ar X iv : m at h / 03 09 13 8 v 2 [ m at h . Q A ] 1 6 A pr 2 00 4 CLUSTER ALGEBRAS AND WEIL - PETERSSON FORMS

In the previous paper [GSV] we have discussed Poisson properties of cluster algebras of geometric type for the case of a nondegenerate matrix of transition exponents. In this paper we consider the case of a general matrix of transition exponents. Our leading idea is that a relevant geometric object in this case is a certain closed 2-form compatible with the cluster algebra structure. The main example is provided by Penner coordinates on the decorated Teichmüller space, in which case the above form coincides with the classical Weil-Petersson symplectic form.