The dimension of an orbitope based on a solution to the Legendre pair problem

Abstract

The Legendre pair problem is a particular case of a rank-$1$ semidefinite description problem that seeks to find a pair of vectors $(\mathbf{u},\mathbf{v})$ each of length $\ell$ such that the vector $(\mathbf{u}^{\top},\mathbf{v}^{\top})^{\top}$ satisfies the rank-$1$ semidefinite description. The group $(\mathbb{Z}_\ell\times\mathbb{Z}_\ell)\rtimes \mathbb{Z}^{\times}_\ell$ acts on the solutions satisfying the rank-$1$ semidefinite description by $((i,j),k)(\mathbf{u},\mathbf{v})=((i,k)\mathbf{u},(j,k)\mathbf{v})$ for each $((i,j),k) \in (\mathbb{Z}_\ell\times\mathbb{Z}_\ell)\rtimes \mathbb{Z}^{\times}_\ell$. By applying the methods based on representation theory in Bulutoglu [Discrete Optim. 45 (2022)], and results in Ingleton [Journal of the London Mathematical Society s(1-31) (1956), 445-460] and Lam and Leung [Journal of Algebra 224 (2000), 91-109], for a given solution $(\mathbf{u}^{\top},\mathbf{v}^{\top})^{\top}$ satisfying the rank-$1$ semidefinite description, we show that the dimension of the convex hull of the orbit of $\mathbf{u}$ under the action of $\mathbb{Z}_{\ell}$ or $\mathbb{Z}_\ell\rtimes\mathbb{Z}^{\times}_\ell$ is $\ell-1$ provided that $\ell=p^n$ or $\ell=pq^i$ for $i=1,2$, any positive integer $n$, and any two odd primes $p,q$. Our results lead to the conjecture that this dimension is $\ell-1$ in both cases. We also show that the dimension of the convex hull of all feasible points of the Legendre pair problem of length $\ell$ is $2\ell-2$ provided that it has at least one feasible point.