The stable solutions of quadratic matrix equations

Abstract

The authors determine which solutions K to the quadratic matrix equation X B X + X A − D X − C = 0 XBX + XA - DX - C = 0 are “stable” in the sense that all small changes in the coefficients of the equation produce equations some of whose solutions are close to K (in the metric determined by the operator norm). Our main result is that a solution is stable if and only if it is an isolated solution. (The isolated solutions already have a simple characterization in terms of the coefficient matrices.) It follows that each equation has only finitely many stable solutions. Equivalently, we identify the stable invariant subspaces for an operator T on a finite-dimensional space as the isolated invariant subspaces.