def _get_x(F_0, F_1, F_i, F_star, Z, alpha):
    """Computes Sundman transformation variable.

    Parameters
    ----------
    F_0: float
        First boundary coefficient.
    F_1: float
        Second boundary coefficient.
    F_i: float
        Third boundary coefficient.
    F_star: float
        Last boundary coefficient.
    Z: float
        Auxiliary constant.
    alpha: float
        Auxiliary constant.

    Returns
    -------
    x: float
        Sundman transformation value.

    Notes
    -----
    This is equation (44) from original report.

    """

    x = (
        (Z * (F_0 - F_star) * (F_1 - F_i))
        / ((F_i - F_star) * (F_1 - F_0) * Z + (F_0 - F_i) * (F_1 - F_star))
    ) ** (1 / alpha)
    return x