THE STATE-FEEDBACK H(INFINITY)-PROBLEM AT OPTIMALITY

We discuss the infinite horizon H(infinity)-control problem by state-f eedback for a linear time invariant plant which is not restricted with respect to its finite or infinite zero structure. We provide an algeb raic characterization for the optimal value to be attained in terms of the solvability of a nonstrict Riccati inequality and propose possibi lities for verifying these criteria. If the optimum is not achieved we give a complete test for the need of high-gain feedback in order to a pproach the optimal value.