\begin{definition}[Definition:Periodic Function/Frequency]
Let $f: X \to X$ be a periodic function, where $X$ is either $\R$ or $\C$.
The '''frequency''' $\nu$ of $f$ is the reciprocal of the period $L$ of $f$:
:$\nu = \dfrac 1 L$
where:
:$\forall x \in X: \map f x = \map f {x + L}$
where $\size L$ is the modulus of $L$.
\end{definition}