Orbits of bounded bijective operators and Gabor frames

This paper is a contribution to frame theory. Frames in a Hilbert space are generalizations of orthonormal bases. In particular, Gabor frames of $L^2(mathbb{R})$, which are made of translations and modulations of one or more windows, are often used in applications. More precisely, the paper deals with a question posed in the last years by Christensen and Hasannasab about the existence of overcomplete Gabor frames, with some ordering over $mathbb{Z}$, which are orbits of bounded operators on $L^2(mathbb{R})$. Two classes of overcomplete Gabor frames which cannot be ordered over $mathbb{Z}$ and represented by orbits of operators in $GL(L^2(mathbb{R}))$ are given. Some results about operator representation are stated in a general context for arbitrary frames, covering also certain wavelet frames.