Schwartz families in tempered distribution spaces

Abstract

In this paper we define Schwartz families in tempered distribution spaces and prove many their properties. Schwartz families are the analogous of infinite dimensional matrices of separable Hilbert spaces, but for the Schwartz test function spaces, having elements (functions) realizable as vectors indexed by real Euclidean spaces (ordered families of scalars indexed by real Euclidean spaces). In the paper, indeed, one of the consequences of the principal result (the characterization of summability for Schwartz families) is that the space of linear continuous operators among Schwartz test function spaces is linearly isomorphic with the space of Schwartz families. It should be noticed that this theorem is possible because of the very good properties of Schwartz test function spaces and because of the particular structures of the Schwartz families viewed as generalized matrices; in fact, any family of tempered distribution, regarded as generalized matrix, has one index belonging to a Euclidean space and one belonging to a test function space, so that any Schwartz family is a matrix in the sense of distributions. Another motivation for the introduction and study of these families is that these are the families which are summable with respect to every tempered system of coefficients, in the sense of superpositions. The Schwartz families we present in this paper are one possible rigorous and simply manageable mathematical model for the infinite matrices used frequently in Quantum Mechanics.