Equiangular lines and large multiplicity of fixed second eigenvalue

Abstract

Answering a question of Jiang and Polyanskii as well as Jiang, Tidor, Yao, Zhang, and Zhao, we show the existence of infinitely many angles $\theta$ for which the maximum number of lines in $\mathbb R^n$ meeting at the origin with pairwise angles $\theta$ exceeds $n+\Omega(\log\log n)$ but is at most $n+o(n)$. To accomplish this, we construct, for various real $\lambda$ and integer $d$, $d$-regular graphs with second eigenvalue exactly $\lambda$ and arbitrarily large second eigenvalue multiplicity. Central to our construction is a distribution on factors of bipartite graphs which possesses concentration properties.