A Constant Factor Approximation Algorithm for Boxicity of Circular Arc Graphs

Boxicity of a graph G(V,E) is the minimum integer k such that G can be represented as the intersection graph of k-dimensional axis parallel rectangles in R. Equivalently, it is the minimum number of interval graphs on the vertex set V such that the intersection of their edge sets is E. It is known that boxicity cannot be approximated even for graph classes like bipartite, co-bipartite and split graphs below O(n)-factor, for any ǫ > 0 in polynomial time unless NP = ZPP . Till date, there is no well known graph class of unbounded boxicity for which even an n-factor approximation algorithm for computing boxicity is known, for any ǫ < 1. In this paper, we study the boxicity problem on Circular Arc graphs intersection graphs of arcs of a circle. We give a (2 + 1 k )-factor polynomial time approximation algorithm for computing the boxicity of any circular arc graph along with a corresponding box representation, where k ≥ 1 is its boxicity. For Normal Circular Arc(NCA) graphs, with an NCA model given, this can be improved to an additive 2-factor approximation algorithm. The time complexity of the algorithms to approximately compute the boxicity is O(mn+ n) in both these cases and in O(mn + kn) = O(n) time we also get their corresponding box representations, where n is the number of vertices of the graph and m is its number of edges. The additive 2-factor algorithm directly works for any Proper Circular Arc graph, since computing an NCA model for it can be done in polynomial time.