2012/05/21 by Mika Göös, Jukka Suomela · 2 citations
Computer Science · #cs.DC #cs.CC #cs.DS
paper · pdf · doi:10.1007/s00446-013-0194-z
11 pages, 5 figures
arxiv created 2012/05/21 · arxiv updated 2013/12/24
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 ε> 0 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 δ> 0 so that no randomised distributed algorithm with running time o(log n) 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.