Re: Problem with sums differentiation

*To*: mathgroup at smc.vnet.net*Subject*: [mg13277] Re: [mg13143] Problem with sums differentiation*From*: "Jens-Peer Kuska" <kuska at linmpi.mpg.de>*Date*: Fri, 17 Jul 1998 03:18:21 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hi Dimitri, The rule that gives the desired result is D[a[x,t],x] /. HoldPattern[Sum[D[a_,x_],iter_]] :>Sum[Evaluate[D[a,x]],iter] the total derivative Dt[] is not needed here. Sum has the attribute HoldAll and will by default not evaluate its arguments. Hope that helps Jens -----Original Message----- From: Dmitri Tcherniak <famdt at pop.dtu.dk> To: mathgroup at smc.vnet.net Subject: [mg13277] [mg13143] Problem with sums differentiation > >I have problems with infinite sums integration, differentiations, >multiplications when the imax is infinity or a symbol. For example > >In: >a[x_,t_]=Sum[uj[t]*Cos[j Pi x],{j,1,Infinity}]; Dt[a[x,t],x] > >gives me >Out: Dt[Sum[uj[t] Cos[j Pi x], {j, 1, Infinity}], x] > >but not something like Sum[-uj[t]*j*Pi*Sin[j Pi x],{j,1,Infinity}] > >Is there a way to move the integration (differentiation) sigh under the >sum sigh and force Mathematica to evaluate the terms? > >Thank you >Dmitri Tcherniak > > >