Re: NDSolve is using Solve?
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- Subject: [mg131624] Re: NDSolve is using Solve?
- From: "vadim.oganesyan at gmail.com" <vadim.oganesyan at gmail.com>
- Date: Fri, 13 Sep 2013 00:34:55 -0400 (EDT)
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On Thursday, March 10, 2011 6:16:45 AM UTC-5, Oliver Ruebenkoenig wrote: > On Wed, 9 Mar 2011, J.Frazer wrote: > > > Hello everyone, > > > > This is my first time posting so my apologies if I'm going about this > > the wrong way. I'm having the following problem: > > > > I'm using NDSolve but its not managing to get to the solution even if I > > leave it running over the weekend. If I greatly simplify one of the > > equations I can get a result but in either case I get the error messages > > > > Solve:: incnst > > Solve:: ifun > > > > This worries me as if its trying to simplify analytically, presumably as > > the equations becomes more complicated this will become incredibly time > > consuming. > > > > Do people agree this may be the cause of the long running time? If so, > > is there a way of stopping NDSolve from using Solve? > > > > Cheers, > > > > Jonny > > > > > Hi Jonny, > > yes NDSolve uses Solve internally but that is only for pre processing > purposes. If this is really the cause for the slow NDSolve (maybe it is a > hard problem...) It were best if you could send some working code, then > we could have a look at it and see if anything can be done. > > Oliver Hi, I have encountered a similar problem, I think: I have re-parametrized a relatively simple system of linear ODEs u'=L.u into something more complicated looking M[v].v'=N[v], but it really is the same problem with new coordinates, so the solution must exist (except near a few possible singularity points of the coordinate change) feeding this or v'=Inverse[M[v]].N[v] into NDSolve has ONLY worked for small enough problems where the inverse can be done analytically. Otherwise, it spends forever in the prep stage for the calculation. I have played around with defining a delayed Inverse etc but nothing worked, so I gave up and coded up a simple RK4 routine and forgot about it, but now I am returning to this problem and would like to get Mathematica to work, which I think it should be able to. Thoughts? Thanks,Vadim