Type II$_1$ von Neumann algebra representations of Hecke operators on Maass forms and the Ramanujan-Petersson conjectures

TL;DR

This paper represents classical Hecke operators on Maass forms as positive maps on II$_1$ factors, leading to spectral bounds consistent with the Ramanujan-Petersson conjectures and offering new insights into their structure.

Contribution

It introduces a novel representation of Hecke operators as completely positive maps on II$_1$ factors, connecting non-commutative algebra with classical automorphic forms.

Findings

Spectral bounds for Hecke operators match Ramanujan-Petersson predictions.

Finite exceptional eigenvalues outside the predicted interval are limited.

Representation via II$_1$ factors provides new structural insights into Hecke operators.

Abstract

Classical Hecke operators on Maass forms are unitarely equivalent, up to a commuting phase, to completely positive maps on II$_1$ factors, associated to a pair of isomorphic subfactors, and an intertwining unitary. This representation is obtained through a quantized representation of the Hecke operators. in this representation, the Hecke operators act on the Berezin's quantization, deformation algebra of the fundamental domain of $\PSL(2,\Z)$ in the upper halfplane. The Hecke operators are inheriting from the ambient, non-commutative algebra on which they act, a rich structure of matrix inequalities. Using this construction we obtain that, for every prime $p$, the essential spectrum of the classical Hecke operator $T_p$ is contained in the interval $[-2\sqrt p, 2\sqrt p]$, predicted by the Ramanujan Petersson conjectures. In particular, given an open interval containing $[-2\sqrt p, 2\sqrt p]$, there are at most a finite number of possible exceptional eigenvalues lying outside this interval. The main tool for obtaining this representation of the Hecke operators (unitarely equivalent to the classical representation, up to commuting phase) is a Schurr type, positive "square root" of the state on $\PGL(2,\Q)$, measuring the displacement of fundamental domain by translations in $\PGL(2,\Q)$. The "square root" is obtained from the matrix coefficients of the discrete series representations of $\PSL(2,\R)$ restricted to $\PGL(2, \Q)$. The methods in this paper may also be applied to any finite index, modular subgroup $\Gamma_0(p^n)$, $n\geq 1$, of $\PSL(2,\Z)$. In this case the essential norm of the Hecke operator is equal to the norm of the corresponding convolution operator on the cosets Hilbert space $\ell^2((\Gamma_0(p^n))\backslash \PSL(2,\Z[1/p])$.