Optimal Shift Invariant Spaces and Their Parseval Frame Generators

Given a set of functions F = {f1, . . . , fm} ⊂ L2(R), we study the problem of finding the shift-invariant space V with n generators {φ1, . . . , φn} that is “closest” to the functions of F in the sense that V = argminV ′∈Vn m X i=1 wi‖fi − PV ′fi‖, where wis are positive weights, and Vn is the set of all shift-invariant spaces that can be generated by n or less generators. The Eckart-Young Theorem uses the singular value decomposition to provide a solution to a related problem in finite dimension. We transform the problem under study into an uncountable set of finite dimensional problems each of which can be solved using an extension of the Eckart-Young Theorem. We prove that the finite dimensional solutions can be patched together and transformed to obtain the optimal shiftinvariant space solution to the original problem, and we produce a Parseval frame for the optimal space. A typical application is the problem of finding a shift-invariant space model that describes a given class of signals or images (e.g., the class of chest X-Rays), from the observation of a set of m signals or images f1, . . . , fm, which may be theoretical samples, or experimental data. Date: May 15, 2007. The research of Akram Aldroubi is supported in part by NSF Grant DMS-0504788. The research Douglas Hardin is supported in part by NSF Grants DMS-0505756 and DMS-0532154. The research of Carlos Cabrelli and Ursula Molter is partially supported by Grants: PICT 15033, CONICET, PIP 5650, UBACyT X058 and X108. 1 2 AKRAM ALDROUBI, CARLOS CABRELLI, DOUGLAS HARDIN, AND URSULA MOLTER