Points of narrowness and uniformly narrow operators

Abstract

It is known that the sum of every two narrow operators on $L_1$ is narrow, however the same is false for $L_p$ with $1 0$ there exists a decomposition $e = e' + e''$ to disjoint elements such that $\|S(e') - S(e'')\| < \varepsilon$ and $\|T(e') - T(e'')\| < \varepsilon$. The standard tool in the literature to prove the narrowness of the sum of two narrow operators $S+T$ is to show that the pair $S,T$ is uniformly narrow. We study the question of whether every pair of narrow operators with narrow sum is uniformly narrow. Having no counterexample, we prove several theorems showing that the answer is affirmative for some partial cases.

Keywords

• narrow operator
• orthogonally additive operator
• kothe banach space