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MathGroup Archive 2002

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sum problem with infinity

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33002] sum problem with infinity
  • From: klepachd at yahoo.com (Doron)
  • Date: Mon, 25 Feb 2002 01:31:18 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

hello,
I am a problem :
I defined ->
\!\(\[Psi][x_, y_] := \(1\/4\) a\^2 - 
      x\^2 - \(\(8 
               a\^2\)\/\[Pi]\^3\) \(\[Sum]\+\(n = \
0\)\%\[Infinity]\(\((\(-1\))\)\^n\/\((2  n + 1)\)\^3\) \(Cosh[k[n]*y]\ \
Cos[k[n]*x]\)\/Cosh[\(1\/2\) k[n]\ b]\)\)
and tried to compute ->
D[\[Psi][x, y], y] /. y -> 0.9 /. x -> 0.8
the answer I got was -> 
General::"ivar": "\!\(0.9`\) is not a valid variable."

General::"stop": "Further output of \!\(General :: \"ivar\"\) will be \
suppressed during this calculation."

\!\(\(-\(\(32\ \(\[Sum]\+\(n = \
0\)\%\[Infinity]\[PartialD]\_0.9`\(\(\((\(-1\))\)\^n\ \((Cosh[
                          k[n]\ 0.9`]\ Cos[
                          k[n]\ 0.8`])\)\)\/\(\((2\ n + 1)\)\^3\ Cosh[
                      1\/2\ k[n]\ b]\)\)\)\)\/\[Pi]\^3\)\)\)
But when I replaced the sums limit from infinity to 99
there was no problem and I got a numerical answer.
Any ideas why ?
Thank you , Doron .


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