$2$-strong uniqueness of a best approximation and of minimal projections in complex polytope norms and their duals

Abstract

We study a property of $2$-strong uniqueness of a best approximation in a class of finite-dimensional complex normed spaces, for which the unit ball is an absolutely convex hull of finite number of points and in its dual class. We prove that, contrary to the real case, these two classes do not coincide but are in fact disjoint. We provide several examples of situations in these two classes, where a uniqueness of an element of a best approximation in a given linear subspace implies its $2$-strong uniqueness. In particular, such a property holds for approximation in an arbitrary subspace of the complex $\ell_1^n$ space, but not of the complex $\ell_{\infty}^n$ space. However, this is true in general under an additional assumption that a subspace has a real basis and an ambient complex normed space is generated by real vectors or functionals. We apply our results and related methods to establish some results concerned with $2$-strongly unique minimal projections in complex normed spaces, proving among other things, that a minimal projection onto a two-dimensional subspace of an arbitrary three-dimensional complex normed space is $2$-strongly unique, if its norm is greater than $1$.