IEEE Transactions on Automatic Control, Vol.53, No.1, 379-384, 2008
An extension of the argument principle and Nyquist criterion to a class of systems with unbounded generators
The Nyquist stability criterion is generalized to systems where the open-loop system has infinite-dimensional input and output spaces and an unbounded infinitesimal generator. The infinitesimal generator is assumed to be a sectorial operator with trace-class resolvent. The main result is obtained through use of the perturbation determinant and an extension of the argument principle to infinitesimal generators with trace-class resolvents.