src/CCL/Lfp.thy
 author Manuel Eberl Wed, 07 Apr 2021 11:05:00 +0200 changeset 73791 16a2b7f582fa parent 67443 3abf6a722518 permissions -rw-r--r--
fixed problematic addition operation in the 'approximation' package (previous version used much too high precision sometimes)
```
(*  Title:      CCL/Lfp.thy
Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
Copyright   1992  University of Cambridge
*)

section \<open>The Knaster-Tarski Theorem\<close>

theory Lfp
imports Set
begin

definition
lfp :: "['a set\<Rightarrow>'a set] \<Rightarrow> 'a set" where \<comment> \<open>least fixed point\<close>
"lfp(f) == Inter({u. f(u) <= u})"

(* lfp(f) is the greatest lower bound of {u. f(u) <= u} *)

lemma lfp_lowerbound: "f(A) <= A \<Longrightarrow> lfp(f) <= A"
unfolding lfp_def by blast

lemma lfp_greatest: "(\<And>u. f(u) <= u \<Longrightarrow> A<=u) \<Longrightarrow> A <= lfp(f)"
unfolding lfp_def by blast

lemma lfp_lemma2: "mono(f) \<Longrightarrow> f(lfp(f)) <= lfp(f)"
by (rule lfp_greatest, rule subset_trans, drule monoD, rule lfp_lowerbound, assumption+)

lemma lfp_lemma3: "mono(f) \<Longrightarrow> lfp(f) <= f(lfp(f))"
by (rule lfp_lowerbound, frule monoD, drule lfp_lemma2, assumption+)

lemma lfp_Tarski: "mono(f) \<Longrightarrow> lfp(f) = f(lfp(f))"
by (rule equalityI lfp_lemma2 lfp_lemma3 | assumption)+

(*** General induction rule for least fixed points ***)

lemma induct:
assumes lfp: "a: lfp(f)"
and mono: "mono(f)"
and indhyp: "\<And>x. \<lbrakk>x: f(lfp(f) Int {x. P(x)})\<rbrakk> \<Longrightarrow> P(x)"
shows "P(a)"
apply (rule_tac a = a in Int_lower2 [THEN subsetD, THEN CollectD])
apply (rule lfp [THEN [2] lfp_lowerbound [THEN subsetD]])
apply (rule Int_greatest, rule subset_trans, rule Int_lower1 [THEN mono [THEN monoD]],
rule mono [THEN lfp_lemma2], rule CollectI [THEN subsetI], rule indhyp, assumption)
done

(** Definition forms of lfp_Tarski and induct, to control unfolding **)

lemma def_lfp_Tarski: "\<lbrakk>h == lfp(f); mono(f)\<rbrakk> \<Longrightarrow> h = f(h)"
apply unfold
apply (drule lfp_Tarski)
apply assumption
done

lemma def_induct: "\<lbrakk>A == lfp(f);  a:A;  mono(f); \<And>x. x: f(A Int {x. P(x)}) \<Longrightarrow> P(x)\<rbrakk> \<Longrightarrow> P(a)"
apply (rule induct [of concl: P a])
apply simp
apply assumption
apply blast
done

(*Monotonicity of lfp!*)
lemma lfp_mono: "\<lbrakk>mono(g); \<And>Z. f(Z) <= g(Z)\<rbrakk> \<Longrightarrow> lfp(f) <= lfp(g)"
apply (rule lfp_lowerbound)
apply (rule subset_trans)
apply (erule meta_spec)
apply (erule lfp_lemma2)
done

end
```