Comments on "Stability tests of N-dimensional discrete time systems using polynomial arrays

For original paper see X. Hu, ibid., vol.42, p.261-8 (1995). In this brief, we wish to point out that Hu overlooked a mistake in the stability test procedure for N-dimensional (N-D, N>2) systems proposed in the above paper, which made the polynomial array approach not general. It is shown that Hu´s test procedure applies only to a very restricted class of N-D stability test problems, and for a general case, instead of necessary and sufficient conditions, it provides only sufficient conditions. A counterexample is also given.