Geometric Aspects of the Daugavet Property

Let X be a closed subspace of a Banach space Y and J be the inclusion map. We say that the pair (X, Y) has the Daugavet property if for every rank one bounded linear operator T from X to Y the equality ‖J+T‖=1+‖T‖ (1)holds. A new characterization of the Daugavet property in terms of weak open sets is given. It is shown that the operators not fixing copies of l1 on a Daugavet pair satisfy (1). Some hereditary properties are found: if X is a Daugavet space and Y is its subspace, then Y is also a Daugavet space provided X/Y has the Radon–Nikodým property; if Y is reflexive, then X/Y is a Daugavet space. Besides, we prove that if (X, Y) has the Daugavet property and Y⊂Z, then Z can be renormed so that (X, Z) possesses the Daugavet property and the equivalent norm coincides with the original one on Y.