Spaces with the maximal projection constant revisited

Abstract

Let $n\ge 2$ be an integer such that an equiangular set of vectors $w_1,\dots ,w_d$ of the maximal possible cardinality (that is, attaining the classical Gerzon upper bound) exists in ${\mathbb{K}}^n$, where $\mathbb{K}=\mathbb{R}$ or $\mathbb{K}=\mathbb{C}$ (so that $d=\frac{n(n+1)}{2}$ in the real case and $d=n^2$ in the complex case). We provide a complete characterization of $n$-dimensional normed spaces whose absolute projection constant is maximal among all $n$-dimensional normed spaces over $\mathbb{K}$. The characterization states that $X$ has the maximal projection constant if and only if it is isometric to a space whose dual unit ball is contained between the absolutely convex hull of the vectors $w_1,\dots ,w_d$ and a suitably rescaled zonotope generated by the same vectors. As a consequence, we obtain that, in the considered situations, $n=2$ with $\mathbb{K}=\mathbb{R}$ is the only case in which there is, up to isometry, a unique norm on ${\mathbb{K}}^n$ with the maximal projection constant. In this case, the unit ball is a linear image of a regular hexagon in ${\mathbb{R}}^2$.