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From: heiko liebhart <heiko.liebh...@googlemail.com>
Newsgroups: sci.image.processing,sci.math.num-analysis,comp.lang.fortran
Subject: fft from numerical recipes
Date: Tue, 17 Apr 2007 22:48:23 +0200
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greetings

i am quite as new to programming in general and to fortran in particular 
as to programming ffts.

as i wanted to apply a fouriertransformation to one of my progs for 
pulseshaping i started with the subroutine four1 provided in numerical 
recipes.

as a start i used a gaussian as input data expecting to get another 
gaussian as output data.

so i generated the input values, filled in the zeros in the imag part in 
order to use a real function, called the subroutine an off we go for the 
crunching.

after rearranging the outputarray in the way according to the book i had 
to realize unfortunately the output i got was not really what i suspected.

what i got was something like a double gaussian. i.e. the sign was 
alternating. like sign of every second value was inverted oszillating 
from plus to minus.

in the next step i took a sin wave as input.
again what i got was not the suspected delta peak at the frequency of 
the oszillation but two antsymmetric peaks in the imaginary part. 
according to the symmetry mentioned in the book this should be right for 
  a real and odd funktion like sin but is not fitting to my 
understanding of fouriertransformation.

i was wondering whats going wrong. is the trick in the rearranging of 
the output or even deeper in the theory of the transformation so that 
maybe i would have to use the product of H and H*.

can somebody please give me a hand to get me through this numerical 
jungle even step by step.

i would appreciate every hint and assistance.

thanks alot