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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: [mg23799] Re: NDSolve error message: Can't find starting value ...
  • From: "Bill Bertram" <wkb at ansto.gov.au>
  • Date: Sat, 10 Jun 2000 02:59:55 -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:
>


Axel,

Using the equivalent boundary conditions R[-3] == 0 and R[3] == 0,
Mathematica (V4.0) comes up with the numerical solution, R[x] = 0 everywhere
which is wrong, but not really surprising. However try DSolve instead of
NDSolve e.g.

DSolve[{0 == R''[x] + DiracDelta[x] - R[x],   R [-3] == 0,
    R [3] == 0}, R[x], x]

This will give you the analytic solution which, as far as I can tell, seems
correct.

Cheers,

    Bill






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