The Lemmens-Seidel conjecture and forbidden subgraphs

17 Mar 2020 · Cao Meng-Yue, H Jack, Koolen, Lin Yen-Chi Roger, Yu Wei-Hsuan ·

In this paper we show that the conjecture of Lemmens and Seidel of 1973 for systems of equiangular lines with common angle $\arccos (1/5)$ is true. Our main tool is forbidden subgraphs for smallest Seidel eigenvalue $-5$.