Optimal Frame Completions with Prescribed Norms for Majorization

Abstract

Given a finite sequence of vectors $$\mathcal F_0$$ in a $$d$$ -dimensional complex Hilbert space $${{\mathcal {H}}}$$ we characterize in a complete and explicit way the optimal completions of $$\mathcal F_0$$ 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 $${\mathcal {F}}_0$$ 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 $${\mathcal {F}}_0$$ 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.