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The distribution of large quadratic character sums and applications

Youness Lamzouri

Vol. 18 (2024), No. 11, 2091–2131
Abstract

We investigate the distribution of the maximum of character sums over the family of primitive quadratic characters attached to fundamental discriminants |d| x. In particular, our work improves results of Montgomery and Vaughan, and gives strong evidence that the Omega result of Bateman and Chowla for quadratic character sums is optimal. We also obtain similar results for real characters with prime discriminants up to x, and deduce the interesting consequence that almost all primes with large Legendre symbol sums are congruent to 3 modulo 4. Our results are motivated by a recent work of Bober, Goldmakher, Granville and Koukoulopoulos, who proved similar results for the family of nonprincipal characters modulo a large prime. However, their method does not seem to generalize to other families of Dirichlet characters. Instead, we use a different and more streamlined approach, which relies mainly on the quadratic large sieve. As an application, we consider a question of Montgomery concerning the positivity of sums of Legendre symbols.

Keywords
character sums, Dirichlet L-functions, Pólya–Vinogradov inequality, quadratic large sieve, Kronecker symbol, Legendre symbol, positivity of partial sums
Mathematical Subject Classification
Primary: 11L40, 11N64
Secondary: 11K65
Milestones
Received: 2 March 2023
Revised: 12 September 2023
Accepted: 27 November 2023
Published: 18 October 2024
Authors
Youness Lamzouri
Institut Élie Cartan de Lorraine
Université de Lorraine, CNRS
Nancy
France
IRL3457 CRM-CNRS
Centre de Recherches Mathématiques
Université de Montréal
Montréal, QC
Canada

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