CONICET | Buscador de Institutos y Recursos Humanos

Given a finite sequence of vectors F0 in a d-dimensional complex Hilbert space H we characterize in a complete and explicit way the optimal completions of F0 obtained by appending a finite sequence of vectors with prescribed norms, where optimality is measured with respect to majorization (of the eigenvalues of the frame operators of the completed sequences). Indeed, we construct (in terms of a fast algorithm) a vector?that depends on the eigenvalues of the frame operator of the initial sequence F0 and the sequence of prescribed norms?that is a minimum for majorization among all eigenvalues of frame operators of completions with prescribed norms. Then, using the eigenspaces of the frame operator of the initial sequence F0 we describe the frame operators of all optimal completions for majorization. Hence, the concrete optimal completions with prescribed norms can be obtained using recent algorithmic constructions related with the Schur-Horn theorem.