Equivalent characterization of invariant subspaces of H2 and applications to the optimal sensitivity problem

This paper gives some equivalent characterizations for invariant subspaces of H², when the underlying structure is specified by the so-called pseudorational transfer functions. This plays a fundamental role in computing the optimal sensitivity for a certain important class of infinite-dimensional systems, including delay systems. A closed formula, easier to compute than the well-known Zhou-Khargonekar formula, is given for optimal sensitivity for such systems. An example is given to illustrate the result.