Frequency of Harmonic Wave

Theorem
Let $\phi$ be a harmonic wave expressed as:
 * $\forall x, t \in \R: \map \phi {x, t} = a \map \cos {\omega \paren {x - c t} }$

The frequency $\nu$ of $\phi$ can be expressed as:
 * $\nu = \dfrac 1 \tau$

where $\tau$ is the period of $\phi$.