public static double LogGamma(double x)
{
// Stirling's formula for the gamma function
List <double> A = new List <double>
{
8.11614167470508450300E-4,
-5.95061904284301438324E-4,
7.93650340457716943945E-4,
-2.77777777730099687205E-3,
8.33333333333331927722E-2
};
List <double> B = new List <double>
{
-1.37825152569120859100E3,
-3.88016315134637840924E4,
-3.31612992738871184744E5,
-1.16237097492762307383E6,
-1.72173700820839662146E6,
-8.53555664245765465627E5
};
List <double> C = new List <double>
{
/* 1.00000000000000000000E0, */
-3.51815701436523470549E2,
-1.70642106651881159223E4,
-2.20528590553854454839E5,
-1.13933444367982507207E6,
-2.53252307177582951285E6,
-2.01889141433532773231E6
};
const double LOGPI = 1.14472988584940017414;
const double LS2PI = 0.91893853320467274178; /* log( sqrt( 2*pi ) ) */
int lsign = 1;
int sign = 1;
double q, w, p, u, z;
int i;
if (x < -34.0)
{
q = -x;
w = LogGamma(q); /* note this modifies sign! */
p = Math.Floor(q);
if (p == q) // argument is negative integer, return NaN
{ // (each singularity goes to +/- infinity depending
// on direction of limit
return(double.NaN);
}
i = (int)p;
// need to translate this code:
// is it computing the parity of i?
lsign = 2 * (i & 1) - 1;
///////////////////////////////
sign = lsign;
z = q - p;
if (z > 0.5)
{
p++;
z = p - q;
}
z = q * Math.Sin(Math.PI * z);
if (z == 0.0)
{ // log singularity
return(double.NaN);
}
z = LOGPI - Math.Log(z) - w;
return(sign * z);
}
if (x < 13.0)
{
z = 1.0;
p = 0.0;
u = x;
while (u >= 3.0)
{
p--;
u = x + p;
z *= u;
}
while (u < 2.0)
{
if (u == 0.0)
{
return(double.NaN);
}
z /= u;
p++;
u = x + p;
}
if (z < 0.0)
{
lsign = -1;
z = -z;
}
else
{
lsign = 1;
}
sign = lsign;
if (u == 2.0)
{
return(Math.Log(z));
}
p -= 2.0;
x = x + p;
p = x * Algebra.PolyEval(x, B) / Algebra.PolyEval(x, C);
return(Math.Log(z) + p);
}
if (x > 2.556348e305)
{
return(sign * double.PositiveInfinity);
}
q = (x - 0.5) * Math.Log(x) - x + LS2PI;
if (x > 1.0e8)
{
return(sign * q);
}
p = 1.0 / (x * x);
if (x >= 1000.0)
{
q += ((7.9365079365079365079365e-4 * p
- 2.7777777777777777777778e-3) * p
+ 0.0833333333333333333333) / x;
}
else
{
q += Algebra.PolyEval(p, A) / x;
}
return(sign * q);
}
public static double Psi(double x)
{
List <double> A = new List <double>
{
8.33333333333333333333E-2,
-2.10927960927960927961E-2,
7.57575757575757575758E-3,
-4.16666666666666666667E-3,
3.96825396825396825397E-3,
-8.33333333333333333333E-3,
8.33333333333333333333E-2
};
double p, q, s, w, y, z;
int i, n;
bool negative = false;
double nz = 0.0;
if (x <= 0.0)
{
negative = true;
q = x;
p = Math.Floor(q);
if (p == q)
{
// throw exception"Psi SINGULARITY");
return(double.MaxValue);
}
/* Remove the zeros of tan(PI x)
* by subtracting the nearest integer from x
*/
nz = q - p;
if (nz != 0.5)
{
if (nz > 0.5)
{
p += 1.0;
nz = q - p;
}
nz = Math.PI / Math.Tan(Math.PI * nz);
}
else
{
nz = 0.0;
}
x = 1.0 - x;
}
/* check for positive integer up to 10 */
if ((x <= 10.0) && (x == Math.Floor(x)))
{
y = 0.0;
n = (int)x;
for (i = 1; i < n; i++)
{
w = i;
y += 1.0 / w;
}
y -= EUL;
if (negative)
{
y -= nz;
}
return(y);
}
s = x;
w = 0.0;
while (s < 10.0)
{
w += 1.0 / s;
s += 1.0;
}
if (s < 1.0e17)
{
z = 1.0 / (s * s);
y = z * Algebra.PolyEval(z, A);
}
else
{
y = 0.0;
}
y = Math.Log(s) - (0.5 / s) - y - w;
if (negative)
{
y -= nz;
}
return(y);
}
