/* Paraboloid centered on (p[0],p[1]), with  

   scale factors (p[2],p[3]) and minimum p[4] */

double

my_f (const gsl_vector *v, void *params)

{

  double x, y;

  double *p = (double *)params;

  x = gsl_vector_get(v, 0);

  y = gsl_vector_get(v, 1);

  return p[2] * (x - p[0]) * (x - p[0]) +

           p[3] * (y - p[1]) * (y - p[1]) + p[4]; 

}

/* The gradient of f, df = (df/dx, df/dy). */

void 

my_df (const gsl_vector *v, void *params, 

       gsl_vector *df)

{

  double x, y;

  double *p = (double *)params;

  x = gsl_vector_get(v, 0);

  y = gsl_vector_get(v, 1);

  gsl_vector_set(df, 0, 2.0 * p[2] * (x - p[0]));

  gsl_vector_set(df, 1, 2.0 * p[3] * (y - p[1]));

}

/* Compute both f and df together. */

void 

my_fdf (const gsl_vector *x, void *params, 

        double *f, gsl_vector *df) 

{

  *f = my_f(x, params); 

  my_df(x, params, df);

}