G. De Nicolao, G. Ferrari-Trecate, and S. Pinzoni. Zeros of continuous-time linear periodic systems. Automatica , 34(12):1651--1655, 1998. Link to Automatica
Zeros of continuous-time linear periodic systems are definedand their properties investigated. Under the assumption thatthe system has uniform relative degree, the zero-dynamics ofthe system is characterized and a closed-form expression ofthe blocking inputs is derived. This leads to the definitionof zeros as unobservable characteristic exponents of asuitably defined periodic pair. The zeros of periodic linearsystems satisfy blocking properties that generalize thewell-known time-invariant case. Finally, an efficient computationalscheme is provided that essentially amounts to solving aneigenvalue problem.