  A rotor-router walk is a deterministic version of a random walk, in which the
walker is routed to each of the neighbouring vertices in some fixed cyclic
order. We consider here directed covers of graphs (called also periodic trees)
and we study several quantities related to rotor-router walks on directed
covers. The quantities under consideration are: order of the rotor-router
group, order of the root element in the rotor-router group and the connection
with random walks.
