Maximality of Seidel matrices and switching roots of graphs

Meng-Yue Cao, Jack H. Koolen, Akihiro Munemasa, Kiyoto Yoshino

Published 2021-02-24Version 1

In this paper, we discuss maximality of Seidel matrices with a fixed largest eigenvalue. We present a classification of maximal Seidel matrices of largest eigenvalue $3$, which gives a classification of maximal equiangular lines in a Euclidean space with angle $\arccos1/3$. Motivated by the maximality of the exceptional root system $E_8$, we define strong maximality of a Seidel matrix, and show that every Seidel matrix achieving the absolute bound is strongly maximal.