\begin{definition}[Definition:Odd Function]
Let $X \subset \R$ be a symmetric set of real numbers:
:$\forall x \in X: -x \in X$
A real function $f: X \to \R$ is an '''odd function''' {{iff}}:
:$\forall x \in X: \map f {-x} = -\map f x$
\end{definition}