在代数中，同态

的余象是域和核的 商:

根据第一同构定理，余象自然同构于像，如果该定理适用。

更一般地，在范畴论，态射的余象是态射的像的对偶表示。如果f : X → Y，则f的余象(如果存在的话)是满同态 c : X → C使得

In algebra, the coimage of a homomorphism

is the quotient

of domain and kernel. The coimage is canonically isomorphic to the image by the first isomorphism theorem, when that theorem applies.

More generally, in category theory, the coimage of a morphism is the dual notion of the image of a morphism. If f : X → Y, then a coimage of f (if it exists) is an epimorphism c : X → C such that