∆ Определение 3.

Let $f: X \rightarrow Y$ and $g: Y \rightarrow Z$ be maps. The composite map is a map that for each element $x\in X$ puts into correspondance element $g(f(x))\in Z$. Notation: $g$ ◦ $f$. I.e. The composite map $g$ ◦ $f$ is defined by applying map $g$ to the result of applying map $f$ to $x$.