MathGroup Archive 2002

[Date Index] [Thread Index] [Author Index]

Search the Archive

sum problem with infinity


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 .


  • Prev by Date: need a function for sums of subsets
  • Next by Date: Re: Iterative application of FindRoot with adjusted started values
  • Previous by thread: need a function for sums of subsets
  • Next by thread: Re: sum problem with infinity