Some results on robustness to unmodeled dynamics of adaptive controllers using normalization factors [see P. Ioannou and K. S. Tsakalis, ibid. 31, 1033-1043 (1986; Zbl 0628.93040); K. S. Narendra and A. M. Annaswamy, ibid. 32, 134-145 (1987; Zbl 0617.93035); R. H. Middleton, G. C. Goodwin, D. J. Hill and D. Q. Mayne, ibid. 33, 55-58 (1988; Zbl 0637.93040); and P. Ioannou and J. Sun, Int. J. Control 47, No.3, 775-813 (1988)] are analyzed in this note.
It is shown that the proofs in the articles loc. cit. fail to establish that the proposed adaptive controllers provide a nonvanishing robustness margin for the considered class of unmodeled dynamics.