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MathGroup Archive 1998

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Problem with sums differentiation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13143] Problem with sums differentiation
  • From: Dmitri Tcherniak <famdt at pop.dtu.dk>
  • Date: Mon, 13 Jul 1998 07:41:56 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

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



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