Re: Problem with FourierTransform

*To*: mathgroup at smc.vnet.net*Subject*: [mg30350] Re: Problem with FourierTransform*From*: Ioan Alexandre Romoscanu <romoscanu at imes.mavt.ethz.ch>*Date*: Sat, 11 Aug 2001 03:40:14 -0400 (EDT)*Organization*: Swiss Federal Institute of Technology (ETHZ)*References*: <9kqjd6$4d9$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

I do not see where you could be wrong. I also faced some quite weird Mathematica behaviour these last times (by plotting functions...weird results, which in addition to this were different before and after a Simplify for instance) cheers A. Oliver Friedrich wrote: > Hallo, > > I found some strange behaviour of FourierTransform > > If I multiply two sine functions and transform it with > > FourierTransform[Cos[a t+b]*Cos[c t+d],t,f] > > I get the correct answer with four DiracDeltas at a+c, a-c, -a+c, -a-d and > some coefficients. > > Now the funny: If I assume both sines without a phaseshift, I can omit b and > d and write > > FourierTransform[a t]*Cos[c t],t,f] > > FourierTransform returns my expression unevaluated. So that algorithm > requires explicitly at least one phase shift argument. If I omit only one, I > get a result (although I've yet not checked for correctness). > Does anybody knows about this behaviour? At the moment my workaround is > > FourierTransform[Cos[a t+b]*Cos[c t+d],t,f]/.{b->0,d->0} > > That seems to be fair enough, but anyway, it should work without. Or didn't > I consider some mathematical traps? > > Best regards > Oliver Friedrich -- ___________________________________________ alexandre ioan romoscanu - institut für mechanik CLA G31, eth zentrum, 8092 zürich, schweiz tel. +41 1 632 77 54, +41 76 323 63 05 fax.+41 1 632 11 45, romoscanu at imes.mavt.ethz.ch ___________________________________________