  The purpose of this short paper is to provide a theoretical analysis for the
consensus problem under nonlinear protocols. A main contribution of this work
is to generalize the previous consensus problems under nonlinear protocols for
networks with undirected graphs to directed graphs (information flow). Our
theoretical result is that if the directed graph is strongly connected and the
nonlinear protocol is strictly increasing, then consensus can be realized. Some
simple examples are also provided to demonstrate the validity of our
theoretical result.
