def simple_power_law(v, a, c, v0):
    """Simple power law:

    .. math::
        S_v =  c \\left( \\frac{v}{v_0} \\right)^a

    Parameters
    ----------
    v : `list`
        Frequency in Hz.
    a : `float`
        Spectral Index.
    c : `float`
        Constant.
    v0 : `float`
        Reference frequency.

    Returns
    -------
    S_v : `list`
        The flux density predicted by the model.
    """
    return c*(v/v0)**a