[Numpy-discussion] Re: Weird (wrong?) real fft 2d behavior
Andrew Jaffe
a.h.jaffe at gmail.com
Fri Jan 27 00:08:04 CST 2006
Dear Warren,
>
> On Wed, 25 Jan 2006, Andrew Jaffe wrote:
>
>> Andrew Jaffe wrote:
>>> If I start with what I thought was an appropriate (n, n/2+1) complex
>>> matrix which should have a real inverse fft, and then take its real
fft,
>>> I don't get back the original matrix.
>>>
>>> Note the presence of very small but nonzero reals in the final matrix,
>
> That's just roundoff.
>
>>> and the fact that the 2d and 4th rows are duplicates. This seems to
be a
>>> mistake somewhere.
>>>
>>> Or am I just misunderstanding/misremembering something about 2d
real ffts?
>
> It looks wrong to me, and I think I wrote those functions. I get the
same
> results in Numeric. I'll try to look into the problem.
>
>> and I should point out that
>> delta_rp = N.dft.real_fft2d(delta_kp)
>> is 'allclose' to the original delta_r (which leads me to believe that I
>> may indeed be misunderstanding something).
>
> "Stable" does not neccessarily imply "correct".
Indeed! And more to the point, it's actually the case that "delta_kp"
doesn't actually have the requisite 16 (non-small) real degrees of
freedom -- so it can't really be right.
Andrew
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