def klypin(x, mvir=1.5e12):
    """Klypin 2011 Bolshoi subhalo mass function.

    Keyword arguments:
    x -- array of peak subhalo halo masses
    mvir -- halo mass of host halo (default 1.5e12)
    """
    tr = 1.7e-3*(mvir)**0.5*(x)**-3
    return tr