def get_aF(r1_norm, r2_norm):
    """
    Computes the semi-major axis of the fundamental ellipse. This value is
    kept constant for all the problem as long as the boundary conditions are not
    changed.

    Parameters
    ----------
    r1_norm: float
        Norm of the initial vector position.
    r2_norm: float
        Norm of the final vector position.

    Returns
    -------
    a_F: float
        Semi-major axis of the fundamental ellipse.

    Notes
    -----
    No labeled equation (appears between [3] and [4]) from Avanzini's report
    [1].

    """

    a_F = (r1_norm + r2_norm) / 2
    return a_F