def secondary_mass_fraction(mu):
    """
    Distribution function of the mass fraction of the secondary in binary systems / SNI
    mu = Mass_secondary / Mass_binary_system
    From: Matteucci, F. & Greggio, L. 1986, A&A, 154, 279
    with Gamma = 2 as Greggio, L., Renzini, A.: 1983a, Astron. Astrophys. 118, 217

    """
    gamma = 2.0
    return 2.0 ** (1.0 + gamma) * (1.0 + gamma) * mu ** gamma
