“…This condition is satisfied, for instance, when T is a metrizable separable space, not necessarily with a Schauder basis (such spaces exist, see for instance [22]), and µ is a Radon measure. In the literature, some deeply investigated particular cases are when X = R n and µ is the Lebesgue measure, and when X is a Banach space with a Schauder basis (see also [22,23,26]). Since the integral of f with respect to µ is an element of X, in general it is not natural to define an inner product, when it is dealt with norm convergence of the involved integrals.…”