## Exact Clustering of Weighted Graphs via Semidefinite Programming

Aleksis Pirinen, Brendan Ames; 20(30):1−34, 2019.

### Abstract

As a model problem for clustering, we consider the densest $k$-disjoint-clique problem of partitioning a weighted complete graph into $k$ disjoint subgraphs such that the sum of the densities of these subgraphs is maximized. We establish that such subgraphs can be recovered from the solution of a particular semidefinite relaxation with high probability if the input graph is sampled from a distribution of clusterable graphs. Specifically, the semidefinite relaxation is exact if the graph consists of $$k$$ large disjoint subgraphs, corresponding to clusters, with weight concentrated within these subgraphs, plus a moderate number of nodes not belonging to any cluster. Further, we establish that if noise is weakly obscuring these clusters, i.e, the between-cluster edges are assigned very small weights, then we can recover significantly smaller clusters. For example, we show that in approximately sparse graphs, where the between-cluster weights tend to zero as the size $n$ of the graph tends to infinity, we can recover clusters of size polylogarithmic in $n$ under certain conditions on the distribution of edge weights. Empirical evidence from numerical simulations is also provided to support these theoretical phase transitions to perfect recovery of the cluster structure.

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