Theory Eventual

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theory Eventual
imports Infinite_Set

(*  Title:      HOLCF/Eventual.thy
Author: Brian Huffman
*)


header {* Eventually-constant sequences *}

theory Eventual
imports Infinite_Set
begin


subsection {* Lemmas about MOST *}

lemma MOST_INFM:
assumes inf: "infinite (UNIV::'a set)"
shows "MOST x::'a. P x ==> INFM x::'a. P x"

unfolding Alm_all_def Inf_many_def
apply (auto simp add: Collect_neg_eq)
apply (drule (1) finite_UnI)
apply (simp add: Compl_partition2 inf)
done

lemma MOST_SucI: "MOST n. P n ==> MOST n. P (Suc n)"
by (rule MOST_inj [OF _ inj_Suc])

lemma MOST_SucD: "MOST n. P (Suc n) ==> MOST n. P n"
unfolding MOST_nat
apply (clarify, rule_tac x="Suc m" in exI, clarify)
apply (erule Suc_lessE, simp)
done

lemma MOST_Suc_iff: "(MOST n. P (Suc n)) <-> (MOST n. P n)"
by (rule iffI [OF MOST_SucD MOST_SucI])

lemma INFM_finite_Bex_distrib:
"finite A ==> (INFM y. ∃x∈A. P x y) <-> (∃x∈A. INFM y. P x y)"

by (induct set: finite, simp, simp add: INFM_disj_distrib)

lemma MOST_finite_Ball_distrib:
"finite A ==> (MOST y. ∀x∈A. P x y) <-> (∀x∈A. MOST y. P x y)"

by (induct set: finite, simp, simp add: MOST_conj_distrib)

lemma MOST_ge_nat: "MOST n::nat. m ≤ n"
unfolding MOST_nat_le by fast

subsection {* Eventually constant sequences *}

definition
eventually_constant :: "(nat => 'a) => bool"
where
"eventually_constant S = (∃x. MOST i. S i = x)"


lemma eventually_constant_MOST_MOST:
"eventually_constant S <-> (MOST m. MOST n. S n = S m)"

unfolding eventually_constant_def MOST_nat
apply safe
apply (rule_tac x=m in exI, clarify)
apply (rule_tac x=m in exI, clarify)
apply simp
apply fast
done

lemma eventually_constantI: "MOST i. S i = x ==> eventually_constant S"
unfolding eventually_constant_def by fast

lemma eventually_constant_comp:
"eventually_constant (λi. S i) ==> eventually_constant (λi. f (S i))"

unfolding eventually_constant_def
apply (erule exE, rule_tac x="f x" in exI)
apply (erule MOST_mono, simp)
done

lemma eventually_constant_Suc_iff:
"eventually_constant (λi. S (Suc i)) <-> eventually_constant (λi. S i)"

unfolding eventually_constant_def
by (subst MOST_Suc_iff, rule refl)

lemma eventually_constant_SucD:
"eventually_constant (λi. S (Suc i)) ==> eventually_constant (λi. S i)"

by (rule eventually_constant_Suc_iff [THEN iffD1])

subsection {* Limits of eventually constant sequences *}

definition
eventual :: "(nat => 'a) => 'a" where
"eventual S = (THE x. MOST i. S i = x)"


lemma eventual_eqI: "MOST i. S i = x ==> eventual S = x"
unfolding eventual_def
apply (rule the_equality, assumption)
apply (rename_tac y)
apply (subgoal_tac "MOST i::nat. y = x", simp)
apply (erule MOST_rev_mp)
apply (erule MOST_rev_mp)
apply simp
done

lemma MOST_eq_eventual:
"eventually_constant S ==> MOST i. S i = eventual S"

unfolding eventually_constant_def
by (erule exE, simp add: eventual_eqI)

lemma eventual_mem_range:
"eventually_constant S ==> eventual S ∈ range S"

apply (drule MOST_eq_eventual)
apply (simp only: MOST_nat_le, clarify)
apply (drule spec, drule mp, rule order_refl)
apply (erule range_eqI [OF sym])
done

lemma eventually_constant_MOST_iff:
assumes S: "eventually_constant S"
shows "(MOST n. P (S n)) <-> P (eventual S)"

apply (subgoal_tac "(MOST n. P (S n)) <-> (MOST n::nat. P (eventual S))")
apply simp
apply (rule iffI)
apply (rule MOST_rev_mp [OF MOST_eq_eventual [OF S]])
apply (erule MOST_mono, force)
apply (rule MOST_rev_mp [OF MOST_eq_eventual [OF S]])
apply (erule MOST_mono, simp)
done

lemma MOST_eventual:
"[|eventually_constant S; MOST n. P (S n)|] ==> P (eventual S)"

proof -
assume "eventually_constant S"
hence "MOST n. S n = eventual S"
by (rule MOST_eq_eventual)
moreover assume "MOST n. P (S n)"
ultimately have "MOST n. S n = eventual S ∧ P (S n)"
by (rule MOST_conj_distrib [THEN iffD2, OF conjI])
hence "MOST n::nat. P (eventual S)"
by (rule MOST_mono) auto
thus ?thesis by simp
qed

lemma eventually_constant_MOST_Suc_eq:
"eventually_constant S ==> MOST n. S (Suc n) = S n"

apply (drule MOST_eq_eventual)
apply (frule MOST_Suc_iff [THEN iffD2])
apply (erule MOST_rev_mp)
apply (erule MOST_rev_mp)
apply simp
done

lemma eventual_comp:
"eventually_constant S ==> eventual (λi. f (S i)) = f (eventual (λi. S i))"

apply (rule eventual_eqI)
apply (rule MOST_mono)
apply (erule MOST_eq_eventual)
apply simp
done

end