Bishop-Phelps-Bollobás Moduli of a Banach space

Abstract

We introduce two Bishop–Phelps–Bollobás moduli of a Banach space which measure, for a given Banach space, what is the best possible Bishop–Phelps–Bollobás theorem in this space. We show that there is a common upper bound for these moduli for all Banach spaces and we present an example showing that this bound is sharp. We prove the continuity of these moduli and an inequality with respect to duality. We calculate the two moduli for Hilbert spaces and also present many examples for which the moduli have the maximum possible value (among them, there are C(K) spaces and L1(μ) spaces). Finally, we show that if a Banach space has the maximum possible value of any of the moduli, then it contains almost isometric copies of the real space l^2_∞ and present an example showing that this condition is not sufficient.