/// <summary>
/// Calculates the specified Gamma function when <paramref name="a"/> is a half integer
/// </summary>
/// <param name="r"></param>
/// <param name="a"></param>
/// <param name="x"></param>
/// <returns></returns>
static double HalfIntegerA(Routine r, double a, double x)
{
Debug.Assert(a - Math.Floor(a) == 0.5);
double sum = 0;
if (a > 1)
{
sum = 1.0;
double term = 1.0;
for (int n = 2; n < a; ++n)
{
term *= x / (n - 0.5);
sum += term;
}
}
double result;
double mult = 2.0 * Constants.RecipSqrtPI * Math.Sqrt(x);
if (r == Routine.Q || r == Routine.Upper)
{
result = Math2.Erfc(Math.Sqrt(x)) + sum * mult * Math.Exp(-x);
return((r == Routine.Q) ? result : result *Math2.Tgamma(a));
}
else
{
result = Math2.Erf(Math.Sqrt(x)) - sum * mult * Math.Exp(-x);
return((r == Routine.P) ? result : result *Math2.Tgamma(a));
}
}
public static double TgammaLowerImp(double a, double x)
{
const Routine routine = Routine.Lower;
// special values
if (x == 0.0)
{
return(0.0);
}
if (double.IsInfinity(x))
{
return(Math2.Tgamma(a));
}
if (a == 1.0)
{
return(-Math2.Expm1(-x)); // 1.0 - Math.Exp(-x);
}
if (a == 0.5)
{
return(Constants.SqrtPI * Math2.Erf(Math.Sqrt(x)));
}
// Process small a, large x with asymptotic series, which is faster than CF
if (x >= Asym_MinLargeZ(a))
{
return(Math2.Tgamma(a) - Asym_SeriesLargeZ(a, x) * Prefix(a, x) / x);
}
// is a small
if ((a < 30) && (x >= a + 1) && (x < DoubleLimits.MaxLogValue))
{
double frac = a - Math.Floor(a);
if (frac == 0)
{
return(IntegerA(routine, a, x));
}
else if (frac == 0.5)
{
return(HalfIntegerA(routine, a, x));
}
}
if (x < (a + 1))
{
if (a < 1)
{
return(SmallA(routine, a, x));
}
return(Prefix(a, x, 1 / a) * LowerSeries(a, x));
}
// Use CF for x > a+1
return(TgammaMinusUpperFraction(a, x));
}
public static double GammaPImp(double a, double x)
{
const Routine routine = Routine.P;
// special values
if (x == 0.0)
{
return(0.0);
}
if (double.IsInfinity(x))
{
return(1.0);
}
if (a == 0)
{
return(1);
}
if (a == 1.0)
{
return(-Math2.Expm1(-x)); // 1.0 - Math.Exp(-x)
}
if (a == 0.5)
{
return(Math2.Erf(Math.Sqrt(x)));
}
// Process small a, large x with asymptotic series, which is faster than CF
if (x >= Asym_MinLargeZ(a))
{
return(1 - Asym_SeriesLargeZ(a, x) * PrefixRegularized(a, x) / x);
}
// is a small
if ((a < 30) && (x >= a + 1) && (x < DoubleLimits.MaxLogValue))
{
double frac = a - Math.Floor(a);
if (frac == 0)
{
return(IntegerA(routine, a, x));
}
else if (frac == 0.5)
{
return(HalfIntegerA(routine, a, x));
}
}
//
// Begin by testing whether we're in the "bad" zone
// where the result will be near 0.5 and the usual
// series and continued fractions are slow to converge:
//
if (a > 20)
{
// This second limit below is chosen so that we use Temme's expansion
// only if the result would be larger than about 10^-6.
// Below that the regular series and continued fractions
// converge OK, and if we use Temme's method we get increasing
// errors from the dominant erfc term as it's (inexact) argument
// increases in magnitude.
double sigma = Math.Abs((x - a) / a);
if (sigma < 0.4 || (a > 200 && 20 / a > sigma * sigma))
{
double t = TemmeSymmetricAsym.GammaP(a, x);
return((x >= a) ? 1 - t : t);
}
}
if (x < (a + 1))
{
if (a < 1)
{
return(SmallA(routine, a, x));
}
return(PrefixRegularized(a, x) * LowerSeries(a, x) / a);
}
// Use CF for x > a+1
return(1.0 - PrefixRegularized(a, x) * UpperFraction(a, x));
}
