Polyhedral faces in Gram spectrahedra of binary forms

Abstract: We analyze both the facial structure of the Gram spectrahedron $\mathrm{Gram}(f)$ and of the Hermitian Gram spectrahedron $\mathcal{H}^{\scriptscriptstyle+}(f)$ of a nonnegative binary form $f \in \mathbb{R}[x, y]{2d}$. We show that if $F \subseteq \mathcal{H}^{\scriptscriptstyle+}(f)$ is a polyhedral face of dimension $k$ then $\binom{k+1}{2} \leq d$. Conversely, for all $k \in \mathbb{N}$ and $d \geq \binom{k+1}{2}$ we show that the Hermitian Gram spectrahedron of a general positive binary form $f \in \mathbb{R}[x, y]{2d}$ with distinct roots contains a face $F$ which is a $k$-simplex and whose extreme points are rank-one tensors. For all $k \in \mathbb{N}$ and $d \geq (k+1)^2$ the (symmetric) Gram spectrahedron of a general positive binary form $f \in \mathbb{R}[x, y]_{2d}$ contains a polyhedral face $F$ with $(\mathrm{rk}(F), \dim(F)) = (2(k+1), k)$.