139 lines
5.2 KiB
Text
139 lines
5.2 KiB
Text
/-
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Copyright (c) 2015 Jakob von Raumer. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Module: types.trunc
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Authors: Jakob von Raumer, Floris van Doorn
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Properties of is_trunc
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-/
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import types.pi types.eq
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open sigma sigma.ops pi function eq equiv eq funext
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namespace is_trunc
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definition is_contr.sigma_char (A : Type) :
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(Σ (center : A), Π (a : A), center = a) ≃ (is_contr A) :=
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begin
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fapply equiv.mk,
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{intro S, apply is_contr.mk, exact S.2},
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{fapply is_equiv.adjointify,
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{intro H, apply sigma.mk, exact (@center_eq A H)},
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{intro H, apply (is_trunc.rec_on H), intro Hint,
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apply (contr_internal.rec_on Hint), intros [H1, H2],
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apply idp},
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{intro S, cases S, apply idp}}
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end
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definition is_trunc.pi_char (n : trunc_index) (A : Type) :
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(Π (x y : A), is_trunc n (x = y)) ≃ (is_trunc (n .+1) A) :=
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begin
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fapply equiv.MK,
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{intro H, apply is_trunc_succ_intro},
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{intros [H, x, y], apply is_trunc_eq},
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{intro H, apply (is_trunc.rec_on H), intro Hint, apply idp},
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{intro P, apply eq_of_homotopy, intro a, apply eq_of_homotopy, intro b,
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esimp [function.id,compose,is_trunc_succ_intro,is_trunc_eq],
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generalize (P a b), intro H, apply (is_trunc.rec_on H), intro H', apply idp},
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end
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definition is_hprop_is_trunc [instance] (n : trunc_index) :
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Π (A : Type), is_hprop (is_trunc n A) :=
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begin
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apply (trunc_index.rec_on n),
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{intro A,
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apply is_trunc_is_equiv_closed, apply equiv.to_is_equiv,
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apply is_contr.sigma_char,
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apply (@is_hprop.mk), intros,
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fapply sigma_eq, apply x.2,
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apply (@is_hprop.elim),
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apply is_trunc_pi, intro a,
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apply is_hprop.mk, intros [w, z],
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have H : is_hset A,
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begin
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apply is_trunc_succ, apply is_trunc_succ,
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apply is_contr.mk, exact y.2
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end,
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fapply (@is_hset.elim A _ _ _ w z)},
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{intros [n', IH, A],
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apply is_trunc_is_equiv_closed,
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apply equiv.to_is_equiv,
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apply is_trunc.pi_char},
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end
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definition is_trunc_succ_of_imp_is_trunc_succ {A : Type} {n : trunc_index} (H : A → is_trunc (n.+1) A)
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: is_trunc (n.+1) A :=
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@is_trunc_succ_intro _ _ (λx y, @is_trunc_eq _ _ (H x) x y)
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definition is_trunc_of_imp_is_trunc_of_leq {A : Type} {n : trunc_index} (Hn : -1 ≤ n)
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(H : A → is_trunc n A) : is_trunc n A :=
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trunc_index.rec_on n (λHn H, empty.rec _ Hn)
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(λn IH Hn, is_trunc_succ_of_imp_is_trunc_succ)
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Hn H
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definition is_hset_of_axiom_K {A : Type} (K : Π{a : A} (p : a = a), p = idp) : is_hset A :=
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is_hset.mk _ (λa b p q, eq.rec_on q K p)
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theorem is_hset_of_relation.{u} {A : Type.{u}} (R : A → A → Type.{u})
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(mere : Π(a b : A), is_hprop (R a b)) (refl : Π(a : A), R a a)
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(imp : Π{a b : A}, R a b → a = b) : is_hset A :=
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is_hset_of_axiom_K
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(λa p,
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have H2 : transport (λx, R a x → a = x) p (@imp a a) = @imp a a, from !apd,
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have H3 : Π(r : R a a), transport (λx, a = x) p (imp r)
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= imp (transport (λx, R a x) p r), from
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to_fun (equiv.symm !heq_pi) H2,
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have H4 : imp (refl a) ⬝ p = imp (refl a), from
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calc
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imp (refl a) ⬝ p = transport (λx, a = x) p (imp (refl a)) : transport_eq_r
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... = imp (transport (λx, R a x) p (refl a)) : H3
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... = imp (refl a) : is_hprop.elim,
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cancel_left H4)
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definition relation_equiv_eq {A : Type} (R : A → A → Type)
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(mere : Π(a b : A), is_hprop (R a b)) (refl : Π(a : A), R a a)
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(imp : Π{a b : A}, R a b → a = b) (a b : A) : R a b ≃ a = b :=
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@equiv_of_is_hprop _ _ _
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(@is_trunc_eq _ _ (is_hset_of_relation R mere refl @imp) a b)
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imp
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(λp, p ▹ refl a)
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local attribute not [reducible]
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definition is_hset_of_double_neg_elim {A : Type} (H : Π(a b : A), ¬¬a = b → a = b)
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: is_hset A :=
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is_hset_of_relation (λa b, ¬¬a = b) _ (λa n, n idp) H
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section
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open decidable
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--this is proven differently in init.hedberg
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definition is_hset_of_decidable_eq (A : Type)
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[H : decidable_eq A] : is_hset A :=
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is_hset_of_double_neg_elim (λa b, by_contradiction)
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end
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definition is_trunc_of_axiom_K_of_leq {A : Type} (n : trunc_index) (H : -1 ≤ n)
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(K : Π(a : A), is_trunc n (a = a)) : is_trunc (n.+1) A :=
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@is_trunc_succ_intro _ _ (λa b, is_trunc_of_imp_is_trunc_of_leq H (λp, eq.rec_on p !K))
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open trunctype equiv equiv.ops
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protected definition trunctype.sigma_char.{l} (n : trunc_index) :
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(trunctype.{l} n) ≃ (Σ (A : Type.{l}), is_trunc n A) :=
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begin
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fapply equiv.MK,
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/--/ intro A, exact (⟨carrier A, struct A⟩),
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/--/ intro S, exact (trunctype.mk S.1 S.2),
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/--/ intro S, apply (sigma.rec_on S), intros [S1, S2], apply idp,
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intro A, apply (trunctype.rec_on A), intros [A1, A2], apply idp,
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end
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protected definition trunctype_eq_equiv (n : trunc_index) (A B : n-Type) :
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(A = B) ≃ (carrier A = carrier B) :=
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calc
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(A = B) ≃ (trunctype.sigma_char n A = trunctype.sigma_char n B) : eq_equiv_fn_eq_of_equiv
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... ≃ ((trunctype.sigma_char n A).1 = (trunctype.sigma_char n B).1) : equiv.symm (!equiv_subtype)
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... ≃ (carrier A = carrier B) : equiv.refl
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end is_trunc
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