Recovering asymmetric communities in the stochastic block model

TL;DR

This paper investigates the limits of community detection in sparse stochastic block models with unbalanced communities, identifying thresholds for algorithmic success and analyzing the role of partial vertex revelation.

Contribution

It characterizes the performance of local algorithms like belief propagation near the Kesten Stigum threshold and introduces the spinodal curve as a new solvability boundary.

Findings

Belief propagation is optimal down to the Kesten Stigum threshold with partial vertex revelation.

Below the Kesten Stigum threshold, community detection becomes impossible below the spinodal curve.

The spinodal curve aligns with the reconstruction threshold on trees, providing a new phase boundary.

Abstract

We consider the sparse stochastic block model in the case where the degrees are uninformative. The case where the two communities have approximately the same size has been extensively studied and we concentrate here on the community detection problem in the case of unbalanced communities. In this setting, spectral algorithms based on the non-backtracking matrix are known to solve the community detection problem (i.e. do strictly better than a random guess) when the signal is sufficiently large namely above the so-called Kesten Stigum threshold. In this regime and when the average degree tends to infinity, we show that if the community of a vanishing fraction of the vertices is revealed, then a local algorithm (belief propagation) is optimal down to Kesten Stigum threshold and we quantify explicitly its performance. Below the Kesten Stigum threshold, we show that, in the large degree limit, there is a second threshold called the spinodal curve below which, the community detection problem is not solvable. The spinodal curve is equal to the Kesten Stigum threshold when the fraction of vertices in the smallest community is above $p^*=\frac{1}{2}-\frac{1}{2\sqrt{3}}$, so that the Kesten Stigum threshold is the threshold for solvability of the community detection in this case. However when the smallest community is smaller than $p^*$, the spinodal curve only provides a lower bound on the threshold for solvability. In the regime below the Kesten Stigum bound and above the spinodal curve, we also characterize the performance of best local algorithms as a function of the fraction of revealed vertices. Our proof relies on a careful analysis of the associated reconstruction problem on trees which might be of independent interest. In particular, we show that the spinodal curve corresponds to the reconstruction threshold on the tree.