We give a geometric realization of cluster categories of type Dn using a polygon with n vertices and one puncture in its center as a model. In this realization, the indecomposable objects of the cluster category correspond to certain homotopy classes of paths between two vertices. , which is only valid in type A n , the cluster category is realized by an ad-hoc method as a category of diagonals of a regular polygon with n + 3 vertices. The morphisms between diagonals are constructed geometrically using so called elementary moves and mesh relations. In that realization, clusters are in one-to-one correspondence with triangulations of the polygon and mutations are given by flips of diagonals in the triangulation. Recently, Baur and Marsh [BM] have generalized this model to m-cluster categories of type A n .
IntroductionIn this paper, we give a geometric realization of the cluster categories of type D n in the spirit of [CCS1]. The polygon with (n + 3) vertices has to be replaced by a polygon with n vertices and one puncture in the center, and instead of looking at diagonals, which are straight lines between two vertices, one has to consider homotopy classes of paths between two vertices, which we will call edges. This punctured polygon model has appeared recently in the work of Fomin, Shapiro and Thurston [FST] on the relation between cluster algebras and triangulated surfaces. Let us point out that they work in a vastly more general context and the punctured polygon is only one example of their theory. We define the cluster category by an ad-hoc method as the category 1