Circular Choosability

In this paper, we study the notion of circular choosability recently introduced by Mohar and Zhu. First, we provide a negative answer to a question of Zhu about circular cliques. We next prove that, for every graph G, cch(G) = O( ch(G) + ln |V(G)| ). We investigate a generalisation of circular choosability, circular f-choosability, when f is a function of the degrees. We also consider the circular choice number of planar graphs. Mohar asked for the value of tau := sup{ cch(G) : G is planar }, and we prove that 6 <= tau <= 8, thereby providing a negative answer to another question of Mohar. Finally, we study the circular choice number of planar and outerplanar graphs with prescribed girth, and graphs with bounded density.