Pólya-Vinogradov bound is not tight for characters of odd, bounded order

You need to know: Set {\mathbb Z} of integers, greatest common divisor (gcd) of 2 integers, set {\mathbb C} of complex numbers, o(1) notation.

Background: Function \chi_q:{\mathbb Z}\to{\mathbb C} is called a Dirichlet character modulo integer q>0 if (i) \chi_q(n)=\chi_q(n+q) for all n, (ii) if \text{gcd}(n,q) > 1 then \chi_q(n)=0; if \text{gcd}(n,q) = 1 then \chi_q(n) \neq 0, and (iii) \chi_q(mn)=\chi_q(m)\chi_q(n) for all integers m and n. An example is character \chi_q^* such that \chi_q^*(n)=1 whenever \text{gcd}(n,q) = 1 and \chi_q^*(n)=0 otherwise. This character is called principal and all other characters – nonprincipal. The order of a Dirichlet character \chi_q is the least positive integer m such that \chi_q(n)^m=\chi_q^*(n) for all n. The character \chi_q is called primitive if there is no integer 0<d<q such that \chi_q(a)=\chi_q(b) whenever \text{gcd}(a,q) = \text{gcd}(b,q) = 1 and a-b is a multiple of d. For any nonprincipal character \chi_q let M(\chi_q) = \max\limits_{1\leq m\leq q}\left|\sum\limits_{n=1}^m \chi_q(n)\right|.

The Theorem: On 2nd March 2005, Andrew Granville and Kannan Soundararajan submitted to the Jornal of the AMS a paper in which they proved that that if \chi_q is a primitive character modulo q of odd order g, then M(\chi_q) \leq C_g \sqrt{q} (\log q)^{1-\frac{\delta_g}{2} + o(1)} where \delta_g = 1 - \frac{g}{\pi}\sin\frac{\pi}{g}, and C_g is a constant depending only on g.

Short context: In 1918, Pólya and Vinogradov independently proved that M(\chi_q)=O(\sqrt{q}\log q) for any nonprincipal Dirichlet character \chi_q. This is known as the Pólya–Vinogradov inequality, has numerous applications, but it is an open question whether a better bound for M(\chi_q) in terms of q is possible. The Theorem provides an improved bound for characters of odd, bounded order.

Links: Free arxiv version of the original paper is here, journal version is here.

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