Colouring graphs when the number of colours is nearly the maximum degree

  • Authors:
  • Michael Molloy;Bruce Reed

  • Affiliations:
  • -;-

  • Venue:
  • STOC '01 Proceedings of the thirty-third annual ACM symposium on Theory of computing
  • Year:
  • 2001

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Abstract

We consider for graphs of maximum degree &Dgr;, the problem of determining whether &khgr;G) &Dgr;-k for various values of k. We obtain sharp theorems characterizing when the barrier to &Dgr;-k colourability must be a local condition, i.e. a small subgraph, and when it can be global. We also show that for large fixed &Dgr;, this problem is either NP-complete or can be solved in linear time, and we determine precisely which values of k correspond to each case prove that Hitting Set with sets of size B is hard to approximate to within a factor $B^{1/19}$. The problem can be approximated to within a factor B [19], and it is the Vertex Cover problem for B=2. The relationship between hardness of approximation and set size seems to have not been explored before.