Coequalizer

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In category theory, a coequalizer (or coequaliser) is a generalization of a quotient by an equivalence relation to objects in an arbitrary category. It is the categorical construction dual to the equalizer (hence the name).

Definition[edit]

A coequalizer is a colimit of the diagram consisting of two objects X and Y and two parallel morphisms f, g : XY.

More explicitly, a coequalizer can be defined as an object Q together with a morphism q : YQ such that qf = qg. Moreover, the pair (Q, q) must be universal in the sense that given any other such pair (Q′, q′) there exists a unique morphism u : QQ′ for which the following diagram commutes:

Coequalizer-01.png

As with all universal constructions, a coequalizer, if it exists, is unique up to a unique isomorphism (this is why, by abuse of language, one sometimes speaks of "the" coequalizer of two parallel arrows).

It can be shown that a coequalizer q is an epimorphism in any category.

Examples[edit]

  • For abelian groups the coequalizer is particularly simple. It is just the factor group Y / im(fg). (This is the cokernel of the morphism fg; see the next section).
  • In the category of topological spaces, the circle object can be viewed as the coequalizer of the two inclusion maps from the standard 0-simplex to the standard 1-simplex.
  • Coequalisers can be large: There are exactly two functors from the category 1 having one object and one identity arrow, to the category 2 with two objects and exactly one non-identity arrow going between them. The coequaliser of these two functors is the monoid of natural numbers under addition, considered as a one-object category. In particular, this shows that while every coequalising arrow is epic, it is not necessarily surjective.

Properties[edit]

  • Every coequalizer is an epimorphism.
  • In a topos, every epimorphism is the coequalizer of its kernel pair.

Special cases[edit]

In categories with zero morphisms, one can define a cokernel of a morphism f as the coequalizer of f and the parallel zero morphism.

In preadditive categories it makes sense to add and subtract morphisms (the hom-sets actually form abelian groups). In such categories, one can define the coequalizer of two morphisms f and g as the cokernel of their difference:

coeq(f, g) = coker(gf).

A stronger notion is that of an absolute coequalizer, this is a coequalizer that is preserved under all functors. Formally, an absolute coequalizer of a pair in a category C is a coequalizer as defined above but with the added property that given any functor F(Q) together with F(q) is the coequalizer of F(f) and F(g) in the category D. Split coequalizers are examples of absolute coequalizers.

See also[edit]

Notes[edit]

  1. ^ Barr, Michael; Wells, Charles (1998). Category theory for computing science (PDF). p. 278. Retrieved 2013-07-25. 

References[edit]

External links[edit]