Generating operators between Banach spaces

Abstract

AbstractWe introduce and study the notion of generating operators as those norm-one operators $$G:X\longrightarrow Y$$ G : X ⟶ Y such that for every $$0<\delta <1$$ 0 < δ < 1 , the set $$\{x\in X:\Vert x\Vert \leqslant 1,\ \Vert Gx\Vert >1-\delta \}$$ { x ∈ X : ‖ x ‖ ⩽ 1 , ‖ G x ‖ > 1 - δ } generates the unit ball of X by closed convex hull. This class of operators includes isometric embeddings, spear operators (actually, operators with the alternative Daugavet property), and other examples like the natural inclusions of $$\ell _1$$ ℓ 1 into $$c_0$$ c 0 and of $$L_\infty [0,1]$$ L ∞ [ 0 , 1 ] into $$L_1[0,1]$$ L 1 [ 0 , 1 ] . We first present a characterization in terms of the adjoint operator, make a discussion on the behaviour of diagonal generating operators on $$c_0$$ c 0 -, $$\ell _1$$ ℓ 1 -, and $$\ell _\infty $$ ℓ ∞ -sums, and present examples in some classical Banach spaces. Even though rank-one generating operators always attain their norm, there are generating operators, even of rank-two, which do not attain their norm. We discuss when a Banach space can be the domain of a generating operator which does not attain its norm in terms of the behaviour of some spear sets of the dual space. Finally, we study when the set of all generating operators between two Banach spaces X and Y generates all non-expansive operators by closed convex hull. We show that this is the case when $$X=L_1(\mu )$$ X = L 1 ( μ ) and Y has the Radon-Nikodým property with respect to $$\mu $$ μ . Therefore, when $$X=\ell _1(\Gamma )$$ X = ℓ 1 ( Γ ) , this is the case for every target space Y. Conversely, we also show that a real finite-dimensional space X satisfies that generating operators from X to Y generate all non-expansive operators by closed convex hull only in the case that X is an $$\ell _1$$ ℓ 1 -space.