Optimal Hardy-Littlewood type inequalities for $m$-linear forms on $\ell_{p}$ spaces with $1\leq p\leq m$

Abstract

The Hardy-Littlewood inequalities for $m$-linear forms on $\ell_{p}$ spaces are stated for $p>m$. In this paper, among other results, we investigate similar results for $1\leq p\leq m.$ Let $\mathbb{K}$ be $% \mathbb{R}$ or $\mathbb{C}$ and $m\geq 2$ be a positive integer. Our main results are the following sharp inequalities: (i) If $\left(r,p\right) \in \left(\lbrack 1,2]\times \lbrack 2,2m)\right) \cup \left(\lbrack 1,\infty)\times \lbrack 2m,\infty \right)) $, then there is a constant $D_{m,r,p}^{\mathbb{K}}>0$ (not depending on $% n $) such that \begin{equation*} \textstyle\left(\sum\limits_{j_{1},...,j_{m}=1}^{n}\left\vert T(e_{j_{1}},...,e_{j_{m}})\right\vert ^{r}\right) ^{\frac{1}{r}}\leq D_{m,r,p}^{\mathbb{K}}n^{\max \left\{ \frac{2mr+2mp-mpr-pr}{2pr},0\right\} }\left\Vert T\right\Vert \end{equation*} for all $m$--linear forms $T:\ell_{p}^{n}\times \cdots \times \ell_{p}^{n}\rightarrow \mathbb{K}$ and all positive integers $n$. (ii) If $\left(r,p\right) \in \lbrack 2,\infty)\times (m,2m]$, then \begin{equation*} \textstyle\left(\sum\limits_{j_{1},...,j_{m}=1}^{n}\left\vert T(e_{j_{1}},...,e_{j_{m}})\right\vert ^{r}\right) ^{\frac{1}{r}}\leq \left(\sqrt{2}\right) ^{m-1}n^{\max \left\{ \frac{p+mr-rp}{pr},0\right\} }\left\Vert T\right\Vert \end{equation*} for all $m$--linear forms $T:\ell_{p}^{n}\times \cdots \times \ell_{p}^{n}\rightarrow \mathbb{K}$ and all positive integers $n.$ Moreover the exponents $\max \{ (2mr+2mp-mpr-pr)/2pr,0 \} $ in (i) and $\max \{(p+mr-rp)/pr,0 \} $ in (ii) are optimal.