def kf_derivative_wrt_density(kf, n):
    """Computes the derivative of kf with respect to density

    It is given by `kf / (3 * n)`

    Parameters
    ----------
    kf : array-like
        The fermi momentum in fm^-1
    n : array-like
        The density in fm^-3

    Returns
    -------
    d(kf)/d(n) : array-like
        In units of fm^2
    """
    return kf / (3 * n)