Speaker: Chris Cox
Title: Nearly orthogonal vectors

Abstract: How can $d+k$ vectors in $\mathbb R^d$ be arranged so that
they are as close to orthogonal as possible? In particular, define
$\theta(d,k):=\min_X\max_{x\neq y\in X}|\langle x,y\rangle|$ where the
minimum is taken over all collections of $d+k$ unit vectors
$X\subseteq\mathbb R^d$. In this work, we focus on the case where $k$ is
fixed and $d\to\infty$. In establishing bounds on $\theta(d,k)$, we find
an intimate connection to the existence of systems of ${k+1\choose 2}$
equiangular lines in $\mathbb R^k$. Using this connection, we are able
to pin down $\theta(d,k)$ whenever $k\in\{1,2,3,7,23\}$ and establish
asymptotics for general $k$. The main tool is an upper bound on $\mathbb
E_{x,y\sim\mu}|\langle x,y\rangle|$ whenever $\mu$ is an isotropic
probability mass on $\mathbb R^k$, which may be of independent interest.
Our results translate naturally to the analogous question in $\mathbb
C^d$. In this case, the question relates to the existence of systems of
$k^2$ equiangular lines in $\mathbb C^k$, also known as SIC-POVM in
physics literature.
Joint work with Boris Bukh.