  In this Letter we propose a method to control a set of arbitrary nodes in a
directed network such that they follow a synchronous trajectory which is, in
general, not shared by the other units of the network. The problem is inspired
to those natural or artificial networks whose proper operating conditions are
associated to the presence of clusters of synchronous nodes. Our proposed
method is based on the introduction of distributed controllers that modify the
topology of the connections in order to generate outer symmetries in the nodes
to be controlled. An optimization problem for the selection of the controllers,
which includes as a special case the minimization of the number of the links
added or removed, is also formulated and an algorithm for its solution is
introduced.
