Relation between pullback/pushout squares and kernel/cokernel sequences #
Consider a commutative square in a preadditive category:
X₁ ⟶ X₂
| |
v v
X₃ ⟶ X₄
In this file, we show that this is a pushout square iff the object X₄
identifies to the cokernel of the difference map X₁ ⟶ X₂ ⊞ X₃
via the obvious map X₂ ⊞ X₃ ⟶ X₄
.
Similarly, it is a pullback square iff the object X₁
identifies to the kernel of the difference map X₂ ⊞ X₃ ⟶ X₄
via the obvious map X₁ ⟶ X₂ ⊞ X₃
.
The cokernel cofork attached to a commutative square in a preadditive category.
Equations
Instances For
The short complex attached to the cokernel cofork of a commutative square.
Equations
- sq.shortComplex = { X₁ := X₁, X₂ := X₂ ⊞ X₃, X₃ := X₄, f := CategoryTheory.Limits.biprod.lift f (-g), g := CategoryTheory.Limits.biprod.desc inl inr, zero := ⋯ }
Instances For
A commutative square in a preadditive category is a pushout square iff
the corresponding diagram X₁ ⟶ X₂ ⊞ X₃ ⟶ X₄ ⟶ 0
makes X₄
a cokernel.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The colimit cokernel cofork attached to a pushout square.
Equations
Instances For
The kernel fork attached to a commutative square in a preadditive category.
Equations
Instances For
The short complex attached to the kernel fork of a commutative square.
(This is similar to CommSq.shortComplex
, but with different signs.)
Equations
- sq.shortComplex' = { X₁ := X₁, X₂ := X₂ ⊞ X₃, X₃ := X₄, f := CategoryTheory.Limits.biprod.lift fst snd, g := CategoryTheory.Limits.biprod.desc f (-g), zero := ⋯ }
Instances For
A commutative square in a preadditive category is a pullback square iff
the corresponding diagram 0 ⟶ X₁ ⟶ X₂ ⊞ X₃ ⟶ X₄ ⟶ 0
makes X₁
a kernel.
Equations
- One or more equations did not get rendered due to their size.
Instances For
The limit kernel fork attached to a pullback square.