The Lemmens-Seidel conjecture and forbidden subgraphs

Abstract

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 $-5$.

28 pages, 2 tables, 6 figures