Comments on "Variable structure control design for uncertain discrete-time systems"

States that the main result (Theorem 2) of the above paper (Wang, Wu, and Yang, IEEE Trans. Autom. Contr., vol.39, no.1, p. 99-102, 1994) is incorrect since the convergence of the system state onto the hyperplane S(x)=Cx=0 is guaranteed only in the region where the nonlinear control component is applied (region A). The authors also state that no stability properties can be inferred in a case when only linear equivalent control U/sub eq/ is used (region B) or when the applied control provokes a permanent switching between A and B regions. The latter case is illustrated by a counterexample showing the instability of the control scheme proposed in the paper.<>