Community Consensus: Converging Locally despite Adversaries and Heterogeneous Connectivity

We introduce the concept of community consensus in the presence of malicious agents using a well-known median-based consensus algorithm. We consider networks that have multiple well-connected regions that we term communities, characterized by specific robustness and minimum degree properties. Prior work derives conditions on properties that are necessary and sufficient for achieving global consensus in a network. This however, requires the minimum degree of the network graph to be proportional to the number of malicious agents in the network, which is not very practical in large networks. In this work we present a natural generalization of this previous result. We characterize cases when although global consensus is not reached, some subsets of agents $V_i$ will still converge to the same values $\mathcal{M}_i$ among themselves. We define more relaxed requirements for this new type of consensus to be reached in terms of the number $k$ of edges connecting an agent in a community to agents external to the community, and the number of malicious agents in each community.