Extremal elements of a sublattice of the majorization lattice and approximate majorization

Abstract

Given a probability vector x with its components sorted in non-increasing order, we consider the closed ball B^p_ε(x) with p ⩾ 1 formed by the probability vectors whose ℓp-norm distance to the center x is less than or equal to a radius epsilon. Here, we provide an order-theoretic characterization of these balls by using the majorization partial order. Unlike the case p = 1 previously discussed in the literature, we find that the extremal probability vectors, in general, do not exist for the closed balls B^p_ε(x) with 1 < p < ∞. On the other hand, we show that B^∞_ε(x) is a complete sublattice of the majorization lattice. As a consequence, this ball also has extremal elements. In addition, we give an explicit characterization of those extremal elements in terms of the radius and the center of the ball. This allows us to introduce some notions of approximate majorization and discuss its relation with previous results of approximate majorization given in terms of the ℓ1-norm. Finally, we apply our results to the problem of approximate conversion of resources within the framework of quantum resource theory of nonuniformity.