diffexp.py 文件源码

def neg_bin_log_pmf(k, mu, phi):
    """ Log(PMF) of negative binomial distribution with mean mu and dispersion phi,
    conveniently parameterized.
    Args:
      k (int) - NB random variable
      mu (float) - mean
      phi (float) - dispersion
    Returns:
      The log of the pmf at k. """
    r = 1.0/phi
    return gammaln(r+k) - (gammaln(r) + gammaln(k+1)) + \
        k * np.log(mu/(r+mu)) + \
        r * np.log(r/(r+mu))