The robust decentralized-control of a servomechanism problem for composite systems with input-output interconnections

In [1], it was shown that under certain mild conditions, there "almost always" is a solution to the robust decentralized servomechanism problem for a composite system consisting of a number of subsystems interconnected in any arbitrary manner, provided that there exists a solution to the servomechanism problem for each subsystem. In this note, it is shown that for the special case when the composite system consists of subsystems interconnected by inputs and outputs, then there always is a solution to the robust decentralized servomechanism problem for the composite system if and only if there exists a solution to the robust servomechanism problem for each subsystem.