A Periodic Function
A very obvious periodic CT function is sin(t + T). The definition of a periodic function is a function which will have the same value at x(t) as at x(t + T) when T > 0. An example of T for a sin function would be any multiple of 2*pi. This is due to a sine wave's nature as it always returns the same value when shifted by 2*pi.
Periodic Function Example
