Critical values of $L$-functions of residual representations of $\mathrm{GL}_4$
Johannes Droschl
TL;DR
This work extends Gröbner–Raghuram-type rationality results for critical $L$-values from cohomological cuspidal representations to non-cuspidal discrete series of ${ m GL}_4$, by lifting from ${ m GL}'_2$ via the Jacquet–Langlands correspondence on a quaternion algebra. It uses Shalika models and zeta-integrals to express critical values as Shalika periods up to a rational multiple, under admissibility and cohomological hypotheses; the analysis hinges on a careful treatment of automorphic rational structures and twists by Hecke characters. The main contributions include (i) a framework for rational structures and Aut$(oldsymbol C)$-compatibility in the residual setting, (ii) a Shalika–period interpretation of critical values for ${ m GL}_4$-residual representations, and (iii) concrete algebraicity statements for twisted and untwisted half-integer critical values within the field ${f Q}(oldsymbol\\Pi',oldsymbol\\chi,oldsymbol\\eta)$. These results illuminate the arithmetic nature of special values of $L$-functions attached to residual automorphic representations and provide tools for explicit algebraicity computations via cohomology and periods.
Abstract
In this paper we prove rationality results of critical values for $L$-functions attached to representations in the residual spectrum of $\mathrm{GL}_4(\mathbb{A})$. We use the Jacquet-Langlands correspondence to describe their partial $L$-functions via cuspidal automorphic representations of the group $\mathrm{GL}_2'(\mathbb{A})$ over a quaternion algebra. Using ideas inspired by results of Grobner and Raghuram we are then able to compute the critical values as a Shalika period up to a rational multiple.
