Could anyone point me to an easily available reference for Wiener's formula used in the following answer ?
I tried the most obvious sources without success.
It is a formula usually attributed to Parseval: http://en.wikipedia.org/wiki/Parsevals_theorem
You can find a proof of this in
W. F. Eberlein, "A note on Fourier-Stiltjes Transforms", Proceedings of the AMS, Vol 6, No 2, April 1955, pp 310-312.
I think the result is older than this paper.