An internal model principle for open systems

The paper explores an extension of the classical internal model principle of Francis and Wonham to cases where the exogeneous signals (references or disturbances) are generated by an open, rather than closed (i.e., autonomous), exosystem. We study this extension both in the linear and nonlinear case, showing that the internal model principle is necessary for robust regulation of a contractive closed-loop system. While preliminary, our results motivate a generalization of nonlinear regulation theory to open exosystems.