Global disturbance rejection of lower triangular systems with unknown exosystem

In this paper, we study the global disturbance rejection problem of nonlinear systems in lower triangular form with unknown exosystem. Solvability conditions are established first. Then the convergence problem of an estimated unknown parameter vector to the true parameter vector is discussed. Finally, the theoretical result is applied to solve the disturbance rejection problem of the controlled uncertain Duffingpsilas system subject to a sinusoidal disturbance with unknown amplitude and unknown frequency.