Journal of the European Mathematical Society

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Volume 20, Issue 7, 2018, pp. 1759–1818
DOI: 10.4171/JEMS/799

Published online: 2018-05-22

The frequency and the structure of large character sums

Jonathan Bober[1], Leo Goldmakher[2], Andrew Granville[3] and Dimitris Koukoulopoulos[4]

(1) University of Bristol, UK
(2) Williams College, Williamstown, USA
(3) Université de Montréal, Canada
(4) Université de Montréal, Canada

Let $M(\chi)$ denote the maximum of $|\sum_{n\le N}\chi(n)|$ for a given non-principal Dirichlet character $\chi$ modulo $q$, and let $N_\chi$ denote a point at which the maximum is attained. In this article we study the distribution of $M(\chi)/\sqrt{q}$ as one varies over characters modulo $q$, where $q$ is prime, and investigate the location of $N_\chi$. We show that the distribution of $M(\chi)/\sqrt{q}$ converges weakly to a universal distribution $\Phi$, uniformly throughout most of the possible range, and get (doubly exponential decay) estimates for $\Phi$'s tail. Almost all $\chi$ for which $M(\chi)$ is large are odd characters that are 1-pretentious. Now, $M(\chi)\ge |\sum_{n\le q/2}\chi(n)| = \frac{|2-\chi(2)|}\pi \sqrt{q} |L(1,\chi)|$, and one knows how often the latter expression is large, which has been how earlier lower bounds on $\Phi$ were mostly proved. We show, though, that for most $\chi$ with $M(\chi)$ large, $N_\chi$ is bounded away from $q/2$, and the value of $M(\chi)$ is little bit larger than $\frac{\sqrt{q}}{\pi} |L(1,\chi)|$.

Keywords: Distribution of character sums, distribution of Dirichlet $L$-functions, pretentious multiplicative functions, random multiplicative functions

Bober Jonathan, Goldmakher Leo, Granville Andrew, Koukoulopoulos Dimitris: The frequency and the structure of large character sums. J. Eur. Math. Soc. 20 (2018), 1759-1818. doi: 10.4171/JEMS/799