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Mathlib.CategoryTheory.Limits.Shapes.Pullback.Pasting

Pasting lemma #

This file proves the pasting lemma for pullbacks. That is, given the following diagram:

  X₁ - f₁ -> X₂ - f₂ -> X₃
  |          |          |
  i₁         i₂         i₃
  ∨          ∨          ∨
  Y₁ - g₁ -> Y₂ - g₂ -> Y₃

if the right square is a pullback, then the left square is a pullback iff the big square is a pullback.

Main results #

def CategoryTheory.Limits.bigSquareIsPullback {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Y₁ : C} {Y₂ : C} {Y₃ : C} (f₁ : X₁ X₂) (f₂ : X₂ X₃) (g₁ : Y₁ Y₂) (g₂ : Y₂ Y₃) (i₁ : X₁ Y₁) (i₂ : X₂ Y₂) (i₃ : X₃ Y₃) (h₁ : CategoryTheory.CategoryStruct.comp i₁ g₁ = CategoryTheory.CategoryStruct.comp f₁ i₂) (h₂ : CategoryTheory.CategoryStruct.comp i₂ g₂ = CategoryTheory.CategoryStruct.comp f₂ i₃) (H : CategoryTheory.Limits.IsLimit (CategoryTheory.Limits.PullbackCone.mk i₂ f₂ h₂)) (H' : CategoryTheory.Limits.IsLimit (CategoryTheory.Limits.PullbackCone.mk i₁ f₁ h₁)) :

Given

X₁ - f₁ -> X₂ - f₂ -> X₃
|          |          |
i₁         i₂         i₃
↓          ↓          ↓
Y₁ - g₁ -> Y₂ - g₂ -> Y₃

Then the big square is a pullback if both the small squares are.

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    def CategoryTheory.Limits.bigSquareIsPushout {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Y₁ : C} {Y₂ : C} {Y₃ : C} (f₁ : X₁ X₂) (f₂ : X₂ X₃) (g₁ : Y₁ Y₂) (g₂ : Y₂ Y₃) (i₁ : X₁ Y₁) (i₂ : X₂ Y₂) (i₃ : X₃ Y₃) (h₁ : CategoryTheory.CategoryStruct.comp i₁ g₁ = CategoryTheory.CategoryStruct.comp f₁ i₂) (h₂ : CategoryTheory.CategoryStruct.comp i₂ g₂ = CategoryTheory.CategoryStruct.comp f₂ i₃) (H : CategoryTheory.Limits.IsColimit (CategoryTheory.Limits.PushoutCocone.mk g₂ i₃ h₂)) (H' : CategoryTheory.Limits.IsColimit (CategoryTheory.Limits.PushoutCocone.mk g₁ i₂ h₁)) :

    Given

    X₁ - f₁ -> X₂ - f₂ -> X₃
    |          |          |
    i₁         i₂         i₃
    ↓          ↓          ↓
    Y₁ - g₁ -> Y₂ - g₂ -> Y₃
    

    Then the big square is a pushout if both the small squares are.

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      def CategoryTheory.Limits.leftSquareIsPullback {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Y₁ : C} {Y₂ : C} {Y₃ : C} (f₁ : X₁ X₂) (f₂ : X₂ X₃) (g₁ : Y₁ Y₂) (g₂ : Y₂ Y₃) (i₁ : X₁ Y₁) (i₂ : X₂ Y₂) (i₃ : X₃ Y₃) (h₁ : CategoryTheory.CategoryStruct.comp i₁ g₁ = CategoryTheory.CategoryStruct.comp f₁ i₂) (h₂ : CategoryTheory.CategoryStruct.comp i₂ g₂ = CategoryTheory.CategoryStruct.comp f₂ i₃) (H : CategoryTheory.Limits.IsLimit (CategoryTheory.Limits.PullbackCone.mk i₂ f₂ h₂)) (H' : CategoryTheory.Limits.IsLimit (CategoryTheory.Limits.PullbackCone.mk i₁ (CategoryTheory.CategoryStruct.comp f₁ f₂) )) :

      Given

      X₁ - f₁ -> X₂ - f₂ -> X₃
      |          |          |
      i₁         i₂         i₃
      ↓          ↓          ↓
      Y₁ - g₁ -> Y₂ - g₂ -> Y₃
      

      Then the left square is a pullback if the right square and the big square are.

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        def CategoryTheory.Limits.rightSquareIsPushout {C : Type u} [CategoryTheory.Category.{v, u} C] {X₁ : C} {X₂ : C} {X₃ : C} {Y₁ : C} {Y₂ : C} {Y₃ : C} (f₁ : X₁ X₂) (f₂ : X₂ X₃) (g₁ : Y₁ Y₂) (g₂ : Y₂ Y₃) (i₁ : X₁ Y₁) (i₂ : X₂ Y₂) (i₃ : X₃ Y₃) (h₁ : CategoryTheory.CategoryStruct.comp i₁ g₁ = CategoryTheory.CategoryStruct.comp f₁ i₂) (h₂ : CategoryTheory.CategoryStruct.comp i₂ g₂ = CategoryTheory.CategoryStruct.comp f₂ i₃) (H : CategoryTheory.Limits.IsColimit (CategoryTheory.Limits.PushoutCocone.mk g₁ i₂ h₁)) (H' : CategoryTheory.Limits.IsColimit (CategoryTheory.Limits.PushoutCocone.mk (CategoryTheory.CategoryStruct.comp g₁ g₂) i₃ )) :

        Given

        X₁ - f₁ -> X₂ - f₂ -> X₃
        |          |          |
        i₁         i₂         i₃
        ↓          ↓          ↓
        Y₁ - g₁ -> Y₂ - g₂ -> Y₃
        

        Then the right square is a pushout if the left square and the big square are.

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          The canonical isomorphism W ×[X] (X ×[Z] Y) ≅ W ×[Z] Y

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            The canonical isomorphism (Y ⨿[X] Z) ⨿[Z] W ≅ Y ×[X] W

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