Ptolemy diagrams and torsion pairs in the cluster categories of Dynkin type D

Abstract

We give a complete classification of torsion pairs in the cluster category of Dynkin type D_n, via a bijection to new combinatorial objects called Ptolemy diagrams of type D. For the latter we give along the way different combinatorial descriptions. One of these allows us to count the number of torsion pairs in the cluster category of type D_n by providing their generating function explicitly.