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MathGroup Archive 2000

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NDSolve and numeric-only computation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21848] NDSolve and numeric-only computation
  • From: bumi314 at aol.com (Carl Bumiller)
  • Date: Sun, 30 Jan 2000 18:53:30 -0500 (EST)
  • Organization: AOL http://www.aol.com
  • Sender: owner-wri-mathgroup at wolfram.com

    I have a question about NDSolve and numeric-only computation.

    First I have defined     RHS=Compile[{{x,_Real}},........]  where ........ 
is some complicated expression, then I ran 
                NDSolve[{x'[t]==RHS[x[t]],x[0]==1},{x},{t,0,1}]
and got the error message

    CompiledFunction::cfr:
            Cannot use compiled code;  Argument x[t] at position 1
                should be a machine-size real number.

This suggests that NDSolve is trying to solve symbolically rather than 
numerically.  How do I get NDSolve to evaluate RHS numerically rather
than symbolically??   When I try Compiled->True or Compiled->False
with NDSolve, I get the same error message.  Since RHS is complicated,
its symbolic evaluation is time comsuming although the equation is 
eventually solved.  I am running Mathematica 3.0 for Digital Unix. 

Carl Bumiller
bumi314 at aol.com


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