Stabilization of linear time-delayed systems by delayed feedback
  In this paper, stability analysis of time delay systems is considered based on decomposition of the systems to subsystems. The decomposition process needs matrices of these systems to be simultaneously block triangularize. We show that a finite set of matrices are simultaneously block triangularize if and only if they have a common invariant subspace. The importance of the decomposition is highlighted by some systems that their characteristic equation has repeated roots on the imaginary axis. For such systems, the cluster treatment method and the direct method cannot be employed to analyze the stability of them, unless, we decompose the systems to subsystems. On the other hand, it has been shown that stabilization of time delay systems can be done by delayed feedback. Moreover, this kind of feedback is applied to design a controller. 
Mohammad Mousa-Abadian and Sayed Hodjatollah Momeni-Masuleh and   Mohammad Haeri
math.OC