  The study of network structural controllability focuses on the minimum number
of driver nodes needed to control a whole network. Despite intensive studies on
this topic, most of them consider static networks only. It is well-known,
however, that real networks are growing, with new nodes and links added to the
system. Here, we analyze controllability of evolving networks and propose a
general rule for the change of driver nodes. We further apply the rule to solve
the problem of network augmentation subject to the controllability constraint.
The findings fill a gap in our understanding of network controllability and
shed light on controllability of real systems.
