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## [Help-gsl] integrating from infinity to origin

 From: Swapnil Tripathi Subject: [Help-gsl] integrating from infinity to origin Date: Mon, 24 Jul 2006 21:23:10 -0500

```Hi Everyone,
I am trying to use the code below to modify it so I may use it to
integrate a function from infinity towards origin ( going in the
direction of decreasing t).The code is a sample program from the section
for ODE solution in GSL manual.

http://www.lsw.uni-heidelberg.de/manuals/gsl-ref-html/gsl-ref_24.html#SEC365

I am unable to figure out the working of this program with the routines
completely. So I am not being able to figure out where to make changes
so as to make it integrate in the negative direction.Can somebody help
me please ?

Thanks
Swapnil

Doctoral Student
Physics

#include <stdio.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_matrix.h>
#include <gsl/gsl_odeiv.h>

int
func (double t, const double y[], double f[],
void *params)
{
double mu = *(double *)params;
f = y;
f = -y - mu*y*(y*y - 1);
return GSL_SUCCESS;
}

int
jac (double t, const double y[], double *dfdy,
double dfdt[], void *params)
{
double mu = *(double *)params;
gsl_matrix_view dfdy_mat
= gsl_matrix_view_array (dfdy, 2, 2);
gsl_matrix * m = &dfdy_mat.matrix;
gsl_matrix_set (m, 0, 0, 0.0);
gsl_matrix_set (m, 0, 1, 1.0);
gsl_matrix_set (m, 1, 0, -2.0*mu*y*y - 1.0);
gsl_matrix_set (m, 1, 1, -mu*(y*y - 1.0));
dfdt = 0.0;
dfdt = 0.0;
return GSL_SUCCESS;
}

int
main (void)
{
const gsl_odeiv_step_type * T
= gsl_odeiv_step_rk8pd;

gsl_odeiv_step * s
= gsl_odeiv_step_alloc (T, 2);
gsl_odeiv_control * c
= gsl_odeiv_control_y_new (1e-6, 0.0);
gsl_odeiv_evolve * e
= gsl_odeiv_evolve_alloc (2);

double mu = 10;
gsl_odeiv_system sys = {func, jac, 2, &mu};

double t = 0.0, t1 = 100.0;
double h = 1e-6;
double y = { 1.0, 0.0 };

gsl_ieee_env_setup();

while (t < t1)
{
int status = gsl_odeiv_evolve_apply (e, c, s,
&sys,
&t, t1,
&h, y);

if (status != GSL_SUCCESS)
break;

printf("%.5e %.5e %.5e\n", t, y, y);
}

gsl_odeiv_evolve_free(e);
gsl_odeiv_control_free(c);
gsl_odeiv_step_free(s);
return 0;
}

```

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