Octahedral norms and convex combination of slices in Banach spaces

Abstract

We study the relation between octahedral norms, Daugavet property and the size of convex combinations of slices in Banach spaces. We prove that the norm of an arbitrary Banach space is octahedral if, and only if, every convex combination of w⁎-slices in the dual unit ball has diameter 2, which answers an open question. As a consequence we get that the Banach spaces with the Daugavet property and its dual spaces have octahedral norms. Also, we show that for every separable Banach space containing ℓ1 and for every ε>0 there is an equivalent norm so that every convex combination of w⁎-slices in the dual unit ball has diameter at least 2−ε.