Twice-ramanujan sparsifiers

  • Authors:
  • Joshua D. Batson;Daniel A. Spielman;Nikhil Srivastava

  • Affiliations:
  • Cambridge University, Cambridge, England UK;Yale University, New Haven, CT, USA;Yale University, New Haven, CT, USA

  • Venue:
  • Proceedings of the forty-first annual ACM symposium on Theory of computing
  • Year:
  • 2009

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Abstract

We prove that every graph has a spectral sparsifier with a number of edges linear in its number of vertices. As linear-sized spectral sparsifiers of complete graphs are expanders, our sparsifiers of arbitrary graphs can be viewed as generalizations of expander graphs. In particular, we prove that for every d 1 and every undirected, weighted graph G = (V,E,w) on n vertices, there exists a weighted graph H=(V,F,~{w}) with at most ⌈d(n-1)⌉ edges such that for every x ∈ RV, [xT LG x ≤ xT LH x ≤ ((d+1+2√d)/(d+1-2√d)) • xT LG x] where LG and LH are the Laplacian matrices of G and H, respectively. Thus, H approximates G spectrally at least as well as a Ramanujan expander with dn/2 edges approximates the complete graph. We give an elementary deterministic polynomial time algorithm for constructing H.