“…The H 1 controller design implies that the resulting closedloop control system is robustly stable and achieves a prescribed level of attenuation from the disturbance input to the output in L 2 /l 2 -norm. A large number of results on this subject have been reported for systems in the general situation, linear or nonlinear (Van der Schaft 1991, 1992Wang, Xie, and De Souza 1992;Xie, Shi, and De Souza 1993;Chen and Tseng 1994;Katebi and Zhang 1995;Nguang 1997;Shi, Shue, Shi, and Agarwal 1999;Fridman 2001;Mahmoud and Boujarwah 2001;Guo and Lam 2002;Gao, Lam, and Wang 2002;Xu and Chen 2002;Xu, Lu, Zhou, and Yang 2004;Hsiao 2007;Mahmoud, Shi, Boukas, and Jain 2008;Xia, Qiu, Zhang, Gao, and Wang 2008;Wang, Ho, Liu, and Liu 2009;Xia, Fu, Shi, Wu, and Zhang 2009;Zhang and Boukas 2009;Zhang and Shi 2009;Zhang, Wang, and Chen 2009). The sufficient conditions for the existence of an H 1 controller, where the control and filter gain matrices satisfy Riccati equations, were obtained for linear systems in Doyle et al (1989).…”