def f(x, ys):
    """ Right part of equation y''' = f(x, y, y', y''),

        ys: (y, y', y''),
        x: independent variable.
    """
    y, dy, d2y = ys
    func = 3 * dy * d2y**2 / (1 + dy**2)  # change function in this line
                                          # x for independent variable
                                          # y for unknown function
                                          # dy for y'
                                          # and d2y for y''
    return [dy, d2y, func]