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Re: [Help-gsl] Boundary value problem for system of ODEs

From: Juan Pablo Amorocho
Subject: Re: [Help-gsl] Boundary value problem for system of ODEs
Date: Thu, 23 Apr 2015 08:33:10 +0200

Hi Ruben, 

regarding your GSL question, take a look here: 

It shows how the jacobian is stored.

I hope this helps.

— Juan Pablo

> On 23 Apr 2015, at 00:30, Ruben Farinelli <address@hidden> wrote:
> Hi,
> I have been working for a long time on a complicate physical problem.
> I have a set of ODEs, three of which are first-order, and the fourth
> is of second order.
> In the latter case unfortunately, I have conditons for its derivative
> R'[0]=0,
> while the second is actually an asymptotic boundary condition, namely
> the function must tend to zero for large value of the variable.
> Of course with a change of variable the system becomes N+1
> first order ODEs.
> Actually I have implemented some kind of shooting method, the main
> issue is that the function has an exponential-decay behavior and
> the result seems to be rather dependent on the adopted integrator.
> Finally I decided to compute the complicate Jacobian to test
> the gsl_odeiv_bsimp which I read should be the most powerful.
> A-part from any welcome suggestion in approaching BVPs, I have
> a doubt. Namely, the right-hand side of the system contains not only
> the functions y_i but also their first derivatives, labeled F_i, or y'_i.
> I mean something like
> F[0]=f_0{y[0]...y[n], F[1].....F[n]}
> F[1]=f_1{y[0]...y[n], F[0].....F[n]}
> etc etc
> The GSL jacobian function arguments are
> (double t, const double y[], double *dfdy, double dtdy[], void *params)
> but I don't see where the functions derivative y'_i are stored.
> They are still present when computing the Jacobian, but apparently
> they are not read.
> Is there something wrong ?
> Thank you for your help !
> Ruben

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