Existence and uniqueness of unmixed solutions of the discrete-time algebraic Riccati equation

A solution X of a discrete-time algebraic Riccati equation is called unmixed if the corresponding closed-loop matrix Φ(X) has the property that the common roots of det (sI−Φ(X)) and det (I− sΦ(X)∗) (if any) are on the unit circle. A necessary and sufficient condition is given for existence and uniqueness of an unmixed solution such that the eigenvalues of Φ(X) lie in a prescribed subset of C. Address for Correspondence: H. Wimmer Mathemat. Institut Universität Würzburg Am Hubland D-97074 Würzburg Germany