Invariant subspace approach to linear H∞-control via measurement feedback

This paper analyzes, and thus reveals the structure of the stable invariant subspace of the related Hamiltonian matrix arising from the measurement feedback H∞-control problem. Using this, it presents another method for the verification of the admissibility of the controller derived by Doyle et al. in 1989. The method not only eliminates unnecessary assumptions on stabilizability and detectability, but also gives deeper insight into the relationship among the stable parts of the associated matrices.