Duality in spaces of finite linear combinations of atoms

In this note we describe the dual and the completion of the space of finite linear combinations of (p,∞)-atoms, 0 < p ≤ 1. As an application, we show an extension result for operators uniformly bounded on (p,∞)-atoms, 0 < p < 1, whose analogue for p = 1 is known to be false. Let 0 < p < 1 and let T be a linear operator defined on the space of finite linear combinations of (p,∞)-atoms, 0 < p < 1, which takes values in a Banach space B. If T is uniformly bounded on (p,∞)-atoms, then T extends to a bounded operator from H(R) into B.