Summability and duality

Abstract

We formalize the observation that the same summability methods converge in a Banach space $X$ and its dual $X^{\ast }$. At the same time we determine conditions under which these methods converge in the weak and weak*-topologies on $X$ and $X^{\ast }$ respectively. We also derive a general limitation theorem, which yields a necessary condition for the convergence of a summability method in $X$. These results are then illustrated by applications to a wide variety of function spaces, including spaces of continuous functions, Lebesgue spaces, the disk algebra, Hardy and Bergman spaces, the BMOA space, the Bloch space, and de Branges-Rovnyak spaces. Our approach shows that all these applications flow from just two abstract theorems.