A morphismis a mapping between sets. The different types of morphism are

  • Homomorphism: preserves the structure (e.g.) whereandare the operations on the domain and codomain respectively. For example exp is a morphism fromtowhereis the set of positive real numbers.

  • Epimorphism: a homomorphism that is surjective  or a homomorphism fromonto the set consisting of the single element 0. (with the operation of either ordinary addition or ordinary multiplication.)

  • Monomorphism: a homomorphism that is injective or one to one. The homomorphismillustrated above is a monomorphism.

  • Isomorphism: a homomorphism that is bijective (one to one and onto); isomorphic objects are an isomorphism fromto the setwith the operation of addition on both domain and codomain.

  • Endomorphism: a homomorphism from a set to a subset of itself. Egis an endomorphism fromto

  • Automorphism: a bijective endomorphism (an isomorphism from an set onto itself, essentially just a re - labeling of elements):where z is a complex number of magnitude 1 is an isomorphism. The effect ofis to rotateby anticlockwise about the origin.

Every morphism send the identity in the domain to the identity in the codomain. This is easy to prove:

Ifis the identity element in the domain with operationthen

sois the identity element in the codomain.

The relationship between the different types of morphism may be written


Endomorphisms do not fit easiliy into this relationship since they are not isomorphisms and must be from a set into the same set.