Stable Trace Formulas and Discrete Series Multiplicities

Let G be a reductive algebraic group over Q, and suppose that Γ ⊂ G(R) is an arithmetic subgroup defined by congruence conditions. A basic problem in arithmetic is to determine the multiplicities of discrete series representations in L2(Γ\G(R)), and in general to determine the traces of Hecke operators on these spaces. In this paper we give a conjectural formula for the traces of Hecke operators, in terms of stable distributions. It is based on a stable version of Arthur’s formula for L2-Lefschetz numbers [3], which is due to Kottwitz [21]. We reduce this formula to the computation of elliptic p-adic orbital integrals and the theory of endoscopic transfer. As evidence for this conjecture, we demonstrate the agreement of the central terms of this formula with the unipotent contributions to the multiplicity coming from Selberg’s trace formula in Wakatsuki [43], in the case G = GSp4 and Γ = GSp4(Z).