The arrows of a category are called morphisms. This is derived from homomorphisms.
Some arrows have special properties that make them extra helpful; and we'll name them:
- A morphism with the same object as source and target.
- A morphism that is left-cancellable. Corresponds to injective functions. We say that f is a monomorphism if for any , the equation implies . In other words, with a concrete perspective, f doesn't introduce additional relations when applied.
- A morphism that is right-cancellable. Corresponds to surjective functions. We say that f is an epimorphism if for any , the equation implies .
Note, by the way, that cancellability does not imply the existence of an inverse. Epi's and mono's that have inverses realizing their cancellability are called split.
- A morphism is an isomorphism if it has an inverse. Split epi and split mono imply isomorphism. Specifically, is an isomorphism if there is a such that and .
- An automorphism is an endomorphism that is an isomorphism.
In a category, we use a different name for the vertices: objects. This comes from the roots in describing concrete categories - thus while objects may be actual mathematical objects, but they may just as well be completely different.
Just as with the morphisms, there are objects special enough to be named. An object v is
- if [v,w] has exactly one element for all other objects w.
- if [w,v] has exactly one element for all other objects w.
- A Zero object
- if it is both initial and terminal.
All initial objects are isomorphic. If are both initial, then there is exactly one map and exactly one map . The two possible compositions are maps and . However, the initiality condition holds even for the morphism set [v,v], so in these, the only existing morphism is and respectively. Hence, the compositions have to be this morphism, which proves the statement.
The same proof carries over, word by word, to the terminal case. This is an illustration of a very commonly occurring phenomenon - dualization.