An inequality of Hardy--Littlewood type for Dirichlet polynomials

Abstract

The $L^q$ norm of a Dirichlet polynomial $F(s)={\sum }_{n=1}^N{a}_n{n}^{-s}$ is defined as \[\| F\|_q:=(\lim_{T\to\infty}\frac{1}{T}\int_{0}^T |F(it)|^qdt)^{1/q}\] for $0<q<\infty$. It is shown that \[ (\sum_{n=1}^{N} |a_n|^2|μ(n)|[d(n)]^{\frac{\log q}{\log 2} -1})^{1/2}\le \| F\|_q \] when $0<q<2$; here $\mu$ is the Möbius function and $d$ the divisor function. This result is used to prove that the $L^q$ norm of $D_N(s):={\sum }_{n=1}^N{n}^{-1/2-s}$ satisfies $\Vert D_N{\Vert }_q\gg (\log N{)}^{q/4}$ for $0<q<\infty$. By Helson's generalization of the M. Riesz theorem on the conjugation operator, the reverse inequality $\Vert D_N{\Vert }_q\ll (\log N{)}^{q/4}$ is shown to be valid in the range $1<q<\infty$. Similar bounds are found for a fairly large class of Dirichlet series including, on one of Selberg's conjectures, the Selberg class of $L$-functions.

This is the final version of this paper, to appear in Journal of Number Theory

References in corpus (1)

• Moments of the Dedekind zeta function and other non-primitive L-functions