  A multi-variable PI (proportional integrating) controller is proved to be
optimal for an important class of control problems where performance is
specified in terms of frequency weighted H-infinity norms. The problem class
includes networked systems with a subsystem in each node and control action
along each edge. For such systems, the optimal PI controller is decentralized
in the sense that control action along a given network edge is entirely
determined by states at nodes connected by that edge.
