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 .