Partitioning into expanders

Let G = (V, E) be an undirected graph, λk be the kth smallest eigenvalue of the normalized laplacian matrix of G. There is a basic fact in algebraic graph theory that λk > 0 if and only if G has at most k -- 1 connected components. We prove a robust version of this fact. If λk > 0, then for some 1 ≤ ℓ ≤ k -- 1, V can be partitioned into ℓ sets P1, ..., Pℓ such that each Pi is a low-conductance set in G and induces a high conductance induced subgraph. In particular, [EQUATION] and [EQUATION]-expander. We make our results algorithmic by designing a simple polynomial time spectral algorithm to find such partitioning of G with a quadratic loss in the inside conductance of Pi's. Unlike the recent results on higher order Cheeger's inequality [6, 9], our results does not use higher order eigenfunctions of G. If there is a sufficiently large gap between λk and λk+1, more precisely if [EQUATION] then our algorithm finds a k partitioning of V into sets P1, ..., Pk such that the induced subgraph G[Pi] has a singnificantly larger conductance than the conductance of Pi in G. Such a partitioning may represent the best k clusterings of G. Our algorithm is a simple local search that only uses the Spectral Partitioning algorithm as a subroutine. We expect to see further applications of this simple algorithm in clustering applications. Let ρ(k) = [EQUATION] be the order k conductance constant of G, in words, ρ(k) is the smallest value of the maximum conductance of any k disjoint subsets of V. Our main technical lemma shows that if (1+ε)ρ(k) < ρ(k+1), then V can be partitioned into k sets P1, ..., Pk such that for each 1 ≤ i ≤ k, φ(G[Pi]) ≳ ε·ρ(k+1)/k and φ((Pi) ≤ k · ρ(k). This significantly improves a recent result of Tanaka [13] who assumed an exponential (in k) gap between ρ(k) and ρ(k + 1).