Statistical function - upper incomplete gamma function

[Grbe]

I need upper incomplete gamma function for C++.. I have here lower incomplete gamma function.. But i need upper incomplete gamma function.

double incgamma (double x, double a){
double sum=0;
double term=1.0/a;
int n=1;
	while (term != 0){
		sum = sum + term;
		term = term*(x/(a+n));
		n++;
		}
return pow(x,a)*exp(-1*x)*sum;
}

Difference between upper and lower functions:
http://en.wikipedia.org/wiki/Incomplete_gamma_function

[Grbe]

Please somobody.. :(

[JLBorges]

boost::math::gamma_q<>() and boost::math::tgamma<>()
http://www.boost.org/doc/libs/1_49_0/libs/math/doc/sf_and_dist/html/math_toolkit/special/sf_gamma/igamma.html

[Grbe]

Is there somewhere a function itself, without the whole package .. as this function incgama