The Yakubovich S-Lemma Revisited: Stability and Contractivity in Non-Euclidean Norms

TL;DR

This paper extends the classical S-Lemma to non-Euclidean norms, providing new stability and contractivity criteria for Lur'e systems using weighted  norms, and offers a novel proof of longstanding conjectures.

Contribution

It introduces a non-polynomial S-Lemma for weighted  norms, enabling stability analysis beyond Euclidean norms and proving key conjectures for positive Lur'e systems.

Findings

Derived constructive criteria for stability using weighted  norms

Provided a new proof of the Aizerman and Kalman conjectures

Established generalized absolute stability and contractivity conditions

Abstract

The celebrated S-Lemma was originally proposed to ensure the existence of a quadratic Lyapunov function in the Lur'e problem of absolute stability. A quadratic Lyapunov function is, however, nothing else than a squared Euclidean norm on the state space (that is, a norm induced by an inner product). A natural question arises as to whether squared non-Euclidean norms $V(x)=\|x\|^2$ may serve as Lyapunov functions in stability problems. This paper presents a novel non-polynomial S-Lemma that leads to constructive criteria for the existence of such functions defined by weighted $\ell_p$ norms. Our generalized S-Lemma leads to new absolute stability and absolute contractivity criteria for Lur'e-type systems, including, for example, a new simple proof of the Aizerman and Kalman conjectures for positive Lur'e systems.