# Community detection thresholds and the weak Ramanujan property

1 INFINE - INFormation NEtworks
Inria Saclay - Ile de France
Abstract : Decelle et al.~\cite{Decelle11} conjectured the existence of a sharp threshold on model parameters for community detection in sparse random graphs drawn from the {\em stochastic block model}. Mossel, Neeman and Sly~\cite{Mossel12} established the negative part of the conjecture, proving impossibility of non-trivial reconstruction below the threshold. In this work we solve the positive part of the conjecture. To that end we introduce a modified adjacency matrix $B$ which counts {\em self-avoiding} paths of a given length $\ell$ between pairs of nodes. We then prove that for logarithmic length $\ell$, the leading eigenvectors of this modified matrix provide a non-trivial reconstruction of the underlying structure, thereby settling the conjecture. A key step in the proof consists in establishing a {\em weak Ramanujan property} of the constructed matrix $B$. Namely, the spectrum of $B$ consists in two leading eigenvalues $\rho(B)$, $\lambda_2$ and $n-2$ eigenvalues of a lower order $O(n^{\epsilon}\sqrt{\rho(B)})$ for all $\epsilon>0$, $\rho(B)$ denoting $B$'s spectral radius.
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Conference papers
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https://hal.archives-ouvertes.fr/hal-00969235
Contributor : Laurent Massoulié <>
Submitted on : Wednesday, April 2, 2014 - 11:48:09 AM
Last modification on : Thursday, February 7, 2019 - 5:34:14 PM
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• HAL Id : hal-00969235, version 1

### Citation

Laurent Massoulié. Community detection thresholds and the weak Ramanujan property. STOC 2014: 46th Annual Symposium on the Theory of Computing, Jun 2014, New York, United States. pp.1-10. ⟨hal-00969235⟩

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