B（Beta）函数定义为：
与函数的关系为：
因此，其计算就直接调用函数。
不完全B（Beta）函数为：
其中
当 时，可采用连分式计算：
其中：
当 时，则利用对称关系计算：
连分式计算过程：
Public Function betacf(ByVal a As Double, ByVal b As Double, ByVal x As Double) As Double
Dim ITMAX, m, m2 As Long
ITMAX = 1000
Dim EPS, FPMIN, aa, C, d, del, h, qab, qam, qap As Double
EPS = getGDelta()
FPMIN = 0.000000000001
qab = a + b
qap = a + 1#
qam = a - 1#
C = 1#
d = 1# - qab * x / qap
If (Abs(d) < FPMIN) Then d = FPMIN
d = 1# / d
h = d
For m = 1 To ITMAX
m2 = 2 * m
aa = m * (b - m) * x / ((qam + m2) * (a + m2))
d = 1# + aa * d
If (Abs(d) < FPMIN) Then d = FPMIN
C = 1# + aa / C
If (Abs(C) < FPMIN) Then C = FPMIN
d = 1# / d
h = h * d * C
aa = -(a + m) * (qab + m) * x / ((a + m2) * (qap + m2))
d = 1# + aa * d
If (Abs(d) < FPMIN) Then d = FPMIN
C = 1# + aa / C
If (Abs(C) < FPMIN) Then C = FPMIN
d = 1# / d
del = d * C
h = h * del
If (Abs(del - 1#) <= EPS) Then Exit For
Next
If (m > ITMAX) Then
betacf = -1
Else
betacf = h
End If
End Function
不完全B函数计算：
Public Function bata(ByVal a As Double, ByVal b As Double, ByVal x As Double) As Double
Dim bt As Double
If (x < 0#) Or (x > 1#) Then
Err.Raise vbObjectError + 513, , "参数错误！ "
End If
If (x = 0#) Or (x = 1#) Then
bt = 0#
Else
bt = Exp(lnGamma(a + b) - lnGamma(a) - lnGamma(b) + a * Log(x) + b * Log(1# - x))
End If
If (x < (a + 1#) / (a + b + 2#)) Then
bata = bt * betacf(a, b, x) / a
Else
bata = 1# - bt * betacf(b, a, 1# - x) / b
End If
End Function
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