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MathGroup Archive 1995

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Re: NDSolve problem

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg921] Re: NDSolve problem
  • From: withoff (David Withoff)
  • Date: Wed, 3 May 1995 00:00:07 -0400
  • Organization: Wolfram Research, Inc.

In article <3nkcth$n02 at news0.cybernetics.net> pitts at mayo.EDU (Todd Pitts) writes:
>I have been using NDSolve[] to investigate the solutions to some large,
>sets of simple, ordinary differential equations.  It is most convenient
>to use subscripted variables/functions and parameters to represent the large number
>of solution functions and system characteristics.  For certain values
>of coefficients the solution proceeds nicely, however, for others I obtain
>
> eq={4.0*D[x[1][t],{t,2}]==x[1][t]-Sin[t]};
> bc={Evaluate[D[x[1][t],t] /. t->0]==0,x[1][0]==0};
> sys=Join[eq,bc];
> NDSolve[sys,x[1],{t,0,10}]
>
>NDSolve::ndnum: Differential equation does not evaluate to a number at t = 0..
>
>Out[32]= {{x[1] -> InterpolatingFunction[{0., 0.}, <>]}}
> 
>Thanks in Advance,
>
>Todd Pitts

Try using

    SetAttributes[x, NProtectedAll]

so that NDSolve doesn't try to convert the index in x[1] into
an inexact number.  This should fix the problem.

In the process of computing numerical values of the derivatives, the
index in an indexed variable like x[1] will sometimes (depending on the
form of the equations) be converted to an inexact number, and the new
variable x[1.] is not recognized as being the same as the original one.
This problem has been fixed for the next version.

Dave Withoff
Research and Development
Wolfram Research


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