1. For the function y=f(x), if there is a non-zero constant t, so that f(x+T)=f(x) holds when x takes every value in the domain, then the function y=f(x) is called a periodic function, and the non-zero constant t is called the period of this function.

2. In fact, any constant kT(k∈Z, k≠0) is its period. And the period t of the periodic function f(x) is a non-zero constant independent of x, and the periodic function does not necessarily have the minimum positive period.

3. If T(≠0) is the period of f(x), then -T is also the period of f(x).

4. If T(≠0) is the period of f(x), then nT(n is an arbitrary non-zero integer) is also the period of f(x).

5. If T 1 and T2 are both periods of f(x), then T 1 T2 is also a period of f(x).