Which functions has a period?

Answer 1

You can invent any function, to make it periodic. Commonly used functions that are periodic include all the trigonometric functions such as sin and cos (period 2 x pi), tan (period pi). Also, when you work with complex numbers, the exponential function (period 2 x pi x i).

Answer 2

Any periodic function. That is, a function for which there is a value p such thatf(x + p) = f(x) for all x.