def get_pF(a_F, ecc_F):
    """
    Computes the orbital parameter (semi-latus) rectum of the fundamental
    ellipse. This value is kept constant for all the problem as long as the
    boundary conditions are not changed.

    Parameters
    ----------
    a_F: float
        Semi-major axis of the fundamental ellipse.
    ecc_F: float
        Eccentricity of the fundamental ellipse.

    Returns
    -------
    p_F: float
        Orbital parameter / semi-latus rectum of the fundamental ellipse.

    Notes
    -----
    No labeled equation (appears between [3] and [4]) from Avanzini's report

    """
    p_F = a_F * (1 - ecc_F ** 2)
    return p_F