Cluster structures for 2-Calabi-Yau categories and unipotent groups

We investigate cluster-tilting objects (and subcategories) in triangulated 2-Calabi-Yau and related categories. In particular., we construct a new class of such categories related to preprojective algebras of non-Dynkin quivers associated with elements ill the Coxeter group. This class of 2-Calabi-Yau categories contains, as special cases., the cluster categories and the stable categories of preprojective algebras of Dynkin graphs. For these 2-Calabi-Yau categories, we construct cluster-tilting objects associated with each reduced expression. The associated quiver is described in terms of the, reduced expression. Motivated by the theory of cluster algebras, we Formulate the notions of (weak) cluster structure and substructure, and give several illustrations of these concepts. We discuss connections with cluster algebras and subcluster algebras is related to unipotent groups, in both the Dynkin and non-Dynkin cases.