- g1 o f = g2 o f implies g1 = g2 for all morphisms g1, g2 : Y → Z.
In the category of sets the epimorphisms are exactly the surjective morphisms. Thus the algebraic and categorical notions are the same. This, however, does not always hold in other concrete categories. For example:
- In the category of monoids, Mon, the inclusion function N → Z is a non-surjective monoid homomorphism, and hence not an algebraic epimorphism. It is, however, a epimorphism in the categorical sense.
- In the category of rings, Ring, the inclusion map Z → Q is a categorical epimorphism but not an algebraic one. (To see this note that any ring homomorphism on Q is determined entirely by its action on Z).
In general, algebraic epimorphisms are always categorical ones but not vice-versa.
There are also useful concepts of regular epimorphism and extremal epimorphism. A regular epimorphism coequalizes some parallel pair of morphisms. An extremal epimorphism is an epimorphism that has no monomorphism as a second factor, unless that monomorphism is an isomorphism.
Notes on Usage
The use of the words epimorphism and monomorphism is somewhat unsettled. They were originally introduced by Bourbaki to mean surjective and injective, respectively. Early category theorists argued that the correct category-theoretic generalization of surjective was the definition of epimorphism given above, and simply gave the word this new, somewhat different, meaning; this made the term ambiguous. Mac Lane attempted to restore its original meanings by using the terms epic morphism and epi to refer to the category-theoretic concept, but this distinction has not caught on. There are many areas where the category-theoretic meaning is well established or at least used very often, e.g. in semigroups or modules.