  The entropy in dynamical systems was introduced by A. Kolmogorov. Initially
dedicated to iterations of one finite measure preserving transformation, the
notion was gradually generalized so as to encompass amenable group actions and
topological actions. L. Bowen (2008) succeeded in breaking the non-amenable
frontier by introducing the sofic entropy. This invariant provides the same
services as the classical entropy for the measured actions of the sofic groups
(a class which contains the residually finite groups). In 2010, D. Kerr et H.
Li established a topological version together with a variational principle.
