75 lines
2.8 KiB
Text
75 lines
2.8 KiB
Text
-- 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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import logic.axioms.funext
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open eq eq.ops
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inductive category [class] (ob : Type) : Type :=
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mk : Π (hom : ob → ob → Type)
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(comp : Π⦃a b c : ob⦄, hom b c → hom a b → hom a c)
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(id : Π {a : ob}, hom a a),
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(Π ⦃a b c d : ob⦄ {h : hom c d} {g : hom b c} {f : hom a b},
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comp h (comp g f) = comp (comp h g) f) →
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(Π ⦃a b : ob⦄ {f : hom a b}, comp id f = f) →
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(Π ⦃a b : ob⦄ {f : hom a b}, comp f id = f) →
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category ob
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namespace category
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variables {ob : Type} [C : category ob]
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variables {a b c d : ob}
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include C
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definition hom [reducible] : ob → ob → Type := rec (λ hom compose id assoc idr idl, hom) C
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-- note: needs to be reducible to typecheck composition in opposite category
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definition compose [reducible] : Π {a b c : ob}, hom b c → hom a b → hom a c :=
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rec (λ hom compose id assoc idr idl, compose) C
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definition id [reducible] : Π {a : ob}, hom a a := rec (λ hom compose id assoc idr idl, id) C
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definition ID [reducible] (a : ob) : hom a a := id
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infixr `∘` := compose
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infixl `⟶`:25 := hom -- input ⟶ using \--> (this is a different arrow than \-> (→))
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variables {h : hom c d} {g : hom b c} {f : hom a b} {i : hom a a}
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theorem assoc : Π ⦃a b c d : ob⦄ (h : hom c d) (g : hom b c) (f : hom a b),
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h ∘ (g ∘ f) = (h ∘ g) ∘ f :=
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rec (λ hom comp id assoc idr idl, assoc) C
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theorem id_left : Π ⦃a b : ob⦄ (f : hom a b), id ∘ f = f :=
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rec (λ hom comp id assoc idl idr, idl) C
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theorem id_right : Π ⦃a b : ob⦄ (f : hom a b), f ∘ id = f :=
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rec (λ hom comp id assoc idl idr, idr) C
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--the following is the only theorem for which "include C" is necessary if C is a variable (why?)
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theorem id_compose (a : ob) : (ID a) ∘ id = id := !id_left
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theorem left_id_unique (H : Π{b} {f : hom b a}, i ∘ f = f) : i = id :=
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calc i = i ∘ id : id_right
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... = id : H
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theorem right_id_unique (H : Π{b} {f : hom a b}, f ∘ i = f) : i = id :=
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calc i = id ∘ i : id_left
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... = id : H
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end category
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inductive Category : Type := mk : Π (ob : Type), category ob → Category
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namespace category
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definition Mk {ob} (C) : Category := Category.mk ob C
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definition MK (a b c d e f g) : Category := Category.mk a (category.mk b c d e f g)
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definition objects [coercion] [reducible] (C : Category) : Type
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:= Category.rec (fun c s, c) C
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definition category_instance [instance] [coercion] [reducible] (C : Category) : category (objects C)
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:= Category.rec (fun c s, s) C
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end category
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open category
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theorem Category.equal (C : Category) : Category.mk C C = C :=
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Category.rec (λ ob c, rfl) C
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