' FB 1.05.0 Win64

Const pi = 3.1415926535897932
Const e  = 2.7182818284590452

Function gammaStirling (x As Double) As Double
  Return Sqr(2.0 * pi / x) * ((x / e) ^ x)
End Function

Function gammaLanczos (x As Double) As Double
  Dim p(0 To 8) As Double = _
  { _
       0.99999999999980993, _
     676.5203681218851, _
   -1259.1392167224028, _			     	
     771.32342877765313, _
    -176.61502916214059, _
      12.507343278686905, _
      -0.13857109526572012, _
       9.9843695780195716e-6, _
       1.5056327351493116e-7 _
  }

  Dim As Integer g = 7
  If x < 0.5 Then Return pi / (Sin(pi * x) * gammaLanczos(1-x))
  x -= 1
  Dim a As Double = p(0)
  Dim t As Double = x + g + 0.5

  For i As Integer = 1 To 8
    a += p(i) / (x + i)
  Next		

  Return Sqr(2.0 * pi) * (t ^ (x + 0.5)) * Exp(-t) * a
End Function

Print " x", "    Stirling",, "    Lanczos"
Print
For i As Integer = 1 To 20
   Dim As Double d = i / 10.0
   Print   Using "#.##"; d;
   Print , Using "#.###############"; gammaStirling(d);
   Print , Using "#.###############"; gammaLanczos(d)
Next
Print
Print "Press any key to quit"
Sleep