Function BETAI(A, B, X)
    If X < 0# Or X > 1# Then Print "bad argument X in BETAI"
    If X = 0# Or X = 1# Then
        BT = 0#
    Else
        AAA = GAMMLN(A + B) - GAMMLN(A) - GAMMLN(B)
        BT = Exp(AAA + A * Log(X) + B * Log(1# - X))
    End If
    If X < (A + 1#) / (A + B + 2#) Then
        BETAI = BT * BETACF(A, B, X) / A
        Exit Function
    Else
        BETAI = 1# - BT * BETACF(B, A, 1# - X) / B
        Exit Function
    End If
End Function