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MathGroup Archive 1996

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Re: Mathematica NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg3876] Re: Mathematica NDSolve
  • From: withoff (David Withoff)
  • Date: Sat, 4 May 1996 02:16:01 -0400
  • Organization: Wolfram Research, Inc.
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4let9r$e4m at dragonfly.wolfram.com> Eric Mockensturm  
<root at mote.ME.berkeley.edu> writes:
> I've used NDSolve quite a few times but I'm getting some results and I 
> can't determine what they mean.  I have a couple of coupled, NLODEs.  
> When I try to solve these guys with NDSolve, I just get {} as the 
> output.  I don't get any error messages or anything.  I'm not sure what 
> to make of this.  I hope it's something stupid and someone can set me 
> straight.  In the meantime, I'll try to remember how to do this with  
Matlab.
> 
> Thanks,
> Eric Mockensturm
> 
> MY NEW EMAIL ADDRESS:  eric at mote.me.berkeley.edu
> 
> Dynamic Stability Lab
> 1113 Etcheverry Hall
> University of California - Berkeley
> (510) 642-6371
> http://mote.berkeley.edu/~eric/eric.html
> 

This behavior means that there are no values of the derivatives that
satisfy the equations, or if there are, Mathematica wasn't able to
find them.  For example:

In[2]:= NDSolve[{f'[x] == 1, f'[x] == 2, f[0] == 1}, f[x], {x, 0, 1}]

Out[2]= {}

As an experiment, try using Solve to solve your equations (without
the initial conditions) for the derivatives.  If that doesn't work,
then NDSolve won't work either.

Unless your equations are such that it is difficult to solve for the
derivatives, my guess is that this is a syntax problem.  As has already
been pointed out, if you could send us your example, someone will
probably be able to spot the problem pretty quickly.

Dave Withoff
Research and Development
Wolfram Research

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