Comments on "Output feedback stabilization for uncertain systems: constrained Riccati approach"

For original paper, by Cheng, see ibid., vol.43, p.81-4 (1998). In the above paper, the constrained Riccati problem (CRP) has been proposed to deal with mismatched uncertainty by extending the concept of the constrained Lyapunov problem. In this note, we show the solvability condition for the CRP only gives a solution for the class of square systems not for the nonsquare systems.