Optimal constants for a mixed Littlewood type inequality

Abstract

For $p\in\lbrack2,\infty]$ a mixed Littlewood-type inequality asserts that there is a constant $C_{(m),p}\geq1$ such that \[ \left( \sum_{i_{1}=1}^{\infty}\left( \sum_{i_{2},...,i_{m}=1}^{\infty }|T(e_{i_{1}},...,e_{i_{m}})|^{2}\right) ^{\frac{1}{2}\frac{p}{p-1}}\right) ^{\frac{p-1}{p}}\leq C_{(m),p}\Vert T\Vert \] for all continuous real-valued $m$-linear forms on $\ell_{p}\times c_{0} \times\dots\times c_{0}$ (when $p=\infty$, $\ell_{p}$ is replaced by $c_{0})$. We prove that for $p>2.18006$ the optimal constants $C_{(m),p}$ are $\left( 2^{\frac{1}{2}-\frac{1}{p}}\right) ^{m-1}.$ When $p=\infty,$ we recover the best constants of the mixed $\left( \ell_{1},\ell_{2}\right) $-Littlewood inequality.