A counterexample of "Comments on 'Stability margin evaluation for uncertain linear systems'"

In the original paper (C. Gong and S. Thompson, ibid., vol.39, p.548-50, 1994) there is given what is claimed to be the tightest bound for all unstructured pertubations such that a specified system keeps its asymptotic stability. J.-H. Su (ibid., vol.39, p.2523-4, 1994) showed that it is not the tightest bound, and gives another bound. That bound is shown, by a counterexample, to be incorrect.