On Enflo and narrow operators acting on $L_p$

TL;DR

This paper investigates conditions under which operators on $L_p$ spaces are narrow, introducing the concept of 'gentle' growth to identify such operators for $1<p<2$, and classifies narrow operators from $L_p$ to sequence spaces for $p>2.

Contribution

It introduces the notion of 'gentle' growth to characterize narrow operators on $L_p$ for $1<p<2$ and classifies narrow operators from $L_p$ to $ ext{ell}_r$ for $p,r>2$, expanding understanding of operator narrowness.

Findings

Operators with 'gentle' growth on $L_p$ are narrow for $1<p<2$.

All operators from $L_p$ to $ ext{ell}_r$ are narrow for $2<p,r<\infty$.

The results complete the classification of narrow operators from $L_p$ to sequence and function spaces.

Abstract

The first part of the paper is inspired by a theorem of H. Rosenthal, that if an operator on $L_1[0,1]$ satisfies the assumption that for each measurable set $A \subseteq [0,1]$ the restriction $T \bigl|_{L_1(A)}$ is not an isomorphic embedding, then the operator is narrow. (Here $L_1(A) = \bigl\{x \in L_1: \,\, {\rm supp} \, x \subseteq A \bigr\}$.) This leads to a natural question of finding mildest possible assumptions for operators on a given space $X$, which will imply that the operator is narrow. We find a partial answer to this question for operators on $L_p(0,1)$ with $1<p<2$. Namely we define a notion of a "gentle" growth of a function and we prove that for $1 < p < 2$ every operator $T$ on $L_p$ which is unbounded from below on $L_p(A)$, $A \subseteq [0,1]$, by means of function having a "gentle" growth, is narrow. In the second part of the paper we consider the question for what Banach spaces $X$, every operator $T:L_p \lra X$ is narrow. We prove that for $2 < p, r < \infty$ every operator $T: L_p\rightarrow\ell_r$ is narrow, which completes the list of results for operators from $L_p$ to sequence and function Lebesgue spaces.