ẋ = Ax + Bu (linear dynamics)
n_D = max(1, N - |maximum matching|)
A system is controllable if any state can be reached from any initial condition. The minimum number of driver nodes equals N minus the size of a maximum matching on the network. Scale-free networks (hubs) surprisingly require more driver nodes than random — hubs create bottlenecks. Highlighted nodes (red) are driver nodes; blue arrows show matched edges.