Jump discontinuity with fourier series

[yoq_bise]

Hi all,
I couldn't find any proof of the following statement: The Fourier series expansion of f(x), which has a discontinuity at y, takes on the mean of the left and right limits
i.e. f(y)= (1/2)(f(y⁺)+f(y^-))

is there anyone who can help me?
Thanks

 

[lurflurf]

Have you tried looking in a book?

 

[yoq_bise]

Yes I looked in Hassani, Arfken and Diprima but I couldn't find
Can anyone suggest other books or web sites?