  This paper deals with strong structural controllability of structured
networks. A structured network is a family of structured systems (called node
systems) that are interconnected by means of a structured interconnection law.
The node systems and their structured interconnection law are given by pattern
matrices. It is shown that a structured network is strongly structurally
controllable if and only if an associated structured system is. This structured
system will in general have a very large state space dimension, and therefore
existing tests for verifying strong structural controllability are not
tractable. The main result of this paper circumvents this problem. We show that
controllability can be tested by replacing the original network by a new
network in which all original node systems have been replaced by (auxiliary)
node systems with state space dimensions either 1 or 2. Hence, controllability
of the original network can be verified by testing controllability of a
structured system with state space dimension at most twice the number of node
systems, regardless of the state space dimensions of the original node systems.
