/// <summary>
/// Computes the Gamma function.
/// </summary>
/// <param name="x">The argument.</param>
/// <returns>The value of Γ(x).</returns>
/// <remarks>
/// <para>The Gamma function is a generalization of the factorial (see <see cref="AdvancedIntegerMath.Factorial"/>) to arbitrary real values.</para>
/// <img src="../images/GammaIntegral.png" />
/// <para>For positive integer arguments, this integral evaluates to Γ(n+1)=n!, but it can also be evaluated for non-integer z.</para>
/// <para>Like the factorial, Γ(x) grows rapidly with increasing x; Γ(x) overflows <see cref="System.Double" />
/// for all x larger than ~171. For arguments in this range, you may find it useful to work with the <see cref="LogGamma" /> method, which
/// returns accurate values for ln(Γ(x)) even in the range for which Γ(x) overflows.</para>
/// <para>To evaluate the Gamma function for a complex argument, use <see cref="AdvancedComplexMath.Gamma" />.</para>
/// <h2>Domain, Range, and Accuracy</h2>
/// <para>The function is defined for all x. It has poles at all negative integers and at zero; the method returns <see cref="Double.NaN"/> for these arguments. For positive
/// arguments, the value of the function increases rapidly with increasing argument. For values of x greater than about 170, the value of the function exceeds
/// <see cref="Double.MaxValue"/>; for these arguments the method returns <see cref="Double.PositiveInfinity"/>. The method is accurate to full precision over its entire
/// domain.</para>
/// </remarks>
/// <seealso cref="AdvancedIntegerMath.Factorial" />
/// <seealso cref="LogGamma" />
/// <seealso cref="AdvancedComplexMath.Gamma" />
/// <seealso href="http://en.wikipedia.org/wiki/Gamma_function" />
/// <seealso href="http://mathworld.wolfram.com/GammaFunction.html" />
/// <seealso href="http://dlmf.nist.gov/5">DLMF on the Gamma Function</seealso>
public static double Gamma(double x)
{
if (x < 0.5)
{
// Use \Gamma(x) \Gamma(1-x) = \frac{\pi}{\sin(\pi x)} to move values close to and left of origin to x > 0
return(Math.PI / MoreMath.SinPi(x) / Gamma(1.0 - x));
}
else if (x < 1.5)
{
return(GammaSeries.GammaOnePlus(x - 1.0));
}
else if (x < 2.5)
{
return(GammaSeries.GammaTwoPlus(x - 2.0));
}
else if (x < 16.0)
{
return(Lanczos.Gamma(x));
}
else if (x < 172.0)
{
return(Stirling.Gamma(x));
}
else if (x <= Double.PositiveInfinity)
{
// For x >~ 172, Gamma(x) overflows.
return(Double.PositiveInfinity);
}
else
{
return(Double.NaN);
}
}
/// <summary>
/// Computes the digamma function.
/// </summary>
/// <param name="x">The argument.</param>
/// <returns>The value of ψ(x).</returns>
/// <remarks>
/// <para>The psi function, also called the digamma function, is the logrithmic derivative of the Γ function.</para>
/// <img src="../images/DiGamma.png" />
/// <para>Because it is defined as a <i>logarithmic</i> derivative, the digamma function does not overflow <see cref="System.Double"/>
/// even for arguments for which <see cref="Gamma(double)"/> does.</para>
/// <para>To evaluate the psi function for complex arguments, use <see cref="AdvancedComplexMath.Psi" />.</para>
/// </remarks>
/// <seealso cref="Gamma(double)"/>
/// <seealso cref="AdvancedComplexMath.Psi"/>
/// <seealso href="http://en.wikipedia.org/wiki/Digamma_function" />
/// <seealso href="http://mathworld.wolfram.com/DigammaFunction.html" />
public static double Psi(double x)
{
if (x < 0.5)
{
return(Psi(1.0 - x) - Math.PI / MoreMath.TanPi(x));
}
else if (x < 1.5)
{
return(GammaSeries.PsiOnePlus(x - 1.0));
}
else if (x < 2.5)
{
return(GammaSeries.PsiTwoPlus(x - 2.0));
}
else if (x < 16.0)
{
return(Lanczos.Psi(x));
}
else if (x <= Double.PositiveInfinity)
{
// For large arguments, the Stirling asymptotic expansion is faster than the Lanzcos approximation
return(Stirling.Psi(x));
}
else
{
return(Double.NaN);
}
}
// one-argument functions
/// <summary>
/// Computes the natural logarithm of the Gamma function.
/// </summary>
/// <param name="x">The argument, which must be positive.</param>
/// <returns>The log Gamma function ln(Γ(x)).</returns>
/// <remarks>
/// <para>Because Γ(x) grows rapidly for increasing positive x, it is often necessary to
/// work with its logarithm in order to avoid overflow. This function returns accurate
/// values of ln(Γ(x)) even for values of x which would cause Γ(x) to overflow.</para>
/// </remarks>
/// <exception cref="ArgumentOutOfRangeException"><paramref name="x"/> is negative.</exception>
/// <seealso cref="Gamma(double)" />
public static double LogGamma(double x)
{
if (x < 0.0)
{
throw new ArgumentOutOfRangeException(nameof(x));
}
else if (x < 0.5)
{
// For small arguments, use the Lanczos approximation.
return(Lanczos.LogGamma(x));
}
else if (x < 1.5)
{
// Use the series expansion near 1.
return(GammaSeries.LogGammaOnePlus(x - 1.0));
}
else if (x < 2.5)
{
// The series expansion can be adapted near 2, too.
return(GammaSeries.LogGammaTwoPlus(x - 2.0));
}
else if (x < 16.0)
{
// In between, we still use Lanczos.
return(Lanczos.LogGamma(x));
}
else if (x < Double.PositiveInfinity)
{
// For large arguments, the asymptotic series is even faster than the Lanczos approximation.
return(Stirling.LogGamma(x));
}
else if (x == Double.PositiveInfinity)
{
// Precisely at infinity x * Math.Log(x) - x => NaN, so special-case it.
return(Double.PositiveInfinity);
}
else
{
return(Double.NaN);
}
}
