@classmethod
           def step(cls, func , t : np.float , u : np.float , dt):

                def f(ti,ui):
                    return  np.array([function(ti,ui) for function in func])

                if cls.first_startup:
                    cls.um3 = u.copy()
                    unext = rungekutta.RK4.step(func,t,cls.um3,dt)
                    t += dt
                    cls.first_startup = False
                elif cls.second_startup:
                    cls.um2 = u.copy()
                    unext = rungekutta.RK4.step(func,t,cls.um2,dt)
                    t+= dt
                    cls.second_startup = False
                elif cls.third_startup:
                    cls.um1 = u.copy()
                    unext = rungekutta.RK4.step(func,t,cls.um1,dt)
                    t += dt
                    cls.third_step = False
                else:  # compute AB 4th order
                    unext = u + dt/24.* ( 55.*f(t,u) - 59.*f(t-dt,cls.um1) + 37.*f(t-dt-dt,cls.um2)
                                                                           - 9.*f(t-dt-dt,cls.um3 ))
                    cls.um3 = cls.um2.copy()
                    cls.um2 = cls.um1.copy()
                    cls.um1 = u.copy()
                return unext