Theory Ramsey (Isabelle2005: October 2005)

# Theory Ramsey

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theory Ramsey
imports Main
begin

    (*  Title:      ZF/ex/ramsey.thy
        ID:         $Id: Ramsey.thy,v 1.12 2005/06/17 14:15:11 haftmann Exp $
        Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
        Copyright   1992  University of Cambridge

    Ramsey's Theorem (finite exponent 2 version)

    Based upon the article
        D Basin and M Kaufmann,
        The Boyer-Moore Prover and Nuprl: An Experimental Comparison.
        In G Huet and G Plotkin, editors, Logical Frameworks.
        (CUP, 1991), pages 89-119

    See also
        M Kaufmann,
        An example in NQTHM: Ramsey's Theorem
        Internal Note, Computational Logic, Inc., Austin, Texas 78703
        Available from the author: kaufmann@cli.com

    This function compute Ramsey numbers according to the proof given below
    (which, does not constrain the base case values at all.

    fun ram 0 j = 1
      | ram i 0 = 1
      | ram i j = ram (i-1) j + ram i (j-1)

    *)

    theory Ramsey imports Main begin
    constdefs
      Symmetric :: "i=>o"
        "Symmetric(E) == (∀x y. <x,y>:E --> <y,x>:E)"

      Atleast :: "[i,i]=>o"  (*not really necessary: ZF defines cardinality*)
        "Atleast(n,S) == (∃f. f ∈ inj(n,S))"

      Clique  :: "[i,i,i]=>o"
        "Clique(C,V,E) == (C ⊆ V) & (∀x ∈ C. ∀y ∈ C. x≠y --> <x,y> ∈ E)"

      Indept  :: "[i,i,i]=>o"
        "Indept(I,V,E) == (I ⊆ V) & (∀x ∈ I. ∀y ∈ I. x≠y --> <x,y> ∉ E)"

      Ramsey  :: "[i,i,i]=>o"
        "Ramsey(n,i,j) == ∀V E. Symmetric(E) & Atleast(n,V) -->
             (∃C. Clique(C,V,E) & Atleast(i,C)) |
             (∃I. Indept(I,V,E) & Atleast(j,I))"

    (*** Cliques and Independent sets ***)

    lemma Clique0 [intro]: "Clique(0,V,E)"
    by (unfold Clique_def, blast)

    lemma Clique_superset: "[| Clique(C,V',E);  V'<=V |] ==> Clique(C,V,E)"
    by (unfold Clique_def, blast)

    lemma Indept0 [intro]: "Indept(0,V,E)"
    by (unfold Indept_def, blast)

    lemma Indept_superset: "[| Indept(I,V',E);  V'<=V |] ==> Indept(I,V,E)"
    by (unfold Indept_def, blast)

    (*** Atleast ***)

    lemma Atleast0 [intro]: "Atleast(0,A)"
    by (unfold Atleast_def inj_def Pi_def function_def, blast)

    lemma Atleast_succD:
        "Atleast(succ(m),A) ==> ∃x ∈ A. Atleast(m, A-{x})"
    apply (unfold Atleast_def)
    apply (blast dest: inj_is_fun [THEN apply_type] inj_succ_restrict)
    done

    lemma Atleast_superset:
        "[| Atleast(n,A);  A ⊆ B |] ==> Atleast(n,B)"
    by (unfold Atleast_def, blast intro: inj_weaken_type)

    lemma Atleast_succI:
        "[| Atleast(m,B);  b∉ B |] ==> Atleast(succ(m), cons(b,B))"
    apply (unfold Atleast_def succ_def)
    apply (blast intro: inj_extend elim: mem_irrefl)
    done

    lemma Atleast_Diff_succI:
         "[| Atleast(m, B-{x});  x ∈ B |] ==> Atleast(succ(m), B)"
    by (blast intro: Atleast_succI [THEN Atleast_superset])

    (*** Main Cardinality Lemma ***)

    (*The #-succ(0) strengthens the original theorem statement, but precisely
      the same proof could be used!!*)
    lemma pigeon2 [rule_format]:
         "m ∈ nat ==>
              ∀n ∈ nat. ∀A B. Atleast((m#+n) #- succ(0), A Un B) -->
                               Atleast(m,A) | Atleast(n,B)"
    apply (induct_tac "m")
    apply (blast intro!: Atleast0, simp)
    apply (rule ballI)
    apply (rename_tac m' n) (*simplifier does NOT preserve bound names!*)
    apply (induct_tac "n", auto)
    apply (erule Atleast_succD [THEN bexE])
    apply (rename_tac n' A B z)
    apply (erule UnE)
    (**case z ∈ B.  Instantiate the '∀A B' induction hypothesis. **)
    apply (drule_tac [2] x1 = A and x = "B-{z}" in spec [THEN spec])
    apply (erule_tac [2] mp [THEN disjE])
    (*cases Atleast(succ(m1),A) and Atleast(succ(k),B)*)
    apply (erule_tac [3] asm_rl notE Atleast_Diff_succI)+
    (*proving the condition*)
    prefer 2 apply (blast intro: Atleast_superset)
    (**case z ∈ A.  Instantiate the '∀n ∈ nat. ∀A B' induction hypothesis. **)
    apply (drule_tac x2="succ(n')" and x1="A-{z}" and x=B
           in bspec [THEN spec, THEN spec])
    apply (erule nat_succI)
    apply (erule mp [THEN disjE])
    (*cases Atleast(succ(m1),A) and Atleast(succ(k),B)*)
    apply (erule_tac [2] asm_rl Atleast_Diff_succI notE)+
    (*proving the condition*)
    apply simp
    apply (blast intro: Atleast_superset)
    done

    (**** Ramsey's Theorem ****)

    (** Base cases of induction; they now admit ANY Ramsey number **)

    lemma Ramsey0j: "Ramsey(n,0,j)"
    by (unfold Ramsey_def, blast)

    lemma Ramseyi0: "Ramsey(n,i,0)"
    by (unfold Ramsey_def, blast)

    (** Lemmas for induction step **)

    (*The use of succ(m) here, rather than #-succ(0), simplifies the proof of
      Ramsey_step_lemma.*)
    lemma Atleast_partition: "[| Atleast(m #+ n, A);  m ∈ nat;  n ∈ nat |]
          ==> Atleast(succ(m), {x ∈ A. ~P(x)}) | Atleast(n, {x ∈ A. P(x)})"
    apply (rule nat_succI [THEN pigeon2], assumption+)
    apply (rule Atleast_superset, auto)
    done

    (*For the Atleast part, proves ~(a ∈ I) from the second premise!*)
    lemma Indept_succ:
        "[| Indept(I, {z ∈ V-{a}. <a,z> ∉ E}, E);  Symmetric(E);  a ∈ V;
            Atleast(j,I) |] ==>
         Indept(cons(a,I), V, E) & Atleast(succ(j), cons(a,I))"
    apply (unfold Symmetric_def Indept_def)
    apply (blast intro!: Atleast_succI)
    done

    lemma Clique_succ:
        "[| Clique(C, {z ∈ V-{a}. <a,z>:E}, E);  Symmetric(E);  a ∈ V;
            Atleast(j,C) |] ==>
         Clique(cons(a,C), V, E) & Atleast(succ(j), cons(a,C))"
    apply (unfold Symmetric_def Clique_def)
    apply (blast intro!: Atleast_succI)
    done

    (** Induction step **)

    (*Published proofs gloss over the need for Ramsey numbers to be POSITIVE.*)
    lemma Ramsey_step_lemma:
       "[| Ramsey(succ(m), succ(i), j);  Ramsey(n, i, succ(j));
           m ∈ nat;  n ∈ nat |] ==> Ramsey(succ(m#+n), succ(i), succ(j))"
    apply (unfold Ramsey_def, clarify)
    apply (erule Atleast_succD [THEN bexE])
    apply (erule_tac P1 = "%z.<x,z>:E" in Atleast_partition [THEN disjE],
           assumption+)
    (*case m*)
    apply (fast dest!: Indept_succ elim: Clique_superset)
    (*case n*)
    apply (fast dest!: Clique_succ elim: Indept_superset)
    done

    (** The actual proof **)

    (*Again, the induction requires Ramsey numbers to be positive.*)
    lemma ramsey_lemma: "i ∈ nat ==> ∀j ∈ nat. ∃n ∈ nat. Ramsey(succ(n), i, j)"
    apply (induct_tac "i")
    apply (blast intro!: Ramsey0j)
    apply (rule ballI)
    apply (induct_tac "j")
    apply (blast intro!: Ramseyi0)
    apply (blast intro!: add_type Ramsey_step_lemma)
    done

    (*Final statement in a tidy form, without succ(...) *)
    lemma ramsey: "[| i ∈ nat;  j ∈ nat |] ==> ∃n ∈ nat. Ramsey(n,i,j)"
    by (blast dest: ramsey_lemma)

    end

lemma Clique0:

      Clique(0, V, E)

lemma Clique_superset:

      [| Clique(C, V', E); V' ⊆ V |] ==> Clique(C, V, E)

lemma Indept0:

      Indept(0, V, E)

lemma Indept_superset:

      [| Indept(I, V', E); V' ⊆ V |] ==> Indept(I, V, E)

lemma Atleast0:

      Atleast(0, A)

lemma Atleast_succD:

      Atleast(succ(m), A) ==> ∃x∈A. Atleast(m, A - {x})

lemma Atleast_superset:

      [| Atleast(n, A); A ⊆ B |] ==> Atleast(n, B)

lemma Atleast_succI:

      [| Atleast(m, B); b ∉ B |] ==> Atleast(succ(m), cons(b, B))

lemma Atleast_Diff_succI:

      [| Atleast(m, B - {x}); x ∈ B |] ==> Atleast(succ(m), B)

lemma pigeon2:

      [| m ∈ nat; n ∈ nat; Atleast(m #+ n #- 1, A ∪ B) |]
      ==> Atleast(m, A) ∨ Atleast(n, B)

lemma Ramsey0j:

      Ramsey(n, 0, j)

lemma Ramseyi0:

      Ramsey(n, i, 0)

lemma Atleast_partition:

      [| Atleast(m #+ n, A); m ∈ nat; n ∈ nat |]
      ==> Atleast(succ(m), {x ∈ A . ¬ P(x)}) ∨ Atleast(n, {x ∈ A . P(x)})

lemma Indept_succ:

      [| Indept(I, {z ∈ V - {a} . ⟨a, z⟩ ∉ E}, E); Symmetric(E); a ∈ V;
         Atleast(j, I) |]
      ==> Indept(cons(a, I), V, E) ∧ Atleast(succ(j), cons(a, I))

lemma Clique_succ:

      [| Clique(C, {z ∈ V - {a} . ⟨a, z⟩ ∈ E}, E); Symmetric(E); a ∈ V;
         Atleast(j, C) |]
      ==> Clique(cons(a, C), V, E) ∧ Atleast(succ(j), cons(a, C))

lemma Ramsey_step_lemma:

      [| Ramsey(succ(m), succ(i), j); Ramsey(n, i, succ(j)); m ∈ nat; n ∈ nat |]
      ==> Ramsey(succ(m #+ n), succ(i), succ(j))

lemma ramsey_lemma:

      i ∈ nat ==> ∀j∈nat. ∃n∈nat. Ramsey(succ(n), i, j)

lemma ramsey:

      [| i ∈ nat; j ∈ nat |] ==> ∃n∈nat. Ramsey(n, i, j)