Hermitian solutions of the discrete algebraic Riccati equation

Hermitian solutions of the discrete algebraic Riccati equation play an important role in the least-squares optimal control problem for discrete linear systems. In this paper we describe the set of hermitian solutions in various ways: in terms of factorizations of rational matrix functions which take hermitian values on the unit circle; in terms of certain invariant subspaces of a matrix which is unitary in an indefinite scalar product; and in terms of all invariant subspaces of a certain matrix. These results are inspired by known results for the algebraic Riccati equation arising in the least-squares optimal control problem for continuous linear systems.