Definition:Composition of Mappings/Warning

Definition
Let $f_1: S_1 \to S_2$ and $f_2: S_2 \to S_3$ be mappings such that:
 * $\Dom {f_2} \ne \Cdm {f_1}$

where $\Dom {f_2}$ and $\Cdm {f_1}$ denote domain and codomain respectively.

Then the composite mapping $f_2 \circ f_1$ is not defined.

Compare with the definition of composition of relations in the context of the fact that a mapping is a special kind of relation.