I start you off.
1)Assume is integrable and -periodic.
2)We will show the forward direction, i.e. is -periodic implies .
3)Say that is a -periodic function.
4)By definition it means .
5)Now #4 is true for all , pick .
7)By definition of we see that .
9)Since is -periodic we use the theorem that .
10)By #9 we see that .