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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: [mg23798] Re: NDSolve error message: Can't find starting value ...
  • From: "Bill Bertram" <wkb at ansto.gov.au>
  • Date: Sat, 10 Jun 2000 02:59:53 -0400 (EDT)
  • Organization: Australian Nuclear Science and Technology Organisation
  • References: <8hfggl$i4h@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Axel Kowald <axel at itb.biologie.hu-berlin.de> wrote in message
news:8hfggl$i4h at smc.vnet.net...
> 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 ?

Axel,

For this case Mathematica requires boundary conditions in terms of R and R'.
Your boundary conditions on R'' should be recast using the DE. Presumably
they would be R[-3] == 0 and R[3] == 0. Mathematica is then quite happy but
I don't know if you'll be, because the solution it gives is R[x] =0 for all
x.

Cheers,
    Bill





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