Quasi-abelian category

In mathematics, specifically in category theory, a quasi-abelian category is a pre-abelian category in which the pushout of a kernel along arbitrary morphisms is again a kernel and, dually, the pullback of a cokernel along arbitrary morphisms is again a cokernel.

Definition

Let be a pre-abelian category. A morphism is a kernel (a cokernel) if there exists a morphism such that is a kernel (cokernel) of . The category is quasi-abelian if for every kernel and every morphism in the pushout diagram

the morphism is again a kernel and, dually, for every cokernel and every morphism in the pullback diagram

the morphism is again a cokernel.

Equivalently, a quasi-abelian category is a pre-abelian category in which the system of all kernel-cokernel pairs forms an exact structure.

Given a pre-abelian category, those kernels, which are stable under arbitrary pushouts, are sometimes called the semi-stable kernels. Dually, cokernels, which are stable under arbitrary pullbacks, are called semi-stable cokernels.[1]

Properties

Let be a morphism in a quasi-abelian category. Then the induced morphism is always a bimorphism, i.e., a monomorphism and an epimorphism. A quasi-abelian category is therefore always semi-abelian.

Examples

Every abelian category is quasi-abelian. Typical non-abelian examples arise in functional analysis.[2]

History

The concept of quasi-abelian category was developed in the 1960s. The history is involved.[3] This is in particular due to Raikov's conjecture, which stated that the notion of a semi-abelian category is equivalent to that of a quasi-abelian category. Around 2005 it turned out that the conjecture is false.[4]

Left and right quasi-abelian categories

By dividing the two conditions in the definition, one can define left quasi-abelian categories by requiring that cokernels are stable under pullbacks and right quasi-abelian categories by requiring that kernels stable under pushouts.[5]

Citations

  1. Richman and Walker, 1977.
  2. Prosmans, 2000.
  3. Rump, 2008, p. 986f.
  4. Rump, 2011, p. 44f.
  5. Rump, 2001.

References

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