On the quadratic controllability and quadratic feedback minimality

In Swei et al. (2000) the quadratic controllability and quadratic feedback minimality of unstructured norm bounded uncertain linear systems have been studied with explicitly verifiable necessary and sufficient solvability conditions in terms of the so-called "essential subsystem." In this note, we give the geometric interpretation of the main results in the above paper, using the strongly controllable subspace and, hence, provide a deeper understanding of the concept of "essential subsystem" in an earlier work.