On cotype and a Grothendieck-type theorem for absolutely summing multilinear operators

TL;DR

This paper investigates the optimal constants for absolutely summing multilinear operators from l_1 to l_2, providing improved bounds and inclusion theorems that extend classical results like Grothendieck's theorem.

Contribution

It establishes the best constant g_n for n-linear operators from l_1 to l_2 and improves previous inclusion theorems for absolutely summing multilinear operators.

Findings

Proves g_n 0; 2/(n+1) for n-linear operators.

Provides an optimal improvement of existing inclusion theorems.

Extends classical linear operator results to multilinear settings.

Abstract

A famous result due to Grothendieck asserts that every continuous linear operator from $\ell_{1}$ to $\ell_{2}$ is absolutely $(1,1)$-summing. If $n\geq2,$ however, it is very simple to prove that every continuous $n$-linear operator from $\ell_{1}\times...\times\ell_{1}$ to $\ell_{2}$ is absolutely $(1;1,...,1) $-summing, and even absolutely $(\frac{2}% {n};1,...,1) $-summing$.$ In this note we deal with the following problem: Given a positive integer $n\geq2$, what is the best constant $g_{n}>0$ so that every $n$-linear operator from $\ell_{1}\times...\times\ell_{1}$ to $\ell_{2}$ is absolutely $(g_{n};1,...,1) $-summing? We prove that $g_{n}\leq\frac{2}{n+1}$ and also obtain an optimal improvement of previous recent results (due to Heinz Juenk $\mathit{et}$ $\mathit{al}$, Geraldo Botelho $\mathit{et}$ $\mathit{al}$ and Dumitru Popa) on inclusion theorems for absolutely summing multilinear operators.