[Rd] Overly restrictive conditions to evaluate beta (PR#10763)

[Jerry.Lewis at biogenidec.com]

Full_Name: Jerry W. Lewis
Version: 2.6.2
OS: Windows XP Professional
Submission from: (NULL) (96.233.108.117)


Currently, beta(a,b) returns NaN if either a or b is negative, but the current
calculation 
  beta(a,b) = gamma(a)*gamma(b)/gamma(a+b)
works equally well if either or both arguments are negative, provided that none
of a, b, a+b is a negative integer.

Also, if n>m>0 are integers, then 
  B(m,-n) = (-1)^m * B(m,n-m+1)