Definition:Functor/Covariant

Definition
Let $\mathbf C$ and $\mathbf D$ be metacategories.

A covariant functor $F : \mathbf C \to \mathbf D$ consists of:


 * An object functor $F_0$ that assigns to each object $X$ of $\mathbf C$ an object $FX$ of $\mathbf D$.


 * An arrow functor $F_1$ that assigns to each arrow $f : X \to Y$ of $\mathbf C$ an arrow $Ff : FX \to FY$ of $\mathbf D$.

These functors must satisfy, for any morphisms $X \stackrel{f}{\longrightarrow} Y \stackrel{g}{\longrightarrow} Z$ in $\mathbf C$:


 * $F(g \circ f) = Fg \circ Ff$, and $F(\operatorname{id}_X) = \operatorname{id}_{FX}$

where $\operatorname{id}_W$ denotes the identity arrow on an object $W$, and $\circ$ is the composition of morphisms.

Also known as
Many sources simply call this a functor.

Also see

 * Contravariant Functor