The distribution of the maximum of cubic character sums

Abstract

For a primitive Dirichlet character $χ\pmod q$ we let \[M(χ):= \frac{1}{\sqrt{q}}\max_{1\leq t \leq q} \Big|\sum_{n \leq t} χ(n) \Big|.\] In this paper, we investigate the distribution of $M(χ)$, as $χ$ ranges over primitive cubic characters $χ\pmod q$ with $(q,3)=1$ and $q\leq Q$. Our first result gives an estimate for the proportion of such characters for which $M(χ)>V$, in a uniform range of $V$, which is best possible under the assumption of the Generalized Riemann Hypothesis. In particular, we show that the distribution of large cubic character sums behaves very differently from those in the family of non-principal characters modulo a large prime, and the family of quadratic characters. We also investigate the location of the number $N_χ$ where the maximum of $|\sum_{n\leq N} χ(n)|$ is attained, and show the surprising result that for almost all primitive cubic characters $χ\pmod q$ with $M(χ)>V$, $N_χ/q$ is very close to a reduced fraction with a large denominator of size $(\log V)^{1/2+o(1)}$. This contradicts the common belief that for an even character $χ$, $N_χ/q$ is located near a rational of small denominator and gives a striking difference with the case of even characters in the other two families mentioned above, for which $N_χ/q\approx 1/3$ or $2/3$ for almost all even $χ$. Furthermore, in the case of cubic characters, the works of Granville-Soundararajan, Goldmakher, and Lamzouri-Mangerel show that if $M(χ)$ is large, then $χ$ pretends to be $ξ(n)n^{it}$ for some small $t$, where $ξ$ is an odd character of small conductor $m$. We show that for almost all such characters, we have $M(χ)=m^{-1/2+o(1)}\big|L(1+it, χ\overlineξ)\big|.$