Equation of Harmonic Wave/Wavelength and Period

Theorem
Let $\phi$ be a harmonic wave which is propagated along the $x$-axis in the positive direction with constant velocity $c$.

Then the disturbance of $\phi$ at point $x$ and time $t$ can be expressed using the equation:
 * $\map \phi {x, t} = a \map \cos {2 \pi \paren {\dfrac x \lambda - \dfrac t \tau} }$

where:
 * $\lambda$ is the wavelength of $\phi$
 * $\tau$ is the period of $\phi$