Gram matrix associated to controlled frames

Abstract

Controlled frames have been recently introduced in Hilbert spaces to improve the numerical efficiency of interactive algorithms for inverting the frame operator. In this paper, unlike the cross-Gram matrix of two different sequences which is not always a diagnostic tool, we define the controlled-Gram matrix of a sequence as a practical implement to diagnose that a given sequence is a controlled Bessel, frame or Riesz basis. Also, we discuss the cases that the operator associated to controlled Gram matrix will be bounded, invertible, Hilbert-Schmidt or a trace-class operator. Similar to standard frames, we present an explicit structure for controlled Riesz bases and show that every $(U,C)$-controlled Riesz basis $\{f_k{\}}_{k=1}^{\infty }$ is in the form $\{U^{-1}CM{e}_k{\}}_{k=1}^{\infty }$, where $M$ is a bijective operator on $H$. Furthermore, we propose an equivalent accessible condition to the sequence $\{f_k{\}}_{k=1}^{\infty }$ being a $(U,C)$-controlled Riesz basis.