\begin{definition}[Definition:Periodic Function/Period]
Let $f: X \to X$ be a periodic function, where $X$ is either $\R$ or $\C$.
The '''period''' of $f$ is the smallest value $\cmod L \in \R_{\ne 0}$ such that:
:$\forall x \in X: \map f x = \map f {x + L}$
where $\cmod L$ is the modulus of $L$.
\end{definition}