src/HOL/Induct/Mutil.thy

tuned;

(*  Title:      HOL/Induct/Mutil.thy
    ID:         $Id$
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1996  University of Cambridge
*)

header {* The Mutilated Chess Board Problem *}

theory Mutil imports Main begin

text {*
  The Mutilated Chess Board Problem, formalized inductively.

  Originator is Max Black, according to J A Robinson.  Popularized as
  the Mutilated Checkerboard Problem by J McCarthy.
*}

inductive_set
  tiling :: "'a set set => 'a set set"
  for A :: "'a set set"
  where
    empty [simp, intro]: "{} \<in> tiling A"
  | Un [simp, intro]:    "[| a \<in> A; t \<in> tiling A; a \<inter> t = {} |] 
                          ==> a \<union> t \<in> tiling A"

inductive_set
  domino :: "(nat \<times> nat) set set"
  where
    horiz [simp]: "{(i, j), (i, Suc j)} \<in> domino"
  | vertl [simp]: "{(i, j), (Suc i, j)} \<in> domino"

text {* \medskip Sets of squares of the given colour*}

definition
  coloured :: "nat => (nat \<times> nat) set" where
  "coloured b = {(i, j). (i + j) mod 2 = b}"

abbreviation
  whites  :: "(nat \<times> nat) set" where
  "whites == coloured 0"

abbreviation
  blacks  :: "(nat \<times> nat) set" where
  "blacks == coloured (Suc 0)"


text {* \medskip The union of two disjoint tilings is a tiling *}

lemma tiling_UnI [intro]:
     "[|t \<in> tiling A; u \<in> tiling A; t \<inter> u = {} |] ==>  t \<union> u \<in> tiling A"
  apply (induct set: tiling)
  apply (auto simp add: Un_assoc)
  done


text {* \medskip Chess boards *}

lemma Sigma_Suc1 [simp]:
     "lessThan (Suc n) \<times> B = ({n} \<times> B) \<union> ((lessThan n) \<times> B)"
  by auto

lemma Sigma_Suc2 [simp]:
     "A \<times> lessThan (Suc n) = (A \<times> {n}) \<union> (A \<times> (lessThan n))"
  by auto

lemma sing_Times_lemma: "({i} \<times> {n}) \<union> ({i} \<times> {m}) = {(i, m), (i, n)}"
  by auto

lemma dominoes_tile_row [intro!]: "{i} \<times> lessThan (2 * n) \<in> tiling domino"
  apply (induct n)
   apply (simp_all add: Un_assoc [symmetric])
  apply (rule tiling.Un)
    apply (auto simp add: sing_Times_lemma)
  done

lemma dominoes_tile_matrix: "(lessThan m) \<times> lessThan (2 * n) \<in> tiling domino"
  by (induct m) auto


text {* \medskip @{term coloured} and Dominoes *}

lemma coloured_insert [simp]:
     "coloured b \<inter> (insert (i, j) t) =
      (if (i + j) mod 2 = b then insert (i, j) (coloured b \<inter> t)
       else coloured b \<inter> t)"
  by (auto simp add: coloured_def)

lemma domino_singletons:
     "d \<in> domino ==>
       (\<exists>i j. whites \<inter> d = {(i, j)}) \<and>
       (\<exists>m n. blacks \<inter> d = {(m, n)})";
  apply (erule domino.cases)
   apply (auto simp add: mod_Suc)
  done

lemma domino_finite [simp]: "d \<in> domino ==> finite d"
  by (erule domino.cases, auto)


text {* \medskip Tilings of dominoes *}

lemma tiling_domino_finite [simp]: "t \<in> tiling domino ==> finite t"
  by (induct set: tiling) auto

declare
  Int_Un_distrib [simp]
  Diff_Int_distrib [simp]

lemma tiling_domino_0_1:
     "t \<in> tiling domino ==> card(whites \<inter> t) = card(blacks \<inter> t)"
  apply (induct set: tiling)
   apply (drule_tac [2] domino_singletons)
   apply auto
  apply (subgoal_tac "\<forall>p C. C \<inter> a = {p} --> p \<notin> t")
    -- {* this lemma tells us that both ``inserts'' are non-trivial *}
   apply (simp (no_asm_simp))
  apply blast
  done


text {* \medskip Final argument is surprisingly complex *}

theorem gen_mutil_not_tiling:
       "t \<in> tiling domino ==>
	 (i + j) mod 2 = 0 ==> (m + n) mod 2 = 0 ==>
	 {(i, j), (m, n)} \<subseteq> t
       ==> (t - {(i, j)} - {(m, n)}) \<notin> tiling domino"
  apply (rule notI)
  apply (subgoal_tac
          "card (whites \<inter> (t - {(i, j)} - {(m, n)})) <
           card (blacks \<inter> (t - {(i, j)} - {(m, n)}))")
   apply (force simp only: tiling_domino_0_1)
  apply (simp add: tiling_domino_0_1 [symmetric])
  apply (simp add: coloured_def card_Diff2_less)
  done

text {* Apply the general theorem to the well-known case *}

theorem mutil_not_tiling:
       "t = lessThan (2 * Suc m) \<times> lessThan (2 * Suc n)
	 ==> t - {(0, 0)} - {(Suc (2 * m), Suc (2 * n))} \<notin> tiling domino"
  apply (rule gen_mutil_not_tiling)
     apply (blast intro!: dominoes_tile_matrix)
    apply auto
  done

end