
    assume t'': "?t'' : ?T";

    let ?e = evnodd;
    have fin: "finite (?e ?t 0)";
      by (rule evnodd_finite, rule tiling_domino_finite, rule t);

    note [simp] = evnodd_iff evnodd_empty evnodd_insert evnodd_Diff;
    have "card (?e ?t'' 0) < card (?e ?t' 0)";
    proof -;
      have "card (?e ?t' 0 - {(2 * m + 1, 2 * n + 1)})
        < card (?e ?t' 0)";
      proof (rule card_Diff1_less);
	show "finite (?e ?t' 0)";
          by (rule finite_subset, rule fin) force;
	show "(2 * m + 1, 2 * n + 1) : ?e ?t' 0"; by simp;
      qed;
      thus ?thesis; by simp;
    qed;
    also; have "... < card (?e ?t 0)";
    proof -;
      have "(0, 0) : ?e ?t 0"; by simp;
      with fin; have "card (?e ?t 0 - {(0, 0)}) < card (?e ?t 0)";
        by (rule card_Diff1_less);
      thus ?thesis; by simp;
    qed;
    also; from t; have "... = card (?e ?t 1)";
      by (rule tiling_domino_01);
    also; have "?e ?t 1 = ?e ?t'' 1"; by simp;
    also; from t''; have "card ... = card (?e ?t'' 0)";
      by (rule tiling_domino_01 [RS sym]);
    finally; have "... < ..."; .; thus False; ..;
  qed;
qed;

end;