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Mathlib.CategoryTheory.Limits.Shapes.Pullback.Assoc

Associativity of pullbacks #

This file shows that pullbacks (and pushouts) are associative up to natural isomorphism.

(X₁ ×[Y₁] X₂) ×[Y₂] X₃ is the pullback (X₁ ×[Y₁] X₂) ×[X₂] (X₂ ×[Y₂] X₃).

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    theorem CategoryTheory.Limits.hasPullback_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Y₁ : C} {Y₂ : C} (f₁ : X₁ Y₁) (f₂ : X₂ Y₁) (f₃ : X₂ Y₂) (f₄ : X₃ Y₂) [CategoryTheory.Limits.HasPullback f₁ f₂] [CategoryTheory.Limits.HasPullback f₃ f₄] [CategoryTheory.Limits.HasPullback (CategoryTheory.CategoryStruct.comp (CategoryTheory.Limits.pullback.snd f₁ f₂) f₃) f₄] :

    X₁ ×[Y₁] (X₂ ×[Y₂] X₃) is the pullback (X₁ ×[Y₁] X₂) ×[X₂] (X₂ ×[Y₂] X₃).

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      The canonical isomorphism (X₁ ×[Y₁] X₂) ×[Y₂] X₃ ≅ X₁ ×[Y₁] (X₂ ×[Y₂] X₃).

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        (X₁ ⨿[Z₁] X₂) ⨿[Z₂] X₃ is the pushout (X₁ ⨿[Z₁] X₂) ×[X₂] (X₂ ⨿[Z₂] X₃).

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          theorem CategoryTheory.Limits.hasPushout_assoc {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Z₁ : C} {Z₂ : C} (g₁ : Z₁ X₁) (g₂ : Z₁ X₂) (g₃ : Z₂ X₂) (g₄ : Z₂ X₃) [CategoryTheory.Limits.HasPushout g₁ g₂] [CategoryTheory.Limits.HasPushout g₃ g₄] [CategoryTheory.Limits.HasPushout (CategoryTheory.CategoryStruct.comp g₃ (CategoryTheory.Limits.pushout.inr g₁ g₂)) g₄] :

          X₁ ⨿[Z₁] (X₂ ⨿[Z₂] X₃) is the pushout (X₁ ⨿[Z₁] X₂) ×[X₂] (X₂ ⨿[Z₂] X₃).

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            theorem CategoryTheory.Limits.hasPushout_assoc_symm {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Z₁ : C} {Z₂ : C} (g₁ : Z₁ X₁) (g₂ : Z₁ X₂) (g₃ : Z₂ X₂) (g₄ : Z₂ X₃) [CategoryTheory.Limits.HasPushout g₁ g₂] [CategoryTheory.Limits.HasPushout g₃ g₄] [CategoryTheory.Limits.HasPushout g₁ (CategoryTheory.CategoryStruct.comp g₂ (CategoryTheory.Limits.pushout.inl g₃ g₄))] :

            The canonical isomorphism (X₁ ⨿[Z₁] X₂) ⨿[Z₂] X₃ ≅ X₁ ⨿[Z₁] (X₂ ⨿[Z₂] X₃).

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