Numeric:FFT inverse_real_fft doesn't trevni!

[Warren B. Focke]

Christian Tismer <tismer at appliedbiometrics.com>:

>mhuster at hotmail.com wrote:
>>I thought that calling inverse_real_fft on the
>> same array as real_fft produced would give back the original array,
>> possibly scaled by N. Not so. I pasted in a sample of a session.

>No, that's a misconception.
>The real fft is just a speedup for cases where your data
>has only real coefficients. Providing an inverse function
>would be useless, since after any modifications of your
>complex result, you will usually not expect a real result.

	That may be the intention of the FFT module, but it is not the
intention of the FFTPACK library on which it is based.  I have written
working FORTRAN programs using FFTPACK, under the assumption that the
rffti function behaves as Michael expected inverse_real_fft to.
	I find that operation extremely useful for doing things like
convolution, interpolation, or differentiation in the Fourier domain.

Warren Focke
moron at glue.umd.edu

-- 
"Telopolies running the Internet would be almost exactly like foxes
guarding the henhouse, except that foxes are smart and agile."
 Bob Metcalfe