No sublogarithmic-time approximation scheme for bipartite vertex cover

  • Authors:
  • Mika Göös;Jukka Suomela

  • Affiliations:
  • Helsinki Institute for Information Technology HIIT, Department of Computer Science, University of Helsinki, Finland;Helsinki Institute for Information Technology HIIT, Department of Computer Science, University of Helsinki, Finland

  • Venue:
  • DISC'12 Proceedings of the 26th international conference on Distributed Computing
  • Year:
  • 2012

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Abstract

König's theorem states that on bipartite graphs the size of a maximum matching equals the size of a minimum vertex cover. It is known from prior work that for every ε0 there exists a constant-time distributed algorithm that finds a (1+ε)-approximation of a maximum matching on 2-coloured graphs of bounded degree. In this work, we show--somewhat surprisingly--that no sublogarithmic-time approximation scheme exists for the dual problem: there is a constant δ0 so that no randomised distributed algorithm with running time o(logn) can find a (1+δ)-approximation of a minimum vertex cover on 2-coloured graphs of maximum degree 3. In fact, a simple application of the Linial---Saks (1993) decomposition demonstrates that this lower bound is tight. Our lower-bound construction is simple and, to some extent, independent of previous techniques. Along the way we prove that a certain cut minimisation problem, which might be of independent interest, is hard to approximate locally on expander graphs.