[Bug-gsl] bug in GSL numerical integration

[Nikhil Jayant Joshi]

Hi,

I am doing integration over (-infty, +infty) using the gsl_integration_qagi
routine. My expectation is that upon translation along the x-axis the
result of integration (area under the curve) should not change. However
that is not, what I observer. Am I making a mistake somewhere? The code is
attached below:

The variable offset basically creates the translation. The area remains
same for offset values of 0, 10.0, 20.0, but suddenly drop to zero after
offset ~ 40.0


double offset=200.0;


double f (double x, void * params) {

    double alpha = *(double *) params;

    x += offset;

    double f = exp(-x*x);

    return f;

}


int main(int argc, const char * argv[])

{



    gsl_integration_workspace * w

    = gsl_integration_workspace_alloc (1000);



    double result, error;

    double expected = -4.0;

    double alpha = 1.0;



    gsl_function F;

    F.function = &f;

    F.params = α



    gsl_integration_qagi (&F, 0, 0.001, 1000,

                          w, &result, &error);



    printf ("result = % .18f\n", result);



    return 0;

}

Thanks in anticipation,
Nikhil
--
My problem:
I have my brain divided into two parts: The left part has nothing right in
it, while the right part has nothing left in it.