chiGser function
The chi Gser function
Implementation
double chiGser(double x, double a) {
// Good for X<A+1.
var t9 = 1 / a;
var g = t9;
var i = 1.0;
while (t9 > g * .00001) {
t9 = t9 * x / (a + i);
g = g + t9;
i = i + 1;
}
return g * exp(a * log(x) - x - lngamma(a));
}