New bounds for equiangular lines and Balla's conjecture

Abstract

Let $N_α(d)$ denote the maximum number of equiangular lines in $\mathbb{R}^d$ with common angle $\arccos(α)$. Balla conjectured that, if the spectral radius order $κ_{\frac{1-α}{2α}}$ of $\frac{1-α}{2α}$ is finite, then $$N_α(d)\leq \max\left\{\frac{(1-α^2)(1-2α^2)}{2α^4},\left\lfloor\frac{κ_{\frac{1-α}{2α}}(d-1)}{κ_{\frac{1-α}{2α}}-1}\right\rfloor\right\},$$ for any $d\geq 1$. The conjecture has previously been verified only for $α\in\left\{\frac{1}{3},\frac{1}{5},\frac{1}{1+2\sqrt{2}}\right\}$. In this paper, we prove that this conjecture holds for $α=\frac{1}{1+2\sqrt{3}}$ and $α=\sqrt{5}-2$. On the other hand, we show that Balla's conjecture fails for infinitely many $α$.