Incremental positivity non-preservation by stability multipliers

Abstract

It is proved that stability multipliers such as Zames-Falb multipliers, Popov multipliers and RL/RC multipliers, known to preserve positivity of monotone memoryless nonlinearities, do not, in general, preserve their incremental positivity. Our result implies that stability results based on these multipliers should be interpreted with caution since without incremental positivity, continuity of the closed-loop system's input-output relation may not be assured.