A semifunctor$F$ from a semicategory$C$ to a semicategory $D$ is a map sending each object$x \in C$ to an object $F(x) \in D$ and each morphism$f : x \to y$ in $C$ to morphism $F(f) : F(x) \to F(y)$ in $D$, such that

$F$ preserves composition: $F(g\circ f) = F(g)\circ F(f)$ whenever the left-hand side is well-defined.

If $C$ is a category, then $F$ need not preserve its identity morphisms, but this axiom does require that it send them to idempotents in $D$.

Examples

A mapping $F$ of a category into another category that sends $id_X$ to a nontrivial idempotent endomorphism of $F(X)$ is a semifunctor but not a functor.

More generally, recall from semicategory that the forgetful functor $U \colon Cat \to Semicat$ has a right adjoint$G$, which sends a semicategory to its category of idempotents, or Karoubi envelope. Thus to give a semifunctor from a category $C$ to a (semi)category $D$ is the same as giving a functor from $C$ to the Karoubi envelope $\bar D$ of $D$ (but beware that this correspondence does not hold for natural transformations).

Revised on November 23, 2012 20:05:02
by Finn Lawler
(86.41.31.29)