  We study the state consensus problem for linear shift-invariant discrete-time
homogeneous multi-agent systems (MASs) over time-varying graphs. A novel
approach based on the small gain theorem is proposed to design the consensus
control protocols for both neutrally stable and neutrally unstable MASs,
assuming the uniformly connected graphs. It is shown that the state consensus
can be achieved for neutrally stable MASs under a weak uniform observability
condition; for neutrally unstable MASs, the state consensus entails a strong
uniform observability condition. Two numerical examples are worked out to
illustrate our consensus results.
