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Topic: Binomial variance for DFT
Replies: 1   Last Post: Dec 27, 2012 10:18 AM

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Cristiano

Posts: 48
Registered: 12/7/12
Binomial variance for DFT
Posted: Dec 27, 2012 7:03 AM
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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



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