# Riesz-Like Bases in Rigged Hilbert Spaces

### Giorgia Bellomonte

Università degli Studi di Palermo, Italy### Camillo Trapani

Università degli Studi di Palermo, Italy

## Abstract

The notions of Bessel sequence, Riesz-Fischer sequence and Riesz basis are generalized to a rigged Hilbert space $\mathcal D[t] \subset \mathcal H \subset \mathcal D^\times[t^\times]$. A Riesz-like basis, in particular, is obtained by considering a sequence $\{\xi_n\} \subset \mathcal D$ which is mapped by a one-to-one continuous operator $T:\mathcal D[t] \to \mathcal H[\| \cdot\|]$ into an orthonormal basis of the central Hilbert space $\mathcal H$ of the triplet. The operator $T$ is, in general, an unbounded operator in $\mathcal H$. If $T$ has a bounded inverse then the rigged Hilbert space is shown to be equivalent to a triplet of Hilbert spaces.

## Cite this article

Giorgia Bellomonte, Camillo Trapani, Riesz-Like Bases in Rigged Hilbert Spaces. Z. Anal. Anwend. 35 (2016), no. 3, pp. 243–265

DOI 10.4171/ZAA/1564