A pseudo-Daugavet property for narrow projections in Lorentz spaces

Abstract

Let $X$ be a rearrangement-invariant space. An operator $T: X\to X$ is called narrow if for each measurable set $A$ and each $\varepsilon > 0$ there exists $x \in X$ with $x^2= \chi_A,\ \int x d \mu = 0$ and $\| Tx \| < \varepsilon$. In particular, all compact operators are narrow. We prove that if $X$ is a Lorentz function space $L_{w,p}$ on [0,1] with $p > 2$, then there exists a constant $k_X > 1$ such that for every narrow projection $P$ on $L_{w,p}$ $\| \operatorname{Id} - P \| \geq k_X. $ This generalizes earlier results on $L_p$ and partially answers a question of E. M. Semenov. Moreover, we prove that every rearrangement-invariant function space $X$ with an absolutely continuous norm contains a complemented subspace isomorphic to $X$ which is the range of a narrow projection and a non-narrow projection. This gives a negative answer to a question of A. Plichko and M. Popov.