On the necessity of sufficient LMI conditions for time-delay systems arising from Legendre approximation

TL;DR

This paper proves that scalable LMI conditions based on Legendre polynomial inequalities become necessary and sufficient for the stability of time-delay systems as the polynomial order increases, addressing a key open question.

Contribution

It demonstrates that stability implies feasibility of scalable LMI conditions at sufficiently large polynomial order, providing an analytic estimate for this order.

Findings

Feasibility of LMI conditions implies system stability at high polynomial order.

Provides an analytic estimate for the necessary polynomial order.

Addresses the open question of convergence of hierarchical LMI methods.

Abstract

This work is dedicated to the stability analysis of time-delay systems with a single constant delay using the Lyapunov-Krasovskii theorem. This approach has been widely used in the literature and numerous sufficient conditions of stability have been proposed and expressed as linear matrix inequalities (LMI). The main criticism of the method that is often pointed out is that these LMI conditions are only sufficient, and there is a lack of information regarding the reduction of the conservatism. Recently, scalable methods have been investigated using Bessel-Legendre inequality or orthogonal polynomial-based inequalities. The interest of these methods relies on their hierarchical structure with a guarantee of reduction of the level of conservatism. However, the convergence is still an open question that will be answered for the first time in this paper. The objective is to prove that the stability of a time-delay system implies the feasibility of these scalable LMI, at a sufficiently large order of the Legendre polynomials. Moreover, the proposed contribution is even able to provide an analytic estimation of this order, giving rise to a necessary and sufficient LMI for the stability of time-delay systems.