header {* Algebraic deflations *}
theory Algebraic
imports Completion Fix Eventual
begin
subsection {* Constructing finite deflations by iteration *}
lemma finite_deflation_imp_deflation:
"finite_deflation d ==> deflation d"
unfolding finite_deflation_def by simp
lemma le_Suc_induct:
assumes le: "i ≤ j"
assumes step: "!!i. P i (Suc i)"
assumes refl: "!!i. P i i"
assumes trans: "!!i j k. [|P i j; P j k|] ==> P i k"
shows "P i j"
proof (cases "i = j")
assume "i = j"
thus "P i j" by (simp add: refl)
next
assume "i ≠ j"
with le have "i < j" by simp
thus "P i j" using step trans by (rule less_Suc_induct)
qed
text {* A pre-deflation is like a deflation, but not idempotent. *}
locale pre_deflation =
fixes f :: "'a -> 'a::cpo"
assumes less: "!!x. f·x \<sqsubseteq> x"
assumes finite_range: "finite (range (λx. f·x))"
begin
lemma iterate_less: "iterate i·f·x \<sqsubseteq> x"
by (induct i, simp_all add: trans_less [OF less])
lemma iterate_fixed: "f·x = x ==> iterate i·f·x = x"
by (induct i, simp_all)
lemma antichain_iterate_app: "i ≤ j ==> iterate j·f·x \<sqsubseteq> iterate i·f·x"
apply (erule le_Suc_induct)
apply (simp add: less)
apply (rule refl_less)
apply (erule (1) trans_less)
done
lemma finite_range_iterate_app: "finite (range (λi. iterate i·f·x))"
proof (rule finite_subset)
show "range (λi. iterate i·f·x) ⊆ insert x (range (λx. f·x))"
by (clarify, case_tac i, simp_all)
show "finite (insert x (range (λx. f·x)))"
by (simp add: finite_range)
qed
lemma eventually_constant_iterate_app:
"eventually_constant (λi. iterate i·f·x)"
unfolding eventually_constant_def MOST_nat_le
proof -
let ?Y = "λi. iterate i·f·x"
have "∃j. ∀k. ?Y j \<sqsubseteq> ?Y k"
apply (rule finite_range_has_max)
apply (erule antichain_iterate_app)
apply (rule finite_range_iterate_app)
done
then obtain j where j: "!!k. ?Y j \<sqsubseteq> ?Y k" by fast
show "∃z m. ∀n≥m. ?Y n = z"
proof (intro exI allI impI)
fix k
assume "j ≤ k"
hence "?Y k \<sqsubseteq> ?Y j" by (rule antichain_iterate_app)
also have "?Y j \<sqsubseteq> ?Y k" by (rule j)
finally show "?Y k = ?Y j" .
qed
qed
lemma eventually_constant_iterate:
"eventually_constant (λn. iterate n·f)"
proof -
have "∀y∈range (λx. f·x). eventually_constant (λi. iterate i·f·y)"
by (simp add: eventually_constant_iterate_app)
hence "∀y∈range (λx. f·x). MOST i. MOST j. iterate j·f·y = iterate i·f·y"
unfolding eventually_constant_MOST_MOST .
hence "MOST i. MOST j. ∀y∈range (λx. f·x). iterate j·f·y = iterate i·f·y"
by (simp only: MOST_finite_Ball_distrib [OF finite_range])
hence "MOST i. MOST j. ∀x. iterate j·f·(f·x) = iterate i·f·(f·x)"
by simp
hence "MOST i. MOST j. ∀x. iterate (Suc j)·f·x = iterate (Suc i)·f·x"
by (simp only: iterate_Suc2)
hence "MOST i. MOST j. iterate (Suc j)·f = iterate (Suc i)·f"
by (simp only: expand_cfun_eq)
hence "eventually_constant (λi. iterate (Suc i)·f)"
unfolding eventually_constant_MOST_MOST .
thus "eventually_constant (λi. iterate i·f)"
by (rule eventually_constant_SucD)
qed
abbreviation
d :: "'a -> 'a"
where
"d ≡ eventual (λn. iterate n·f)"
lemma MOST_d: "MOST n. P (iterate n·f) ==> P d"
using eventually_constant_iterate by (rule MOST_eventual)
lemma f_d: "f·(d·x) = d·x"
apply (rule MOST_d)
apply (subst iterate_Suc [symmetric])
apply (rule eventually_constant_MOST_Suc_eq)
apply (rule eventually_constant_iterate_app)
done
lemma d_fixed_iff: "d·x = x <-> f·x = x"
proof
assume "d·x = x"
with f_d [where x=x]
show "f·x = x" by simp
next
assume f: "f·x = x"
have "∀n. iterate n·f·x = x"
by (rule allI, rule nat.induct, simp, simp add: f)
hence "MOST n. iterate n·f·x = x"
by (rule ALL_MOST)
thus "d·x = x"
by (rule MOST_d)
qed
lemma finite_deflation_d: "finite_deflation d"
proof
fix x :: 'a
have "d ∈ range (λn. iterate n·f)"
using eventually_constant_iterate
by (rule eventual_mem_range)
then obtain n where n: "d = iterate n·f" ..
have "iterate n·f·(d·x) = d·x"
using f_d by (rule iterate_fixed)
thus "d·(d·x) = d·x"
by (simp add: n)
next
fix x :: 'a
show "d·x \<sqsubseteq> x"
by (rule MOST_d, simp add: iterate_less)
next
from finite_range
have "finite {x. f·x = x}"
by (rule finite_range_imp_finite_fixes)
thus "finite {x. d·x = x}"
by (simp add: d_fixed_iff)
qed
end
lemma pre_deflation_d_f:
assumes "finite_deflation d"
assumes f: "!!x. f·x \<sqsubseteq> x"
shows "pre_deflation (d oo f)"
proof
interpret d: finite_deflation d by fact
fix x
show "!!x. (d oo f)·x \<sqsubseteq> x"
by (simp, rule trans_less [OF d.less f])
show "finite (range (λx. (d oo f)·x))"
by (rule finite_subset [OF _ d.finite_range], auto)
qed
lemma eventual_iterate_oo_fixed_iff:
assumes "finite_deflation d"
assumes f: "!!x. f·x \<sqsubseteq> x"
shows "eventual (λn. iterate n·(d oo f))·x = x <-> d·x = x ∧ f·x = x"
proof -
interpret d: finite_deflation d by fact
let ?e = "d oo f"
interpret e: pre_deflation "d oo f"
using `finite_deflation d` f
by (rule pre_deflation_d_f)
let ?g = "eventual (λn. iterate n·?e)"
show ?thesis
apply (subst e.d_fixed_iff)
apply simp
apply safe
apply (erule subst)
apply (rule d.idem)
apply (rule antisym_less)
apply (rule f)
apply (erule subst, rule d.less)
apply simp
done
qed
subsection {* Type constructor for finite deflations *}
defaultsort profinite
typedef (open) 'a fin_defl = "{d::'a -> 'a. finite_deflation d}"
by (fast intro: finite_deflation_approx)
instantiation fin_defl :: (profinite) sq_ord
begin
definition
sq_le_fin_defl_def:
"op \<sqsubseteq> ≡ λx y. Rep_fin_defl x \<sqsubseteq> Rep_fin_defl y"
instance ..
end
instance fin_defl :: (profinite) po
by (rule typedef_po [OF type_definition_fin_defl sq_le_fin_defl_def])
lemma finite_deflation_Rep_fin_defl: "finite_deflation (Rep_fin_defl d)"
using Rep_fin_defl by simp
interpretation Rep_fin_defl: finite_deflation "Rep_fin_defl d"
by (rule finite_deflation_Rep_fin_defl)
lemma fin_defl_lessI:
"(!!x. Rep_fin_defl a·x = x ==> Rep_fin_defl b·x = x) ==> a \<sqsubseteq> b"
unfolding sq_le_fin_defl_def
by (rule Rep_fin_defl.lessI)
lemma fin_defl_lessD:
"[|a \<sqsubseteq> b; Rep_fin_defl a·x = x|] ==> Rep_fin_defl b·x = x"
unfolding sq_le_fin_defl_def
by (rule Rep_fin_defl.lessD)
lemma fin_defl_eqI:
"(!!x. Rep_fin_defl a·x = x <-> Rep_fin_defl b·x = x) ==> a = b"
apply (rule antisym_less)
apply (rule fin_defl_lessI, simp)
apply (rule fin_defl_lessI, simp)
done
lemma Abs_fin_defl_mono:
"[|finite_deflation a; finite_deflation b; a \<sqsubseteq> b|]
==> Abs_fin_defl a \<sqsubseteq> Abs_fin_defl b"
unfolding sq_le_fin_defl_def
by (simp add: Abs_fin_defl_inverse)
subsection {* Take function for finite deflations *}
definition
fd_take :: "nat => 'a fin_defl => 'a fin_defl"
where
"fd_take i d = Abs_fin_defl (eventual (λn. iterate n·(approx i oo Rep_fin_defl d)))"
lemma Rep_fin_defl_fd_take:
"Rep_fin_defl (fd_take i d) =
eventual (λn. iterate n·(approx i oo Rep_fin_defl d))"
unfolding fd_take_def
apply (rule Abs_fin_defl_inverse [unfolded mem_Collect_eq])
apply (rule pre_deflation.finite_deflation_d)
apply (rule pre_deflation_d_f)
apply (rule finite_deflation_approx)
apply (rule Rep_fin_defl.less)
done
lemma fd_take_fixed_iff:
"Rep_fin_defl (fd_take i d)·x = x <->
approx i·x = x ∧ Rep_fin_defl d·x = x"
unfolding Rep_fin_defl_fd_take
by (rule eventual_iterate_oo_fixed_iff
[OF finite_deflation_approx Rep_fin_defl.less])
lemma fd_take_less: "fd_take n d \<sqsubseteq> d"
apply (rule fin_defl_lessI)
apply (simp add: fd_take_fixed_iff)
done
lemma fd_take_idem: "fd_take n (fd_take n d) = fd_take n d"
apply (rule fin_defl_eqI)
apply (simp add: fd_take_fixed_iff)
done
lemma fd_take_mono: "a \<sqsubseteq> b ==> fd_take n a \<sqsubseteq> fd_take n b"
apply (rule fin_defl_lessI)
apply (simp add: fd_take_fixed_iff)
apply (simp add: fin_defl_lessD)
done
lemma approx_fixed_le_lemma: "[|i ≤ j; approx i·x = x|] ==> approx j·x = x"
by (erule subst, simp add: min_def)
lemma fd_take_chain: "m ≤ n ==> fd_take m a \<sqsubseteq> fd_take n a"
apply (rule fin_defl_lessI)
apply (simp add: fd_take_fixed_iff)
apply (simp add: approx_fixed_le_lemma)
done
lemma finite_range_fd_take: "finite (range (fd_take n))"
apply (rule finite_imageD [where f="λa. {x. Rep_fin_defl a·x = x}"])
apply (rule finite_subset [where B="Pow {x. approx n·x = x}"])
apply (clarify, simp add: fd_take_fixed_iff)
apply (simp add: finite_fixes_approx)
apply (rule inj_onI, clarify)
apply (simp add: expand_set_eq fin_defl_eqI)
done
lemma fd_take_covers: "∃n. fd_take n a = a"
apply (rule_tac x=
"Max ((λx. LEAST n. approx n·x = x) ` {x. Rep_fin_defl a·x = x})" in exI)
apply (rule antisym_less)
apply (rule fd_take_less)
apply (rule fin_defl_lessI)
apply (simp add: fd_take_fixed_iff)
apply (rule approx_fixed_le_lemma)
apply (rule Max_ge)
apply (rule finite_imageI)
apply (rule Rep_fin_defl.finite_fixes)
apply (rule imageI)
apply (erule CollectI)
apply (rule LeastI_ex)
apply (rule profinite_compact_eq_approx)
apply (erule subst)
apply (rule Rep_fin_defl.compact)
done
interpretation fin_defl: basis_take sq_le fd_take
apply default
apply (rule fd_take_less)
apply (rule fd_take_idem)
apply (erule fd_take_mono)
apply (rule fd_take_chain, simp)
apply (rule finite_range_fd_take)
apply (rule fd_take_covers)
done
subsection {* Defining algebraic deflations by ideal completion *}
typedef (open) 'a alg_defl =
"{S::'a fin_defl set. sq_le.ideal S}"
by (fast intro: sq_le.ideal_principal)
instantiation alg_defl :: (profinite) sq_ord
begin
definition
"x \<sqsubseteq> y <-> Rep_alg_defl x ⊆ Rep_alg_defl y"
instance ..
end
instance alg_defl :: (profinite) po
by (rule sq_le.typedef_ideal_po
[OF type_definition_alg_defl sq_le_alg_defl_def])
instance alg_defl :: (profinite) cpo
by (rule sq_le.typedef_ideal_cpo
[OF type_definition_alg_defl sq_le_alg_defl_def])
lemma Rep_alg_defl_lub:
"chain Y ==> Rep_alg_defl (\<Squnion>i. Y i) = (\<Union>i. Rep_alg_defl (Y i))"
by (rule sq_le.typedef_ideal_rep_contlub
[OF type_definition_alg_defl sq_le_alg_defl_def])
lemma ideal_Rep_alg_defl: "sq_le.ideal (Rep_alg_defl xs)"
by (rule Rep_alg_defl [unfolded mem_Collect_eq])
definition
alg_defl_principal :: "'a fin_defl => 'a alg_defl" where
"alg_defl_principal t = Abs_alg_defl {u. u \<sqsubseteq> t}"
lemma Rep_alg_defl_principal:
"Rep_alg_defl (alg_defl_principal t) = {u. u \<sqsubseteq> t}"
unfolding alg_defl_principal_def
by (simp add: Abs_alg_defl_inverse sq_le.ideal_principal)
interpretation alg_defl:
ideal_completion sq_le fd_take alg_defl_principal Rep_alg_defl
apply default
apply (rule ideal_Rep_alg_defl)
apply (erule Rep_alg_defl_lub)
apply (rule Rep_alg_defl_principal)
apply (simp only: sq_le_alg_defl_def)
done
text {* Algebraic deflations are pointed *}
lemma finite_deflation_UU: "finite_deflation ⊥"
by default simp_all
lemma alg_defl_minimal:
"alg_defl_principal (Abs_fin_defl ⊥) \<sqsubseteq> x"
apply (induct x rule: alg_defl.principal_induct, simp)
apply (rule alg_defl.principal_mono)
apply (induct_tac a)
apply (rule Abs_fin_defl_mono)
apply (rule finite_deflation_UU)
apply simp
apply (rule minimal)
done
instance alg_defl :: (bifinite) pcpo
by intro_classes (fast intro: alg_defl_minimal)
lemma inst_alg_defl_pcpo: "⊥ = alg_defl_principal (Abs_fin_defl ⊥)"
by (rule alg_defl_minimal [THEN UU_I, symmetric])
text {* Algebraic deflations are profinite *}
instantiation alg_defl :: (profinite) profinite
begin
definition
approx_alg_defl_def: "approx = alg_defl.completion_approx"
instance
apply (intro_classes, unfold approx_alg_defl_def)
apply (rule alg_defl.chain_completion_approx)
apply (rule alg_defl.lub_completion_approx)
apply (rule alg_defl.completion_approx_idem)
apply (rule alg_defl.finite_fixes_completion_approx)
done
end
instance alg_defl :: (bifinite) bifinite ..
lemma approx_alg_defl_principal [simp]:
"approx n·(alg_defl_principal t) = alg_defl_principal (fd_take n t)"
unfolding approx_alg_defl_def
by (rule alg_defl.completion_approx_principal)
lemma approx_eq_alg_defl_principal:
"∃t∈Rep_alg_defl xs. approx n·xs = alg_defl_principal (fd_take n t)"
unfolding approx_alg_defl_def
by (rule alg_defl.completion_approx_eq_principal)
subsection {* Applying algebraic deflations *}
definition
cast :: "'a alg_defl -> 'a -> 'a"
where
"cast = alg_defl.basis_fun Rep_fin_defl"
lemma cast_alg_defl_principal:
"cast·(alg_defl_principal a) = Rep_fin_defl a"
unfolding cast_def
apply (rule alg_defl.basis_fun_principal)
apply (simp only: sq_le_fin_defl_def)
done
lemma deflation_cast: "deflation (cast·d)"
apply (induct d rule: alg_defl.principal_induct)
apply (rule adm_subst [OF _ adm_deflation], simp)
apply (simp add: cast_alg_defl_principal)
apply (rule finite_deflation_imp_deflation)
apply (rule finite_deflation_Rep_fin_defl)
done
lemma finite_deflation_cast:
"compact d ==> finite_deflation (cast·d)"
apply (drule alg_defl.compact_imp_principal, clarify)
apply (simp add: cast_alg_defl_principal)
apply (rule finite_deflation_Rep_fin_defl)
done
interpretation cast: deflation "cast·d"
by (rule deflation_cast)
lemma "cast·(\<Squnion>i. alg_defl_principal (Abs_fin_defl (approx i)))·x = x"
apply (subst contlub_cfun_arg)
apply (rule chainI)
apply (rule alg_defl.principal_mono)
apply (rule Abs_fin_defl_mono)
apply (rule finite_deflation_approx)
apply (rule finite_deflation_approx)
apply (rule chainE)
apply (rule chain_approx)
apply (simp add: cast_alg_defl_principal Abs_fin_defl_inverse finite_deflation_approx)
done
text {* This lemma says that if we have an ep-pair from
a bifinite domain into a universal domain, then e oo p
is an algebraic deflation. *}
lemma
assumes "ep_pair e p"
constrains e :: "'a::profinite -> 'b::profinite"
shows "∃d. cast·d = e oo p"
proof
interpret ep_pair e p by fact
let ?a = "λi. e oo approx i oo p"
have a: "!!i. finite_deflation (?a i)"
apply (rule finite_deflation_e_d_p)
apply (rule finite_deflation_approx)
done
let ?d = "\<Squnion>i. alg_defl_principal (Abs_fin_defl (?a i))"
show "cast·?d = e oo p"
apply (subst contlub_cfun_arg)
apply (rule chainI)
apply (rule alg_defl.principal_mono)
apply (rule Abs_fin_defl_mono [OF a a])
apply (rule chainE, simp)
apply (subst cast_alg_defl_principal)
apply (simp add: Abs_fin_defl_inverse a)
apply (simp add: expand_cfun_eq lub_distribs)
done
qed
text {* This lemma says that if we have an ep-pair
from a cpo into a bifinite domain, and e oo p is
an algebraic deflation, then the cpo is bifinite. *}
lemma
assumes "ep_pair e p"
constrains e :: "'a::cpo -> 'b::profinite"
assumes d: "!!x. cast·d·x = e·(p·x)"
obtains a :: "nat => 'a -> 'a" where
"!!i. finite_deflation (a i)"
"(\<Squnion>i. a i) = ID"
proof
interpret ep_pair e p by fact
let ?a = "λi. p oo cast·(approx i·d) oo e"
show "!!i. finite_deflation (?a i)"
apply (rule finite_deflation_p_d_e)
apply (rule finite_deflation_cast)
apply (rule compact_approx)
apply (rule sq_ord_less_eq_trans [OF _ d])
apply (rule monofun_cfun_fun)
apply (rule monofun_cfun_arg)
apply (rule approx_less)
done
show "(\<Squnion>i. ?a i) = ID"
apply (rule ext_cfun, simp)
apply (simp add: lub_distribs)
apply (simp add: d)
done
qed
end