Hall algebras and graphs of Hecke operators for elliptic curves

Abstract

The graph of a Hecke operator encodes all information about the action of this operator on automorphic forms over a global function field. These graphs were introduced by Lorscheid in [15] for PGL 2 and generalized to GL n in [1]. After reviewing some general properties, we explain the connection to the Hall algebra of the function field. In the case of an elliptic function field, we can use structure results of Burban-Schiffmann and Fratila to develop an algorithm which explicitly calculate these graphs. We apply this algorithm to determine some structure constants and provide explicitly the rank two case in the last section.Contents 18 5. Calculating structure constants 23 6. The case of rank 2 29 References 37