Definition 0.1 A morphism in a category is an isomorphism when there exists an inverse morphism of in , denoted by
, such that
.
One also writes: , expressing the fact that the object A is isomorphic with object B under the isomorphism .
Note also that an isomorphism is both a monomorphism and an epimorphism; moreover, an isomorphism is both a section and a retraction. However, an isomorphism is not the same as an equivalence relation.