Periodic Functions
According to Wikipedia a periodic function is "a function that repeats its values after some definite period has been added to its independent variable." That is to say that x(t) = x(t + T) or x[n] = x[n + N].
An example of a periodic function is the sine function because it repeats every 2*pi. sin(t) = sin(t + 2*pi)
An example of a non-periodic function is $ e^n $ because there is no interval N that would satisfy the condition e^n = e^(n+N).
