def n_eff(dlnsdlnm):
    """
    Return the power spectral slope at the scale of the halo radius, 
    
    Parameters
    ----------
    dlnsdlnm : array
        The derivative of log sigma with log M
    
    Returns
    -------
    n_eff : float

    Notes
    -----
    Uses eq. 42 in Lukic et. al 2007.
    """

    n_eff = -3.0 * (2.0 * dlnsdlnm + 1.0)

    return n_eff