122 lines
4.5 KiB
Text
122 lines
4.5 KiB
Text
/-
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Copyright (c) 2014 Floris van Doorn. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Floris van Doorn
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Ported from Coq HoTT
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Theorems about pi-types (dependent function spaces)
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-/
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import types.sigma
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open eq equiv is_equiv funext
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namespace pi
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universe variables l k
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variables {A A' : Type.{l}} {B : A → Type.{k}} {C : Πa, B a → Type}
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{D : Πa b, C a b → Type}
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{a a' a'' : A} {b b₁ b₂ : B a} {b' : B a'} {b'' : B a''} {f g : Πa, B a}
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/- Paths -/
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/- Paths [p : f ≈ g] in a function type [Πx:X, P x] are equivalent to functions taking values in path types, [H : Πx:X, f x ≈ g x], or concisely, [H : f ∼ g].
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This equivalence, however, is just the combination of [apD10] and function extensionality [funext], and as such, [path_forall], et seq. are given in axioms.funext and path: -/
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/- Now we show how these things compute. -/
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definition apD10_path_pi [H : funext] (h : f ∼ g) : apD10 (path_pi h) ∼ h :=
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apD10 (retr apD10 h)
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definition path_pi_eta [H : funext] (p : f = g) : path_pi (apD10 p) = p :=
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sect apD10 p
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definition path_pi_idp [H : funext] : path_pi (λx : A, refl (f x)) = refl f :=
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!path_pi_eta
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/- The identification of the path space of a dependent function space, up to equivalence, is of course just funext. -/
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definition path_equiv_homotopy [H : funext] (f g : Πx, B x) : (f = g) ≃ (f ∼ g) :=
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equiv.mk _ !funext.ap
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definition is_equiv_path_pi [instance] [H : funext] (f g : Πx, B x)
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: is_equiv (@path_pi _ _ _ f g) :=
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inv_closed apD10
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definition homotopy_equiv_path [H : funext] (f g : Πx, B x) : (f ∼ g) ≃ (f = g) :=
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equiv.mk _ !is_equiv_path_pi
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/- Transport -/
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protected definition transport (p : a = a') (f : Π(b : B a), C a b)
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: (transport (λa, Π(b : B a), C a b) p f)
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∼ (λb, transport (C a') !transport_pV (transportD _ _ p _ (f (p⁻¹ ▹ b)))) :=
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eq.rec_on p (λx, idp)
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/- A special case of [transport_pi] where the type [B] does not depend on [A],
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and so it is just a fixed type [B]. -/
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definition transport_constant {C : A → A' → Type} (p : a = a') (f : Π(b : A'), C a b)
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: (eq.transport (λa, Π(b : A'), C a b) p f) ∼ (λb, eq.transport (λa, C a b) p (f b)) :=
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eq.rec_on p (λx, idp)
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/- Maps on paths -/
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/- The action of maps given by lambda. -/
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definition ap_lambdaD [H : funext] {C : A' → Type} (p : a = a') (f : Πa b, C b) :
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ap (λa b, f a b) p = path_pi (λb, ap (λa, f a b) p) :=
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begin
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apply (eq.rec_on p),
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apply inverse,
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apply path_pi_idp
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end
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/- Dependent paths -/
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/- with more implicit arguments the conclusion of the following theorem is
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(Π(b : B a), transportD B C p b (f b) = g (eq.transport B p b)) ≃
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(eq.transport (λa, Π(b : B a), C a b) p f = g) -/
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definition dpath_pi [H : funext] (p : a = a') (f : Π(b : B a), C a b) (g : Π(b' : B a'), C a' b')
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: (Π(b : B a), p ▹D (f b) = g (p ▹ b)) ≃ (p ▹ f = g) :=
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eq.rec_on p (λg, !homotopy_equiv_path) g
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section open sigma sigma.ops
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/- more implicit arguments:
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(Π(b : B a), eq.transport C (sigma.path p idp) (f b) = g (p ▹ b)) ≃
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(Π(b : B a), transportD B (λ(a : A) (b : B a), C ⟨a, b⟩) p b (f b) = g (eq.transport B p b)) -/
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definition dpath_pi_sigma {C : (Σa, B a) → Type} (p : a = a')
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(f : Π(b : B a), C ⟨a, b⟩) (g : Π(b' : B a'), C ⟨a', b'⟩) :
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(Π(b : B a), (sigma.path p idp) ▹ (f b) = g (p ▹ b)) ≃ (Π(b : B a), p ▹D (f b) = g (p ▹ b)) :=
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eq.rec_on p (λg, !equiv.refl) g
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end
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/- truncation -/
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open truncation
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definition trunc_pi [instance] [H : funext.{l k}] (B : A → Type.{k}) (n : trunc_index)
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[H : ∀a, is_trunc n (B a)] : is_trunc n (Πa, B a) :=
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begin
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reverts (B, H),
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apply (trunc_index.rec_on n),
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intros (B, H),
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fapply is_contr.mk,
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intro a, apply center,
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intro f, apply path_pi,
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intro x, apply (contr (f x)),
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intros (n, IH, B, H),
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fapply is_trunc_succ, intros (f, g),
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fapply trunc_equiv',
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apply equiv.symm, apply path_equiv_homotopy,
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apply IH,
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intro a,
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show is_trunc n (f a = g a), from
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succ_is_trunc n (f a) (g a)
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end
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definition trunc_path_pi [instance] [H : funext.{l k}] (n : trunc_index) (f g : Πa, B a)
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[H : ∀a, is_trunc n (f a = g a)] : is_trunc n (f = g) :=
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begin
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apply trunc_equiv', apply equiv.symm,
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apply path_equiv_homotopy
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end
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end pi
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