# Periodic function

We say a function ${\displaystyle f(t)}$ is ${\displaystyle T}$-periodic if and only if ${\displaystyle f(t+T)=f(t)}$.
Simple examples of periodic functions include familiar ${\displaystyle 2\pi }$-periodic functions from trigonometry
${\displaystyle \cos(\omega t+2\pi )=\cos(\omega t)}$
${\displaystyle \sin(\omega t+2\pi )=\sin(\omega t)}$
where ${\displaystyle \omega }$ is the angular frequency and ${\displaystyle t}$ is time.