Comments on "Invariance of the aperiodic property for polynomials with perturbed coefficients by C.B. Soh

In the above-titled paper (ibid., vol.35, p.616, May 1990), C.B. Soh presents a procedure for calculating the maximal intervals for the coefficients of a perturbed polynomial for persistence of the strict aperiodicity property. Both continuous-time and discrete-time aperiodicity are dealt with. The commenters note that, in view of the results of C.B. Soh and C.S. Berger (1989), very closely related results appear in Example 3.6 of a later paper by the commenters (1990), based on the concept of a ´guardian map´. Guardian maps associate scalar quantities with polynomials (or matrices) and vanish exactly when an open set of interest is exited.<>