Large independent sets in triangle-free planar graphs

Abstract

Every triangle-free planar graph on n vertices has an independent set of size at least (n+1)/3, and this lower bound is tight. We give an algorithm that, given a triangle-free planar graph G on n vertices and an integer k≥0, decides whether G has an independent set of size at least (n+k)/3, in time 20(√k). Thus, the problem is fixed-parameter tractable when parameterized by k. Furthermore, as a corollary of the result used to prove the correctness of the algorithm, we show that there exists ε>0 such that every planar graph of girth at least five on n vertices has an independent set of size at least n/(3-ε). © 2014 Springer-Verlag Berlin Heidelberg.