  The exact mean time between encounters of a given particle in a system
consisting of many particles undergoing random walks in discrete time is
calculated, on both regular and complex networks. Analytical results are
obtained both for independent walkers, where any number of walkers can occupy
the same site, and for walkers with an exclusion interaction, when no site can
contain more than one walker. These analytical results are then compared with
numerical simulations, showing very good agreement.
