# The $H_\infty$ control problem with zeros on the boundary of the stability domain

A.A. Stoorvogel

In this paper we study the discrete and continuous time $H_\infty$ control problems without any assumptions on the system parameters. Our approach yields necessary and sufficient conditions for the existence of a suitable controller. Keywords: $H_\infty$ optimal control, Riccati equation, discrete time systems, continuous time systems.