[Help-gsl] gsl_min_fminimizer_f_minimum

[David Doria]

I am trying to minimize a distance function.  The problem is, the function
Distance is squared distance, so it should always be positive.  I output the
value that Distance returns and it is indeed positive.  However when i
output gsl_min_fminimizer_f_minimum(s) at every iteration it is negative!!??
My code is below - am I doing something wrong?

double MinimizeDistance(MyParam_T &MyParams)
{
    size_t iter = 0;
    int MAX_ITER = 20;

    int status;
    const gsl_min_fminimizer_type *T;
    gsl_min_fminimizer *s;

    double a = -10;
    double b = 10;
    double m = 2;
    double val;

    gsl_function F;
    F.function = &Distance; //my function is double Distance(double T, void
*params)
    F.params = &MyParams;

    T = gsl_min_fminimizer_goldensection;

    s = gsl_min_fminimizer_alloc(T);
    gsl_min_fminimizer_set(s, &F, m, a, b);

    do
    {
        iter++;
        status = gsl_min_fminimizer_iterate(s);
        a = gsl_min_fminimizer_x_lower(s);
        b = gsl_min_fminimizer_x_upper(s);
        m = gsl_min_fminimizer_x_minimum(s);
        val = gsl_min_fminimizer_f_minimum(s);

        status = gsl_min_test_interval(a, b, .1, 0);

        if(verbosity >= 2)
            printf("Iter: [%u %u] Min: %f Val: %f\n", a, b, m, val);
    }
    while ( (status == GSL_CONTINUE) && (iter < MAX_ITER) );

    double fmin = gsl_min_fminimizer_f_minimum(s);

    gsl_min_fminimizer_free(s);

    return fmin;

}


-- 
Thanks,

David