exact period

The smallest n for which a point x_0 is a periodic point of a function f so that f^n(x_0) = x_0. For example, for the function f(x) = - x, all points x have period 2 (including x = 0). However, x = 0 has a least period of 1. The analogous concept exists for a periodic sequence, but not for a periodic function. The least period is also called the exact period.