“…Such systems have received a lot of attention in the last 30 years, e.g., in satellite attitude control, helicopter design, harmonic balance methods in nonlinear circuit design, and many more, see, e.g., [6,10,17,18,30,33]. Some control-theoretic concepts including controllability and observability, Gramians, Hankel singular values and balanced realization have been extended to periodic discrete-time descriptor systems with singular E k , see [8,9,24].…”