  We study cluster synchronization in networks and show that the stability of
all possible cluster synchronization patterns depends on a small set of
Lyapunov exponents. Our approach can be applied to clusters corresponding to
both orbital partitions of the network nodes (symmetry-cluster synchronization)
and equitable partitions of the network nodes (input-cluster synchronization.)
Our results are verified experimentally in networks of coupled opto-electronic
oscillators.
