On the Number of α-Orientations

We deal with the enumeration of combinatorial structures on planar maps. Prominent instances of such problems are the enumeration of spanning trees, bipartite perfect matchings and ice models. The notion of an α-orientation unifies many different combinatorial structures, including the afore mentioned. We ask for the number of α-orientations and also for special instances thereof, such as Schnyder woods and bipolar orientations. The main focus of this paper are bounds for the maximum number of such structures that a planar map with n vertices can have. We give examples of triangulations with 2.37n Schnyder woods, 3-connected planar maps with 3.209n Schnyder woods and inner triangulations with 2.91n bipolar orientations. These lower bounds are accompanied by upper bounds of 3.56n, 8n and 3.97n respectively. We also show that for any planar map M and any α the number of α-orientations is bounded from above by 3.73n and give a family of maps which have at least 2.598n α-orientations for n big enough. AMS Math Subject Classification: 05A16, 05C20, 05C30