  The paper studies the output-feedback synchronization problem for a network
of identical, linear time-invariant systems. A criterion to test network
synchronization is derived and the class of output-feedback synchronizable
systems is introduced and characterized by sufficient and necessary conditions.
In particular it is observed that output-feedback stabilizability is sufficient
but not necessary for output-feedback synchronizability. In the special case of
single-input single-output systems, conditions are derived in the frequency
domain. The theory is illustrated with several examples.
