Extreme points of the unit ball of $\mathcal{L}(X{)}_w^{\ast }$ and best approximation in $\mathcal{L}(X{)}_w$

Abstract

We study the geometry of $\mathcal{L}(X{)}_w,$ the space of all bounded linear operators on a Banach space $X,$ endowed with the numerical radius norm, whenever the numerical radius defines a norm. We obtain the form of the extreme points of the unit ball of the dual space of $\mathcal{L}(X{)}_w.$ Using this structure, we explore Birkhoff-James orthogonality, best approximation and deduce distance formula in $\mathcal{L}(X{)}_w.$ A special attention is given to the case of operators satisfying a notion of smoothness. Finally, we obtain an equivalence between Birkhoff-James orthogonality in $\mathcal{L}(X{)}_w$ and that in $X.$