Frequency conditions for the global stability of nonlinear delay equations with several equilibria

Abstract: In our adjacent work, we developed a spectral comparison principle for compound cocycles generated by delay equations. It allows to derive frequency conditions (inequalities) for the uniform exponential stability of such cocycles by means of their comparison with stationary problems. Such inequalities are hard to verify analytically, and in this work we develop approximation schemes to verify some of the arising frequency inequalities. Besides some general results, we mainly stick to the case of scalar equations. By means of the Suarez-Schopf delayed oscillator and the Mackey-Glass equations, we demonstrate applications of the theory to reveal regions in the space of parameters where the absence of closed invariant contours can be guaranteed. Since our conditions are robust, so close systems also satisfy them, we expect them to actually imply the global stability, as in known finite-dimensional results utilizing variants of the Closing Lemma which is still awaiting developments in infinite dimensions.