Question

I want to integrate a probability density function from (-\infty, a] because the cdf is not available in closed form. But I’m not sure how to do this in C++.

This task is pretty simple in Mathematica; All I need to do is define the function,

f[x_, lambda_, alpha_, beta_, mu_] := 
   Module[{gamma}, 
     gamma = Sqrt[alpha^2 - beta^2]; 
     (gamma^(2*lambda)/((2*alpha)^(lambda - 1/2)*Sqrt[Pi]*Gamma[lambda]))*
      Abs[x - mu]^(lambda - 1/2)*
      BesselK[lambda - 1/2, alpha Abs[x - mu]] E^(beta (x - mu))
   ];

and then call the NIntegrate Routine to numerically integrate it.

F[x_, lambda_, alpha_, beta_, mu_] := 
    NIntegrate[f[t, lambda, alpha, beta, mu], {t, -\[Infinity], x}] 

Now I want to achieve the same thing in C++. I using the routine gsl_integration_qagil from the gsl numerics library. It is designed to integrate functions on the semi infinite intervals (-\infty, a] which is just what I want. But unfortunately I can’t get it to work.

This is the density function in C++,

density(double x)
{
using namespace boost::math;

if(x == _mu)
    return std::numeric_limits<double>::infinity();

    return pow(_gamma, 2*_lambda)/(pow(2*_alpha, _lambda-0.5)*sqrt(_pi)*tgamma(_lambda))* pow(abs(x-_mu), _lambda - 0.5) * cyl_bessel_k(_lambda-0.5, _alpha*abs(x - _mu)) * exp(_beta*(x - _mu));

}  

Then I try and integrate to obtain the cdf by calling the gsl routine.

cdf(double x)
{
gsl_integration_workspace * w = gsl_integration_workspace_alloc (1000);

    double result, error;      
    gsl_function F;
    F.function = &density;

    double epsabs = 0;
    double epsrel = 1e-12;

    gsl_integration_qagil (&F, x, epsabs, epsrel, 1000, w, &result, &error);

    printf("result          = % .18f\n", result);
    printf ("estimated error = % .18f\n", error);
    printf ("intervals =  %d\n", w->size);

    gsl_integration_workspace_free (w);

    return result;

}

However gsl_integration_qagil returns an error, number of iterations was insufficient.

 double mu = 0.0f;
 double lambda = 3.0f;
 double alpha = 265.0f;
 double beta = -5.0f;

 cout << cdf(0.01) << endl;

If I increase the size of the workspace then the bessel function will not evaluate.

I was wondering if there was anyone that could give me any insight to my problem. A call to the corresponding Mathematica function F above with x = 0.01 returns 0.904384.

Could it be that the density is concentrated around a very small interval (i.e. outside of [-0.05, 0.05] the density is almost 0, a plot is given below). If so what can be done about this. Thanks for reading.

Answer 1

Re: integrating to +/- infinity:

I would use Mathematica to find an empirical bound for |x – μ| >> K, where K represents the “width” around the mean, and K is a function of alpha, beta, and lambda — for example F is less than and approximately equal to a(x-μ)^-2 or ae^-b(x-μ)2 or whatever. These functions have known integrals out to infinity, for which you can evaluate empirically. Then you can integrate numerically out to K, and use the bounded approximation to get from K to infinity.

Figuring out K may be a bit tricky; I’m not very familiar with Bessel functions so I can’t help you much there.

In general, I’ve found that for numerical calculation that’s not obvious, the best way is to do as much analytical math as you can before you do numerical evaluation. (Kind of like an autofocus camera — get it close to where you want, then let the camera do the rest.)