functions in gammp.i - b

             betai(a, b, x)  
 
    return I_x(a,x) = int[0 to x]{ du * u^(a-1)*(1-u)^(b-1) } / beta(a,b)  
    the incomplete beta function  
    
    betai(a,b,x) = 1 - betai(b,a,1-x)  
    
    Note that Student's t-distribution is  
      A(t|nu) = 1 - betai(0.5*nu,0.5, nu/(nu+t^2))  
    The F-distribution is  
      Q(F|nu1,nu2) = betai(0.5*nu2,0.5*nu1, nu2/(nu2+F*nu1))  
    
interpreted function, defined at i/gammp.i   line 135