Date: Dec 27, 2012 7:03 AM
Author: Cristiano
Subject: Binomial variance for DFT

I have n/2 real numbers.

I calculate a threshold T (a real number) for which p * n/2 numbers (0 < 
p < 1) are expected to fall below T.

Then I count how many numbers are less than T.

That count should have a binomial distribution, right? Hence, its 
variance should be
    n/2 * p * (1-p).

But the n/2 numbers are the modulus of the first n/2 complex numbers 
obtained from a discrete Fourier transform and the variance should be
    n/2 * p * (1-p) / 2
as explained here (page 10):
http://eprint.iacr.org/2004/018.pdf

Unfortunately, the value given in the paper is significantly different 
from the one obtained by extensive simulations.

Does anybody now a procedure or a formula to calculate the exact 
variance of the counts?

Thanks
Cristiano