2D FFT using fftpack5

Hello -

I'm really hoping someone on this list can help out. I have a question
regarding fftpack5, but the e-mail address contact listed on the fftpack5
webpage is no longer valid.

The basic issue is that I'm using the 2D real to complex forward transform
to analyze fluctuations in a surface. The basic behavior I expect is that
the time average of the _square_ of the fourier transform should simply be a
function of the magnitude of k.

For the most part the results I obtain bear this out. Nearly all the points
lie on a single curve. However a certain subset of those points only lie on
that curve when multiplied by a factor of 4. These datapoints, following
the notation in the documentation for RFFT2F, correspond to

* i=0 with any j, and
* i=L/2 with any j.

The high order of the points that are different made me wonder if a factor
of 2 was left out somewhere in fftpack5 for those edge lines (leading to the
factor of 4 difference in the square magnitude of the transform).

I appreciate anyone's time in taking a look at this. Even suggestions of
what to look for in the library code would be helpful. And please let me
know I should send you any other info.

Thanks so much,
Jocelyn Rodgers

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Received on Wed Nov 12 2008 - 16:43:52 MST