Definition:Periodic Function/Periodic Element

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

Let $L \in X_{\ne 0}$.

Then $L$ is a periodic element of $f$ :
 * $\forall x \in X: f \left({x}\right) = f \left({x + L}\right)$