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MathGroup Archive 1998

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Mantain unevaluated

  • To: mathgroup at smc.vnet.net
  • Subject: [mg14551] Mantain unevaluated
  • From: Guillermo de Paz <guipaz at dali.eis.uva.es>
  • Date: Fri, 30 Oct 1998 03:07:25 -0500
  • Organization: Universidad de Valladolid - Spain
  • Sender: owner-wri-mathgroup at wolfram.com

I'm using Mathematica to obtain the differential equations of some
mechanical systems and to integrate them numerically. When constructing
the equations I need to invert a matrix. This matrix is not very big
(lets say 8x8) but the elements al very long trigonometric expressions,
the inverse takes a long time. By the other side, it is possible that
for some values of the dependent variables the rank of the matrix is
not complete, and the size of the problem decreases. For these reasons
I would like to have unevaluated this inverse until the numerical
function NDSolve uses it (not to compute symbolically the matrix, but
compute it numerically inside NDSolve in each integration step).
I have done some attempts with := and with Compile, but I cant avoid the
symbolic  computation of the inverse. Could somebody help me?

Guillermo de Paz
Universidad de Valladolid.
e-mail: guipaz at dali.eis.uva.es



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