Robust stabilizability of linear time invariant plants: Nonlinear time-varying controllers

This paper is addressed to the problem of robust stabilization of imprecisely known linear time-invariant (LTI) plants, using nonlinear time-varying (NLTV) controllers. We consider a general family of LTI plants described by a nominal plant model with unstructured uncertainty, and show that a necessary and sufficient condition for such a family to be robustly stabilizable (by an arbitrary NLTV controller) is that it be robustly stabilizable by an LTI controller.