mirror of https://github.com/nmvdw/HITs-Examples
Added proof: Bishop finite => Kuratowski finite
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Niels
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@ -1,6 +1,7 @@
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(* Bishop-finiteness is that "default" notion of finiteness in the HoTT library *)
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Require Import HoTT.
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Require Import Sub notation.
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Require Import Sub notation variations.k_finite.
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Require Import fsets.properties.
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Section finite_hott.
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Variable A : Type.
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@ -132,4 +133,255 @@ Section finite_hott.
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** refine (px @ _ @ py^). symmetry. auto.
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** apply (px @ py^).
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Defined.
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Section empty.
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Variable (X : A -> hProp)
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(Xequiv : {a : A & a ∈ X} <~> Fin 0).
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Lemma X_empty : X = ∅.
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Proof.
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apply path_forall.
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intro z.
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apply path_iff_hprop ; try contradiction.
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intro x.
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destruct Xequiv as [f fequiv].
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contradiction (f(z;x)).
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Defined.
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End empty.
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Section split.
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Variable (X : A -> hProp)
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(n : nat)
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(Xequiv : {a : A & a ∈ X} <~> Fin n + Unit).
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Definition split : {X' : A -> hProp & {a : A & a ∈ X'} <~> Fin n}.
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Proof.
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destruct Xequiv as [f [g fg gf adj]].
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unfold Sect in *.
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pose (fun x : A => sig (fun y : Fin n => x = (g(inl y)).1 )) as X'.
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assert (forall a : A, IsHProp (X' a)).
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{
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intros.
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unfold X'.
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apply hprop_allpath.
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intros [x px] [y py].
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simple refine (path_sigma _ _ _ _ _).
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* cbn.
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pose (f(g(inl x))) as fgx.
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cut (g(inl x) = g(inl y)).
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{
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intros q.
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pose (ap f q).
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rewrite !fg in p.
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refine (path_sum_inl _ p).
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}
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apply path_sigma with (px^ @ py).
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apply path_ishprop.
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* apply path_ishprop.
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}
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pose (fun a => BuildhProp(X' a)) as Y.
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exists Y.
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unfold Y, X'.
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cbn.
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unshelve esplit.
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- intros [a [y p]]. apply y.
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- apply isequiv_biinv.
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unshelve esplit.
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* exists (fun x => (( (g(inl x)).1 ;(x;idpath)))).
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unfold Sect.
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intros [a [y p]].
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apply path_sigma with p^.
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simpl.
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rewrite transport_sigma.
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simple refine (path_sigma _ _ _ _ _) ; simpl.
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** rewrite transport_const ; reflexivity.
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** apply path_ishprop.
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* exists (fun x => (( (g(inl x)).1 ;(x;idpath)))).
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unfold Sect.
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intros x.
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reflexivity.
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Defined.
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Definition new_el : {a' : A & forall z, X z =
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lor (split.1 z) (merely (z = a'))}.
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Proof.
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exists ((Xequiv^-1 (inr tt)).1).
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intros.
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apply path_iff_hprop.
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- intros Xz.
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pose (Xequiv (z;Xz)) as fz.
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pose (c := fz).
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assert (c = fz). reflexivity.
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destruct c as [fz1 | fz2].
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* refine (tr(inl _)).
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unfold split ; simpl.
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exists fz1.
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rewrite X0.
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unfold fz.
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destruct Xequiv as [? [? ? sect ?]].
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compute.
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rewrite sect.
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reflexivity.
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* refine (tr(inr(tr _))).
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destruct fz2.
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rewrite X0.
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unfold fz.
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rewrite eissect.
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reflexivity.
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- intros X0.
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strip_truncations.
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destruct X0 as [Xl | Xr].
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* unfold split in Xl ; simpl in Xl.
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destruct Xequiv as [f [g fg gf adj]].
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destruct Xl as [m p].
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rewrite p.
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apply (g (inl m)).2.
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* strip_truncations.
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rewrite Xr.
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apply ((Xequiv^-1(inr tt)).2).
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Defined.
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End split.
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Definition bfin_to_kfin : forall (X : Sub A), Bfin X -> Kf_sub _ X.
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Proof.
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intros X BFinX.
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unfold Bfin in BFinX.
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destruct BFinX as [n iso].
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strip_truncations.
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revert iso ; revert X.
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induction n ; unfold Kf_sub, Kf_sub_intern.
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- intros.
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exists ∅.
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apply path_forall.
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intro z.
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simpl in *.
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apply path_iff_hprop ; try contradiction.
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destruct iso as [f f_equiv].
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apply (fun Xz => f(z;Xz)).
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- intros.
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simpl in *.
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destruct (new_el X n iso) as [a HXX'].
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destruct (split X n iso) as [X' X'equiv].
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destruct (IHn X' X'equiv) as [Y HY].
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exists (Y ∪ {|a|}).
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unfold map in *.
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apply path_forall.
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intro z.
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rewrite union_isIn.
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rewrite <- (ap (fun h => h z) HY).
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rewrite HXX'.
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cbn.
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reflexivity.
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Defined.
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Context `{A_deceq : DecidablePaths A}.
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(*
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Lemma kfin_is_bfin : closedUnion Bfin.
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Proof.
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intros X Y HX HY.
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unfold Bfin in *.
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destruct HX as [n Xequiv].
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revert X Xequiv.
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induction n.
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- intros.
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strip_truncations.
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rewrite (X_empty X Xequiv).
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assert(∅ ∪ Y = Y).
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{ apply path_forall ; intro z.
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compute-[lor].
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eauto with lattice_hints typeclass_instances.
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}
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rewrite X0.
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apply HY.
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- simpl in *.
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intros.
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destruct HY as [m Yequiv].
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strip_truncations.
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destruct (new_el X n Xequiv) as [a HXX'].
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destruct (split X n Xequiv) as [X' X'equiv].
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destruct (IHn X' (tr X'equiv)) as [k Hk].
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strip_truncations.
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cbn in *.
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rewrite (path_forall _ _ HXX').
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assert
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(forall a0,
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BuildhProp (Trunc (-1) (X' a0 ∨ merely (a0 = a) + Y a0))
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=
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BuildhProp (hor (Trunc (-1) (X' a0 + Y a0)) (merely (a0 = a)))
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).
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{
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intros.
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apply path_iff_hprop.
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* intros X0.
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strip_truncations.
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destruct X0 as [X0 | X0].
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** strip_truncations.
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destruct X0 as [X0 | X0].
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*** refine (tr(inl(tr _))).
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apply (inl X0).
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*** refine (tr(inr X0)).
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** refine (tr(inl(tr _))).
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apply (inr X0).
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* intros X0.
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strip_truncations.
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destruct X0 as [X0 | X0].
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** strip_truncations.
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destruct X0 as [X0 | X0].
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*** refine (tr(inl(tr(inl X0)))).
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*** refine (tr(inr X0)).
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** refine (tr(inl(tr(inr X0)))).
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}
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(* rewrite (path_forall _ _ X0). *)
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assert
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(
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{a0 : A & hor (Trunc (-1) (X' a0 + Y a0)) (merely (a0 = a))}
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=
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{a0 : A & Trunc (-1) (X' a0 + Y a0)}
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+
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{a0 : A & (merely (a0 = a))}
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).
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{
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assert ({a0 : A & Trunc (-1) (X' a0 + Y a0)} + {a0 : A & merely (a0 = a)} ->
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{a0 : A & hor (Trunc (-1) (X' a0 + Y a0)) (merely (a0 = a))}).
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{
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intros.
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destruct X1.
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* destruct s as [c p].
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exists c.
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apply tr.
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left.
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apply p.
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* destruct s as [c p].
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exists c.
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apply tr.
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right. apply p.
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simple refine (path_universe _).
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* intros [a0 p].
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destruct (dec (a0 = a)).
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** right. exists a0. apply (tr p0).
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** left.
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exists a0.
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strip_truncations.
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destruct p ; strip_truncations.
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*** apply (tr t).
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*** contradiction (n0 t).
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* apply isequiv_biinv.
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unfold BiInv.
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split.
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**
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exists a0
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}
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rewrite X1.
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apply finite_sum.
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* simple refine (Build_Finite _ k (tr Hk)).
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* apply singleton.
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Admitted.
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*)
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End finite_hott.
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