  Closed-loop positivity of feedback interconnections of positive monotone
nonlinear systems is investigated. It is shown that an instantaneous gain
condition on the open-loop systems which implies feedback well-posedness also
guarantees feedback positivity. Furthermore, the notion of integral linear
constraints (ILC) is utilised as a tool to characterise uncertainty in positive
feedback systems. Robustness analysis of positive linear time-varying and
nonlinear feedback systems is studied using ILC, paralleling the well-known
results based on integral quadratic constraints.
