A Tight Lower Bound on the Controllability of Networks with Multiple   Leaders
  In this paper we study the controllability of networked systems with static network topologies using tools from algebraic graph theory. Each agent in the network acts in a decentralized fashion by updating its state in accordance with a nearest-neighbor averaging rule, known as the consensus dynamics. In order to control the system, external control inputs are injected into the so called leader nodes, and the influence is propagated throughout the network. Our main result is a tight topological lower bound on the rank of the controllability matrix for such systems with arbitrary network topologies and possibly multiple leaders. 
Ahmet Yasin Yazicioglu, Waseem Abbas, and Magnus Egerstedt
cs.SY math.OC