Comments on the computation of interval Routh approximants

In recent papers [6], [7], the Routh approximation method was extended to d erive reduced order Interval models for linear interval systems. In this pa per, the authors show that: 1) interval Routh approximants to a high-order interval transfer function depend on the implementation of interval Routh e xpansion and inversion algorithms; 2) interval Routh expansion algorithms c annot guarantee the success in generating a full interval Routh array; 3) s ome interval Routh approximants may not be robustly stable even if the orig inal interval system is robustly stable; and 4) an interval Routh approxima nt is in general not useful for robust controller design because its dynami c uncertainties tin terms of robust frequency responses) do not cover those of the original interval system.