def fast_rep(para_no, infer_fac=4, freq_step=2):
    """Number of iterations needed for the FAST algorithm.

    Parameters
    ----------
    para_no : :class:`int`
        Number of parameters in the model.
    infer_fac : :class:`int`, optional
        The inference fractor. Default: 4
    freq_step : :class:`int`, optional
        The frequency step. Default: 2
    """
    return 2 * int(
        para_no * (1 + 4 * infer_fac ** 2 * (1 + (para_no - 2) * freq_step))
    )