Dimensions of spaces of level one automorphic forms for split classical groups using the trace formula

TL;DR

This paper develops an algorithm to compute the dimensions of automorphic form spaces for split classical groups at level one, using the trace formula and orbital integrals, enabling explicit dimension calculations and classification of unramified automorphic representations.

Contribution

It introduces an algorithm for calculating orbital integrals at torsion elements, facilitating explicit dimension formulas for automorphic forms on split classical groups.

Findings

Computed the geometric side of Arthur's trace formula for specific groups.

Derived explicit dimension formulas for vector-valued Siegel modular forms.

Enabled classification of unramified automorphic representations with given discrete series at infinity.

Abstract

We consider the problem of explicitly computing dimensions of spaces of automorphic or modular forms in level one, for a split classical group $\mathbf{G}$ over $\mathbb{Q}$ such that $\mathbf{G}(\R)$ has discrete series. Our main contribution is an algorithm calculating orbital integrals for the characteristic function of $\mathbf{G}(\mathbb{Z}_p)$ at torsion elements of $\mathbf{G}(\mathbb{Q}_p)$. We apply it to compute the geometric side in Arthur's specialisation of his invariant trace formula involving stable discrete series pseudo-coefficients for $\mathbf{G}(\mathbb{R})$. Therefore we explicitly compute the Euler-Poincar\'e characteristic of the level one discrete automorphic spectrum of $\mathbf{G}$ with respect to a finite-dimensional representation of $\mathbf{G}(\mathbb{R})$. For such a group $\mathbf{G}$, Arthur's endoscopic classification of the discrete spectrum allows to analyse precisely this Euler-Poincar\'e characteristic. For example one can deduce the number of everywhere unramified automorphic representations $\pi$ of $\mathbf{G}$ such that $\pi_{\infty}$ is isomorphic to a given discrete series representation of $\mathbf{G}(\mathbb{R})$. Dimension formulae for the spaces of vector-valued Siegel modular forms are easily derived.