  The present paper is devoted to estimating the speed of convergence towards
consensus for a general class of discrete-time multi-agent systems. In the
systems considered here, both the topology of the interconnection graph and the
weight of the arcs are allowed to vary as a function of time. Under the
hypothesis that some spanning tree structure is preserved along time, and that
some nonzero minimal weight of the information transfer along this tree is
guaranteed, an estimate of the contraction rate is given. The latter is
expressed explicitly as the spectral radius of some matrix depending upon the
tree depth and the lower bounds on the weights.
