∆ Определение 1.

Let $X$ and $Y$ be sets. If for each $x\in X$ there is a unique corresponding $f(x)\in Y$, then $f$ is called a mapping or a map from $X$ to $Y$. If $f(x)=y$, then $y$ is called the image of $x$ and $x$ is called a preimage of $y$. Notation: $f: X \rightarrow Y$.