Graph Theory Seminar
Thursday, October 1, 2015 - 1:30pm
1 hour (actually 50 minutes)
There has been extensive research on cycle lengths in graphs with large minimum degree. In this talk, we will present several new and tight results in this area. Let G be a graph with minimum degree at least k+1. We prove that if G is bipartite, then there are k cycles in G whose lengths form an arithmetic progression with common difference two. For general graph G, we show that G contains \lfloor k/2\rfloor cycles with consecutive even lengths, and in addition, if G is 2-connected and non-bipartite, then G contains \lfloor k/2\rfloor cycles with consecutive odd lengths. Thomassen (1983) made two conjectures on cycle lengths modulo a fixed integer k: (1) every graph with minimum degree at least k+1 contains cycles of all even lengths modulo k; (2) every 2-connected non-bipartite graph with minimum degree at least $k+1$ contains cycles of all lengths modulo k. These two conjectures, if true, are best possible. Our results confirm both conjectures! when k is even. And when k is odd, we show that minimum degree at least $+4 suffices. Moreover, our results derive new upper bounds of the chromatic number in terms of the longest sequence of cycles with consecutive (even or odd) lengths. This is a joint work with Chun-Hung Liu.