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MathGroup Archive 2000

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Re: NDSolve error message: Can't find starting value ...

  • To: mathgroup at smc.vnet.net
  • Subject: [mg23772] Re: NDSolve error message: Can't find starting value ...
  • From: Rob Knapp <rknapp at wolfram.com>
  • Date: Sat, 10 Jun 2000 02:59:18 -0400 (EDT)
  • Organization: Wolfram Research, Inc.
  • References: <8hfggl$i4h@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Axel Kowald wrote:
> 
> Hello,
> 
> I try to solve the following second order ODE with NDSolve
> 
> NDSolve[{0 == R''[x] + DiracDelta[x] - R[x],         R''[-3] == 0,
> R''[3] == 0}, R, {x, -3, 3}]
> 
> and I get the following error message:
> 
> NDSolve::"ndsv": "Cannot find starting value for the variable x."
> 
> I couldn't find any description of this message in the Mathematica book, so I'm
> not sure what to do. I have a second order ODE with two boundary
> conditions, what's missing ?
> 
> Many thanks,
> 
>                Axel Kowald
> 
> P.S. Btw., this is done with Mathematica 4.

The problem is that initial or boundary values must be specified for the
functions with differential order less than that of the differential
equation itself. i.e., if you used

In[1]:= NDSolve[{0 == R''[x] + DiracDelta[x] - R[x],R[-3] ==
DiracDelta[-3], R[3] == DiracDelta[3]},R,{x,-3,3}]

Out[1]= {{R -> InterpolatingFunction[{{-3., 3.}}, <>]}}


NDSolve will be much happier.

In a future version, the message will be somewhat clearer as to the real
problem, or the capability of resolving the problem using the equation
automatically will be implemented.

Rob Knapp
Wolfram Research


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