Equiangular lines via matrix projection

In 1973, Lemmens and Seidel posed the problem of determining the maximum number of equiangular lines in R r with angle arccos ⁡ ( α ) and provided a good partial answer in the regime r ≤ 1 / α 2 − 2 . At the other extreme where r is at least exponential in 1 / α 2 , recent breakthroughs have led to an almost complete resolution of this problem. In this paper, we introduce a new method for obtaining upper bounds which unifies and improves upon previous approaches, thereby yielding bounds which bridge the gap between the aforementioned regimes and are best possible either exactly or up to a factor of two. Our approach relies on orthogonal projection of matrices with respect to the Frobenius inner product and as a byproduct, it yields the first extension of the Alon–Boppana theorem to dense graphs, with equality for strongly regular graphs corresponding to families of ( r + 1 2 ) equiangular lines in R r . Applications of our method in the complex setting will be discussed as well.