def abc_psd(nu, a, b, c):
    """Lorentzian model of a Power Spectral Density.

    Parameters
    ----------
    nu : `numpy.ndarray` or `float`
        spatial frequency
    a : `float`
        a coefficient
    b : `float`
        b coefficient
    c : `float`
        c coefficient

    Returns
    -------
    `numpy.ndarray`
        value of PSD model

    """
    return a / (1 + (nu / b) ** 2) ** (c / 2)
