Information limits for recovering a hidden community

We study the problem of recovering a hidden community of cardinality K from an n × n symmetric data matrix A, where for distinct indices i; j, A_ij ∼ P if i; j both belong to the community and A_ij ∼ Q otherwise, for two known probability distributions P and Q depending on n. We focus on two types of asymptotic recovery guarantees as n → ∞: (1) weak recovery: expected number of classification errors is o(K); (2) exact recovery: probability of classifying all indices correctly converges to one. Under mild assumptions on P and Q, and allowing the community size to scale sublinearly with n, we derive a set of sufficient conditions and a set of necessary conditions for recovery, which are asymptotically tight with sharp constants. The results hold in particular for the Gaussian case (P = N(μ, 1) and Q = N(0; 1)), and for the case of bounded log likelihood ratio, including the Bernoulli case (P = Bern(p) and Q = Bern(q)) whenever p/q and 1-p/1-q are bounded away from zero and infinity. An important algorithmic implication is that, whenever exact recovery is information theoretically possible, any algorithm that provides weak recovery when the community size is concentrated near K can be upgraded to achieve exact recovery in linear additional time by a simple voting procedure.