  Minimal input/output selection is investigated in this paper for each
subsystem of a networked system. Some novel sufficient conditions are derived
respectively for the controllability and observability of a networked system,
as well as some necessary conditions. These conditions only depend separately
on parameters of each subsystem and its in/out-degrees. It is proven that in
order to be able to construct a controllable/observable networked system, it is
necessary and sufficient that each subsystem is controllable/observable. In
addition, both sparse and dense subsystem connections are helpful in making the
whole system controllable/observable. An explicit formula is given for the
smallest number of inputs/outputs for each subsystem required to guarantee
controllability/observability of the whole system.
