Frames and Stable Basesfor Shift - Invariant Subspaces of L 2 ( IRd )

Let X be a countable fundamental set in a Hilbert space H, and let T be the operator X x2X c(x)x: Whenever T is well-deened and bounded, X is said to be a Bessel sequence. If, in addition, ran T is closed, then X is a frame. Finally, a frame whose corresponding T is injective is a stable basis (also known as a Riesz basis). This paper considers the above three properties for subspaces H of L 2 (IR d), and for sets X of the form with either a singleton, a nite set, or, more generally, a countable set. The analysis is performed on the Fourier domain, where the two operators TT and T T are decomposed into a collection of simpler \\ber" operators. The main theme of the entire analysis is the characterization of each of the above three properties in terms of the analogous property of these simpler operators.