def ab_psd(nu, a, b):
    """Inverse power model of a Power Spectral Density.

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

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

    """
    return a * nu ** -b
