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|\mu(n)|[d(n)]^{\frac{\log q}{\log 2} -1})^{1/2}\le | F|q ] when $0<q<2$; here $\mu$ is the M\"{o}bius 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 $|D_N|_q\gg (\log N)^{q/4}$ for $0<q<\infty$. By Helson's generalization of the M. Riesz theorem on the conjugation operator, the reverse inequality $|D_N|_q \ll (\log N)^{q/4}$ 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.