Pullbacks of synthetic categories
Content created by Ivan Kobe.
Created on 2024-09-25.
Last modified on 2024-11-17.
{-# OPTIONS --guardedness #-} module synthetic-category-theory.pullbacks-synthetic-categories where
Imports
open import foundation.cartesian-product-types open import foundation.dependent-pair-types open import foundation.universe-levels open import globular-types.globular-types open import synthetic-category-theory.cone-diagrams-synthetic-categories open import synthetic-category-theory.cospans-synthetic-categories open import synthetic-category-theory.equivalences-synthetic-categories open import synthetic-category-theory.synthetic-categories
Idea
Consider a cospan diagram S of synthetic categories. The pullback¶ of S is a cone diagram cᵤ = (pr₀, pr₁, τᵤ) over S with apex P that is universal in the sense that:
1) for every cone diagram c = (t₀, t₁, τ) over S with apex T there exists a functor
(t₀, t₁) : T → P together with an isomorphism of cone diagrams c ≅ (t₀, t₁)*(cᵤ)
2) given two functors f,g : T → P equipped with an isomorphism of cone diagrams
s*(cᵤ) ≅ t*(cᵤ), there exists a natural isomorphism s ≅ t that induces the said
isomorphism of cone diagrams.
module _ {l : Level} where record pullback-Synthetic-Category-Theory (κ : language-Synthetic-Category-Theory l) (μ : composition-Synthetic-Category-Theory κ) (ι : identity-Synthetic-Category-Theory κ) (ν : inverse-Synthetic-Category-Theory κ μ ι) (Α : associative-composition-Synthetic-Category-Theory κ μ) (Χ : horizontal-composition-Synthetic-Category-Theory κ μ) (Λ : left-unit-law-composition-Synthetic-Category-Theory κ ι μ) (Ρ : right-unit-law-composition-Synthetic-Category-Theory κ ι μ) (Ξ : preserves-associativity-composition-horizontal-composition-Synthetic-Category-Theory κ μ Α Χ) (I : interchange-composition-Synthetic-Category-Theory κ μ Χ) (M : preserves-isomorphism-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ) (N : preserves-identity-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ) : UU l where coinductive field apex-pullback-Synthetic-Category-Theory : {C D E : category-Synthetic-Category-Theory κ} → cospan-Synthetic-Category-Theory κ C D E → category-Synthetic-Category-Theory κ cone-diagram-pullback-Synthetic-Category-Theory : {C D E : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C D E) → cone-diagram-Synthetic-Category-Theory κ μ S ( apex-pullback-Synthetic-Category-Theory S) universality-functor-pullback-Synthetic-Category-Theory : {C D E : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C D E) {T : category-Synthetic-Category-Theory κ} (c : cone-diagram-Synthetic-Category-Theory κ μ S T) → functor-Synthetic-Category-Theory κ T ( apex-pullback-Synthetic-Category-Theory S) universality-iso-pullback-Synthetic-Category-Theory : {C D E : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C D E) (T : category-Synthetic-Category-Theory κ) (c : cone-diagram-Synthetic-Category-Theory κ μ S T) → iso-of-cone-diagrams-Synthetic-Category-Theory κ μ ι Χ ( c) ( induced-cone-diagram-Synthetic-Category-Theory κ μ ι ν Χ Α S ( cone-diagram-pullback-Synthetic-Category-Theory S) ( universality-functor-pullback-Synthetic-Category-Theory S c)) triviality-iso-of-cone-diagrams-pullback-Synthetic-Category-Theory : {C D E : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C D E) {T : category-Synthetic-Category-Theory κ} (s t : functor-Synthetic-Category-Theory κ T ( apex-pullback-Synthetic-Category-Theory S)) (H : iso-of-cone-diagrams-Synthetic-Category-Theory κ μ ι Χ (induced-cone-diagram-Synthetic-Category-Theory κ μ ι ν Χ Α S ( cone-diagram-pullback-Synthetic-Category-Theory S) ( s)) (induced-cone-diagram-Synthetic-Category-Theory κ μ ι ν Χ Α S ( cone-diagram-pullback-Synthetic-Category-Theory S) ( t))) → Σ ( isomorphism-Synthetic-Category-Theory κ s t) λ α → iso-of-isos-of-cone-diagrams-Synthetic-Category-Theory κ μ ι ν Χ M ( induced-iso-cone-diagram-Synthetic-Category-Theory κ μ ι ν Α Χ Λ Ρ Ξ I M N S ( cone-diagram-pullback-Synthetic-Category-Theory S) s t α) ( H) open pullback-Synthetic-Category-Theory public
The left and right projection functors with domain the apex of the pullback cone
module _ {l : Level} where left-functor-pullback-Synthetic-Category-Theory : (κ : language-Synthetic-Category-Theory l) (μ : composition-Synthetic-Category-Theory κ) {ι : identity-Synthetic-Category-Theory κ} {ν : inverse-Synthetic-Category-Theory κ μ ι} {Α : associative-composition-Synthetic-Category-Theory κ μ} {Χ : horizontal-composition-Synthetic-Category-Theory κ μ} {Λ : left-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ρ : right-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ξ : preserves-associativity-composition-horizontal-composition-Synthetic-Category-Theory κ μ Α Χ} {I : interchange-composition-Synthetic-Category-Theory κ μ Χ} {Μ : preserves-isomorphism-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} {Ν : preserves-identity-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} (PB : pullback-Synthetic-Category-Theory κ μ ι ν Α Χ Λ Ρ Ξ I Μ Ν) {C E D : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C E D) → functor-Synthetic-Category-Theory κ ( apex-pullback-Synthetic-Category-Theory PB S) ( left-source-cospan-Synthetic-Category-Theory κ S) left-functor-pullback-Synthetic-Category-Theory κ μ PB S = left-functor-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S) right-functor-pullback-Synthetic-Category-Theory : (κ : language-Synthetic-Category-Theory l) (μ : composition-Synthetic-Category-Theory κ) {ι : identity-Synthetic-Category-Theory κ} {ν : inverse-Synthetic-Category-Theory κ μ ι} {Α : associative-composition-Synthetic-Category-Theory κ μ} {Χ : horizontal-composition-Synthetic-Category-Theory κ μ} {Λ : left-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ρ : right-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ξ : preserves-associativity-composition-horizontal-composition-Synthetic-Category-Theory κ μ Α Χ} {I : interchange-composition-Synthetic-Category-Theory κ μ Χ} {Μ : preserves-isomorphism-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} {Ν : preserves-identity-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} (PB : pullback-Synthetic-Category-Theory κ μ ι ν Α Χ Λ Ρ Ξ I Μ Ν) {C E D : category-Synthetic-Category-Theory κ} (S : cospan-Synthetic-Category-Theory κ C E D) → functor-Synthetic-Category-Theory κ ( apex-pullback-Synthetic-Category-Theory PB S) ( right-source-cospan-Synthetic-Category-Theory κ S) right-functor-pullback-Synthetic-Category-Theory κ μ PB S = right-functor-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S)
Functoriality of pullbacks
Taking pullbacks is functorial in the sense that given cospans S and S’ and a transformations of cospans S → S’, there exists a preffered functor between the pullback of S and the pullback of S’.
module _ {l : Level} where functor-pullback-Synthetic-Category-Theory : (κ : language-Synthetic-Category-Theory l) (μ : composition-Synthetic-Category-Theory κ) (ι : identity-Synthetic-Category-Theory κ) (ν : inverse-Synthetic-Category-Theory κ μ ι) (Α : associative-composition-Synthetic-Category-Theory κ μ) (Χ : horizontal-composition-Synthetic-Category-Theory κ μ) {Λ : left-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ρ : right-unit-law-composition-Synthetic-Category-Theory κ ι μ} {Ξ : preserves-associativity-composition-horizontal-composition-Synthetic-Category-Theory κ μ Α Χ} {I : interchange-composition-Synthetic-Category-Theory κ μ Χ} {M : preserves-isomorphism-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} {N : preserves-identity-horizontal-composition-Synthetic-Category-Theory κ ι μ Χ} (PB : pullback-Synthetic-Category-Theory κ μ ι ν Α Χ Λ Ρ Ξ I M N) {C C' E E' D D' : category-Synthetic-Category-Theory κ} {S : cospan-Synthetic-Category-Theory κ C E D} {S' : cospan-Synthetic-Category-Theory κ C' E' D'} → transformation-cospan-Synthetic-Category-Theory κ μ S S' → functor-Synthetic-Category-Theory κ ( apex-pullback-Synthetic-Category-Theory PB S) ( apex-pullback-Synthetic-Category-Theory PB S') functor-pullback-Synthetic-Category-Theory κ μ ι ν Α Χ PB {S = S} {S' = S'} H = universality-functor-pullback-Synthetic-Category-Theory PB S' ( comp-functor-Synthetic-Category-Theory μ ( right-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( right-functor-pullback-Synthetic-Category-Theory κ μ PB S) , ( comp-functor-Synthetic-Category-Theory μ ( left-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( left-functor-pullback-Synthetic-Category-Theory κ μ PB S)) , ( comp-iso-Synthetic-Category-Theory μ ( associative-comp-functor-Synthetic-Category-Theory Α ( left-functor-cospan-Synthetic-Category-Theory κ S') ( left-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( left-functor-pullback-Synthetic-Category-Theory κ μ PB S)) ( comp-iso-Synthetic-Category-Theory μ ( horizontal-comp-iso-Synthetic-Category-Theory Χ ( left-commuting-square-transformation-cospan-Synthetic-Category-Theory κ μ H) ( id-iso-Synthetic-Category-Theory ι ( left-functor-pullback-Synthetic-Category-Theory κ μ PB S))) ( comp-iso-Synthetic-Category-Theory μ ( inv-iso-Synthetic-Category-Theory ν ( associative-comp-functor-Synthetic-Category-Theory Α ( middle-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( left-functor-cospan-Synthetic-Category-Theory κ S) ( left-functor-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S)))) ( comp-iso-Synthetic-Category-Theory μ ( horizontal-comp-iso-Synthetic-Category-Theory Χ ( id-iso-Synthetic-Category-Theory ι ( middle-functor-transformation-cospan-Synthetic-Category-Theory κ μ H)) ( iso-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S))) ( comp-iso-Synthetic-Category-Theory μ ( associative-comp-functor-Synthetic-Category-Theory Α ( middle-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( right-functor-cospan-Synthetic-Category-Theory κ S) ( right-functor-pullback-Synthetic-Category-Theory κ μ PB S)) ( comp-iso-Synthetic-Category-Theory μ ( horizontal-comp-iso-Synthetic-Category-Theory Χ ( inv-iso-Synthetic-Category-Theory ν ( right-commuting-square-transformation-cospan-Synthetic-Category-Theory κ μ H)) ( id-iso-Synthetic-Category-Theory ι ( right-functor-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S)))) ( inv-iso-Synthetic-Category-Theory ν ( associative-comp-functor-Synthetic-Category-Theory Α ( right-functor-cospan-Synthetic-Category-Theory κ S') ( right-functor-transformation-cospan-Synthetic-Category-Theory κ μ H) ( right-functor-cone-diagram-Synthetic-Category-Theory κ μ ( cone-diagram-pullback-Synthetic-Category-Theory PB S)))))))))))
Recent changes
- 2024-11-17. Egbert Rijke. chore: Moving files about globular types to a new namespace (#1223).
- 2024-09-25. Ivan Kobe. Pullbacks of synthetic categories (#1183).