Definition:Periodic Real Function/Period

Definition
Let $f: X \to X$ be a periodic function, where $X$ is either $\R$ or $\C$.

The period of $f$ is the smallest value $\left|{L}\right| \in \R_{\ne 0}$ such that:
 * $\forall x \in X: f \left({x}\right) = f \left({x + L}\right)$

where $\left|{L}\right|$ is the modulus of $L$.