def _FracInt(x,y,z,a,b,c,tau,n):
    """Returns
                1                     x^2       y^2       z^2
    -------------------------- (1 - ------- - ------- - -------)^n
    sqrt(tau+a)(tau+b)(tau+c))       tau+a     tau+b     tau+c
    """
    denom = np.sqrt((a + tau)*(b + tau)*(c + tau))
    return (1. - x**2/(a + tau) - y**2/(b + tau) - z**2/(c + tau))**n / denom