Authors´ reply

Dr. M. S. de Queiroz made some comments on our technique note (Su et al., 2007). The major points in the author´s comment are as follows. 1) The C² condition in assumptions Al and A2 are not necessary. 2) Although there is a slight difference in the way the observer equations by Su et al. (2007) and Xian et al. (2004), the two observers are structurally similar. 3) The Lyapunov-like function of Su et al. includes an extra term to allow use of the LaSalle´s invariance principle in proving the expected convergence. However, as shown Xian et al., this is not necessary since one can use the LaSalle-Yoshizawa-like theorem in Lemma 2 of Xian et al. to prove the same result. We agree that the C² condition in Assumptions A1 and A2 can be changed to the C¹ condition. This observation does not challenge the proposed observer, but it certainly is insightful, and thus, permits wider application.