Wave Number 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 wave number $k$ of $\phi$ can be expressed as:
 * $k = \dfrac 1 \lambda$

where $\lambda$ is the wavelength of $\phi$.