“…Although this problem is also known NP-hard (Syrmos et al, 1997), the curious fact to note here is that these early negative results have not prevented researchers from studying output feedback problems. In fact, there are a lot of existing works addressing this problem using different approaches, say, for example, Riccati equation approach, rank-constrained conditions, approach based on structural properties, bilinear matrix inequality (BMI) approaches and min-max optimization techniques (e.g., Bara & Boutayeb (2005;; Benton (Jr.); Gadewadikar et al (2006) ;Geromel, de Oliveira & Hsu (1998) ;Geromel et al (1996); Ghaoui et al (2001); Henrion et al (2005); Kučera & Souza (1995); Syrmos et al (1997) and the references therein). Nevertheless, the LMI approaches for this problem remain popular (Bara & Boutayeb, 2005;Cao & Sun, 1998;Geromel, de Oliveira & Hsu, 1998;Geromel et al, 1996;Prempain & Postlethwaite, 2001;Yu, 2004;Zečević & Šiljak, 2004) due to simplicity and efficiency.…”