Three-block exceptional collections over Del Pezzo surfaces

Abstract

We study complete exceptional collections of coherent sheaves over Del Pezzo surfaces, which consist of three blocks such that inside each block all Ext groups between the sheaves are zero. We show that the ranks of all sheaves in such a block are the same and the three ranks corresponding to a complete 3-block exceptional collection satisfy a Markov-type Diophantine equation that is quadratic in each variable. For each Del Pezzo surface, there is a finite number of these equations; the complete list is given. The 3-string braid group acts by mutations on the set of complete 3-block exceptional collections. We describe this action. In particular, any orbit contains a 3-block collection with the sum of ranks that is minimal for the solutions of the corresponding Markov-type equation, and the orbits can be obtained from each other via tensoring by an invertible sheaf and with the action of the Weyl group. This allows us to compute the number of orbits up to twisting.Comment: LaTex v2.09, 32 pages with 1 figure. To appear in Izvestiya Mat