Quelle Fixedpt.thy Sprache: Isabelle

(*  Title:      ZF/Fixedpt.thy
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1992  University of Cambridge
*)

section\<open>Least and Greatest Fixed Points; the Knaster-Tarski Theorem\<close>

theory Fixedpt imports equalities begin

definition
  (*monotone operator from Pow(D) to itself*)
  bnd_mono :: "[i,i\i]\o" where
     "bnd_mono(D,h) \ h(D)<=D \ (\W X. W<=X \ X<=D \ h(W) \ h(X))"

definition
  lfp      :: "[i,i\i]\i" where
     "lfp(D,h) \ \({X: Pow(D). h(X) \ X})"

definition
  gfp      :: "[i,i\i]\i" where
     "gfp(D,h) \ \({X: Pow(D). X \ h(X)})"

text\<open>The theorem is proved in the lattice of subsets of \<^term>\<open>D\<close>,
      namely \<^term>\<open>Pow(D)\<close>, with Inter as the greatest lower bound.\<close>

subsection\<open>Monotone Operators\<close>

lemma bnd_monoI:
    "\h(D)<=D;
        \<And>W X. \<lbrakk>W<=D;  X<=D;  W<=X\<rbrakk> \<Longrightarrow> h(W) \<subseteq> h(X)
\<rbrakk> \<Longrightarrow> bnd_mono(D,h)"
by (unfold bnd_mono_def, clarify, blast)

lemma bnd_monoD1: "bnd_mono(D,h) \ h(D) \ D"
  unfolding bnd_mono_def
apply (erule conjunct1)
done

lemma bnd_monoD2: "\bnd_mono(D,h); W<=X; X<=D\ \ h(W) \ h(X)"
by (unfold bnd_mono_def, blast)

lemma bnd_mono_subset:
    "\bnd_mono(D,h); X<=D\ \ h(X) \ D"
by (unfold bnd_mono_def, clarify, blast)

lemma bnd_mono_Un:
     "\bnd_mono(D,h); A \ D; B \ D\ \ h(A) \ h(B) \ h(A \ B)"
  unfolding bnd_mono_def
apply (rule Un_least, blast+)
done

(*unused*)
lemma bnd_mono_UN:
     "\bnd_mono(D,h); \i\I. A(i) \ D\
      \<Longrightarrow> (\<Union>i\<in>I. h(A(i))) \<subseteq> h((\<Union>i\<in>I. A(i)))"
  unfolding bnd_mono_def
apply (rule UN_least)
apply (elim conjE)
apply (drule_tac x="A(i)" in spec)
apply (drule_tac x="(\i\I. A(i))" in spec)
apply blast
done

(*Useful??*)
lemma bnd_mono_Int:
     "\bnd_mono(D,h); A \ D; B \ D\ \ h(A \ B) \ h(A) \ h(B)"
apply (rule Int_greatest)
apply (erule bnd_monoD2, rule Int_lower1, assumption)
apply (erule bnd_monoD2, rule Int_lower2, assumption)
done

subsection\<open>Proof of Knaster-Tarski Theorem using \<^term>\<open>lfp\<close>\<close>

(*lfp is contained in each pre-fixedpoint*)
lemma lfp_lowerbound:
    "\h(A) \ A; A<=D\ \ lfp(D,h) \ A"
by (unfold lfp_def, blast)

(*Unfolding the defn of Inter dispenses with the premise bnd_mono(D,h)!*)
lemma lfp_subset: "lfp(D,h) \ D"
by (unfold lfp_def Inter_def, blast)

(*Used in datatype package*)
lemma def_lfp_subset:  "A \ lfp(D,h) \ A \ D"
apply simp
apply (rule lfp_subset)
done

lemma lfp_greatest:
    "\h(D) \ D; \X. \h(X) \ X; X<=D\ \ A<=X\ \ A \ lfp(D,h)"
by (unfold lfp_def, blast)

lemma lfp_lemma1:
    "\bnd_mono(D,h); h(A)<=A; A<=D\ \ h(lfp(D,h)) \ A"
apply (erule bnd_monoD2 [THEN subset_trans])
apply (rule lfp_lowerbound, assumption+)
done

lemma lfp_lemma2: "bnd_mono(D,h) \ h(lfp(D,h)) \ lfp(D,h)"
apply (rule bnd_monoD1 [THEN lfp_greatest])
apply (rule_tac [2] lfp_lemma1)
apply (assumption+)
done

lemma lfp_lemma3:
    "bnd_mono(D,h) \ lfp(D,h) \ h(lfp(D,h))"
apply (rule lfp_lowerbound)
apply (rule bnd_monoD2, assumption)
apply (rule lfp_lemma2, assumption)
apply (erule_tac [2] bnd_mono_subset)
apply (rule lfp_subset)+
done

lemma lfp_unfold: "bnd_mono(D,h) \ lfp(D,h) = h(lfp(D,h))"
apply (rule equalityI)
apply (erule lfp_lemma3)
apply (erule lfp_lemma2)
done

(*Definition form, to control unfolding*)
lemma def_lfp_unfold:
    "\A\lfp(D,h); bnd_mono(D,h)\ \ A = h(A)"
apply simp
apply (erule lfp_unfold)
done

subsection\<open>General Induction Rule for Least Fixedpoints\<close>

lemma Collect_is_pre_fixedpt:
    "\bnd_mono(D,h); \x. x \ h(Collect(lfp(D,h),P)) \ P(x)\
     \<Longrightarrow> h(Collect(lfp(D,h),P)) \<subseteq> Collect(lfp(D,h),P)"
by (blast intro: lfp_lemma2 [THEN subsetD] bnd_monoD2 [THEN subsetD]
                 lfp_subset [THEN subsetD])

(*This rule yields an induction hypothesis in which the components of a
  data structure may be assumed to be elements of lfp(D,h)*)
lemma induct:
    "\bnd_mono(D,h); a \ lfp(D,h);
        \<And>x. x \<in> h(Collect(lfp(D,h),P)) \<Longrightarrow> P(x)
\<rbrakk> \<Longrightarrow> P(a)"
apply (rule Collect_is_pre_fixedpt
              [THEN lfp_lowerbound, THEN subsetD, THEN CollectD2])
apply (rule_tac [3] lfp_subset [THEN Collect_subset [THEN subset_trans]],
       blast+)
done

(*Definition form, to control unfolding*)
lemma def_induct:
    "\A \ lfp(D,h); bnd_mono(D,h); a:A;
        \<And>x. x \<in> h(Collect(A,P)) \<Longrightarrow> P(x)
\<rbrakk> \<Longrightarrow> P(a)"
by (rule induct, blast+)

(*This version is useful when "A" is not a subset of D
  second premise could simply be h(D \<inter> A) \<subseteq> D or \<And>X. X<=D \<Longrightarrow> h(X)<=D *)
lemma lfp_Int_lowerbound:
    "\h(D \ A) \ A; bnd_mono(D,h)\ \ lfp(D,h) \ A"
apply (rule lfp_lowerbound [THEN subset_trans])
apply (erule bnd_mono_subset [THEN Int_greatest], blast+)
done

(*Monotonicity of lfp, where h precedes i under a domain-like partial order
  monotonicity of h is not strictly necessary; h must be bounded by D*)
lemma lfp_mono:
  assumes hmono: "bnd_mono(D,h)"
      and imono: "bnd_mono(E,i)"
      and subhi: "\X. X<=D \ h(X) \ i(X)"
    shows "lfp(D,h) \ lfp(E,i)"
apply (rule bnd_monoD1 [THEN lfp_greatest])
apply (rule imono)
apply (rule hmono [THEN [2] lfp_Int_lowerbound])
apply (rule Int_lower1 [THEN subhi, THEN subset_trans])
apply (rule imono [THEN bnd_monoD2, THEN subset_trans], auto)
done

(*This (unused) version illustrates that monotonicity is not really needed,
  but both lfp's must be over the SAME set D;  Inter is anti-monotonic!*)
lemma lfp_mono2:
    "\i(D) \ D; \X. X<=D \ h(X) \ i(X)\ \ lfp(D,h) \ lfp(D,i)"
apply (rule lfp_greatest, assumption)
apply (rule lfp_lowerbound, blast, assumption)
done

lemma lfp_cong:
     "\D=D'; \X. X \ D' \ h(X) = h'(X)\ \ lfp(D,h) = lfp(D',h')"
apply (simp add: lfp_def)
apply (rule_tac t=Inter in subst_context)
apply (rule Collect_cong, simp_all)
done

subsection\<open>Proof of Knaster-Tarski Theorem using \<^term>\<open>gfp\<close>\<close>

(*gfp contains each post-fixedpoint that is contained in D*)
lemma gfp_upperbound: "\A \ h(A); A<=D\ \ A \ gfp(D,h)"
  unfolding gfp_def
apply (rule PowI [THEN CollectI, THEN Union_upper])
apply (assumption+)
done

lemma gfp_subset: "gfp(D,h) \ D"
by (unfold gfp_def, blast)

(*Used in datatype package*)
lemma def_gfp_subset: "A\gfp(D,h) \ A \ D"
apply simp
apply (rule gfp_subset)
done

lemma gfp_least:
    "\bnd_mono(D,h); \X. \X \ h(X); X<=D\ \ X<=A\ \
     gfp(D,h) \<subseteq> A"
  unfolding gfp_def
apply (blast dest: bnd_monoD1)
done

lemma gfp_lemma1:
    "\bnd_mono(D,h); A<=h(A); A<=D\ \ A \ h(gfp(D,h))"
apply (rule subset_trans, assumption)
apply (erule bnd_monoD2)
apply (rule_tac [2] gfp_subset)
apply (simp add: gfp_upperbound)
done

lemma gfp_lemma2: "bnd_mono(D,h) \ gfp(D,h) \ h(gfp(D,h))"
apply (rule gfp_least)
apply (rule_tac [2] gfp_lemma1)
apply (assumption+)
done

lemma gfp_lemma3:
    "bnd_mono(D,h) \ h(gfp(D,h)) \ gfp(D,h)"
apply (rule gfp_upperbound)
apply (rule bnd_monoD2, assumption)
apply (rule gfp_lemma2, assumption)
apply (erule bnd_mono_subset, rule gfp_subset)+
done

lemma gfp_unfold: "bnd_mono(D,h) \ gfp(D,h) = h(gfp(D,h))"
apply (rule equalityI)
apply (erule gfp_lemma2)
apply (erule gfp_lemma3)
done

(*Definition form, to control unfolding*)
lemma def_gfp_unfold:
    "\A\gfp(D,h); bnd_mono(D,h)\ \ A = h(A)"
apply simp
apply (erule gfp_unfold)
done

subsection\<open>Coinduction Rules for Greatest Fixed Points\<close>

(*weak version*)
lemma weak_coinduct: "\a: X; X \ h(X); X \ D\ \ a \ gfp(D,h)"
by (blast intro: gfp_upperbound [THEN subsetD])

lemma coinduct_lemma:
    "\X \ h(X \ gfp(D,h)); X \ D; bnd_mono(D,h)\ \
     X \<union> gfp(D,h) \<subseteq> h(X \<union> gfp(D,h))"
apply (erule Un_least)
apply (rule gfp_lemma2 [THEN subset_trans], assumption)
apply (rule Un_upper2 [THEN subset_trans])
apply (rule bnd_mono_Un, assumption+)
apply (rule gfp_subset)
done

(*strong version*)
lemma coinduct:
     "\bnd_mono(D,h); a: X; X \ h(X \ gfp(D,h)); X \ D\
      \<Longrightarrow> a \<in> gfp(D,h)"
apply (rule weak_coinduct)
apply (erule_tac [2] coinduct_lemma)
apply (simp_all add: gfp_subset Un_subset_iff)
done

(*Definition form, to control unfolding*)
lemma def_coinduct:
    "\A \ gfp(D,h); bnd_mono(D,h); a: X; X \ h(X \ A); X \ D\ \
     a \<in> A"
apply simp
apply (rule coinduct, assumption+)
done

(*The version used in the induction/coinduction package*)
lemma def_Collect_coinduct:
    "\A \ gfp(D, \w. Collect(D,P(w))); bnd_mono(D, \w. Collect(D,P(w)));
        a: X;  X \<subseteq> D;  \<And>z. z: X \<Longrightarrow> P(X \<union> A, z)\<rbrakk> \<Longrightarrow>
     a \<in> A"
apply (rule def_coinduct, assumption+, blast+)
done

(*Monotonicity of gfp!*)
lemma gfp_mono:
    "\bnd_mono(D,h); D \ E;
        \<And>X. X<=D \<Longrightarrow> h(X) \<subseteq> i(X)\<rbrakk> \<Longrightarrow> gfp(D,h) \<subseteq> gfp(E,i)"
apply (rule gfp_upperbound)
apply (rule gfp_lemma2 [THEN subset_trans], assumption)
apply (blast del: subsetI intro: gfp_subset)
apply (blast del: subsetI intro: subset_trans gfp_subset)
done

end

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