Attenuation of Disturbances in Nonlinear Control Systems

In the last few years, the solution of the H ∞ (sub)optimal control problem via state-space methods was developed by several authors (for a rather comprehensive coverage of this subject the reader may consult the paper [1] and the theses [2] [3]). In the state-space formulation, the problem of minimizing the H ∞ norm (or, equivalently, the L 2 gain) of a closed loop system is viewed as a two-person, zero sum, differential game and, thus, the existence of the desired controller can be related to the existence of a solution of the algebraic Riccati equations arising in linear quadratic differential game theory (see, e.g. [4], [5] and [6]).