Re: Problem with FourierTransform
- To: mathgroup at smc.vnet.net
- Subject: [mg30349] Re: Problem with FourierTransform
- From: "Oliver Friedrich" <oliver.friedrich at tz-mikroelektronik.de>
- Date: Sat, 11 Aug 2001 03:40:14 -0400 (EDT)
- References: <9kqjd6$4d9$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Sorry, I did a writing error, the following is the correct input. So take look, regard the phase shift b and d FourierTransform[Cos[a t+b]*Cos[c t+d],t,f] works fine FourierTransform[Cos[a t]*Cos[c t+d],t,f] works fine FourierTransform[Cos[a t+b]*Cos[c t],t,f] works fine FourierTransform[Cos[a t]*Cos[c t],t,f] doesn't work at all Any ideas? "Oliver Friedrich" <oliver.friedrich at tz-mikroelektronik.de> schrieb im Newsbeitrag news:9kqjd6$4d9$1 at smc.vnet.net... > 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 > >