  Differential passivity is a property that allows to check with a pointwise
criterion that a system is incrementally passive, a property that is relevant
to study interconnected systems in the context of regulation, synchronization,
and estimation. The paper investigates how restrictive is the property,
focusing on a class of open gradient systems encountered in the coenergy
modeling framework of physical systems, in particular the Brayton-Moser
formalism for nonlinear electrical circuits.
