Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Xu, Jianhong


In this dissertation, we investigate the Riccati diagonal stability and explore some extensions of this notion. Riccati diagonal stability plays an important role in the stability analysis of linear time-delay systems. It is known that if a linear time-delay system is Riccati diagonally stable then it admits a diagonal Lyapunov-Krasovskii functional. The existence of such a functional implies the asymptotic stability of the linear time-delay system. This diagonal stability problem has other applications in applied areas such as physical sciences and population dynamics. We also study the Lyapunov diagonal stability, which has a clear connection to the Riccati diagonal stability. Using a separation theorem, we first provide new proofs for some existing results on the Lyapunov-type diagonal stability. We also construct a new, shorter, and more transparent proof for a well-known result by Kraaijevanger that gives explicit conditions for the Lyapunov diagonal stability on matrices in $\mathbb{R}^{3 \times 3}$. In addition, we give several necessary and sufficient conditions for matrices in $\mathbb{R}^{3 \times 3}$ to be Lyapunov diagonally stable. Furthermore, we present an extension of the so-called Riccati diagonal stability to the Riccati $\alpha$-scalar stability. We derive two new characterizations regarding the Riccati $\alpha$-scalar solution of the Riccati matrix inequality so as to expand and broaden the relevant existing results. We also generalize this notion to consider a common $\alpha$-scalar solution for a family of Riccati matrix inequalities. We shall refer to this new generalization as common Riccati $\alpha$-scalar stability. As an application for the main results, we further explore families of block triangular matrices. Finally, motivated by recent developments, we formulate the problem of Riccati $\alpha$-stability. We present a necessary and sufficient condition for this type of stability and study the connection between Riccati $\alpha$-stability of a pair of $\alpha$-block matrices and Riccati stability of the diagonal block pairs. Moreover, we generalize the Riccati $\alpha$-stability by considering a family of pairs of $\alpha$-block matrices and give a new characterization for this new case.

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