Exact and Approximate Bandwidth

  • Authors:
  • Marek Cygan;Marcin Pilipczuk

  • Affiliations:
  • Dept. of Mathematics, Computer Science and Mechanics, University of Warsaw, Poland;Dept. of Mathematics, Computer Science and Mechanics, University of Warsaw, Poland

  • Venue:
  • ICALP '09 Proceedings of the 36th International Colloquium on Automata, Languages and Programming: Part I
  • Year:
  • 2009

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Abstract

In this paper we gather several improvements in the field of exact and approximate exponential-time algorithms for the Bandwidth problem. For graphs with treewidth t we present a O (n O (t ) 2 n ) exact algorithm. Moreover for the same class of graphs we introduce a subexponential constant-approximation scheme --- for any *** 0 there exists a (1 + *** )-approximation algorithm running in $O(\exp(c(t + \sqrt{n/\alpha})\log n))$ time where c is a universal constant. These results seem interesting since Unger has proved that Bandwidth does not belong to APX even when the input graph is a tree (assuming P *** NP). So somewhat surprisingly, despite Unger's result it turns out that not only a subexponential constant approximation is possible but also a subexponential approximation scheme exists. Furthermore, for any positive integer r , we present a (4r *** 1)-approximation algorithm that solves Bandwidth for an arbitrary input graph in $O^*(2^{n\over r})$ time and polynomial space. Finally we improve the currently best known exact algorithm for arbitrary graphs with a O (4.473 n ) time and space algorithm. In the algorithms for the small treewidth we develop a technique based on the Fast Fourier Transform, parallel to the Fast Subset Convolution techniques introduced by Björklund et al. This technique can be also used as a simple method of finding a chromatic number of all subgraphs of a given graph in O *(2 n ) time and space, what matches the best known results.