Orthogonal signed graphs of degree 5

Abstract

An orthogonal signed graph is a connected signed graph whose signed adjacency matrix has pairwise orthogonal rows. They are closely related to Hadamard matrices, maximal arrangements of equiangular lines, bipartite Ramanujan graphs, and the remarkable resolution of the Sensitivity Conjecture. Orthogonal signed graphs of degree at most 4 have been completely classified, and partial results were known for degree 5. We complete the classification of orthogonal signed graphs of maximum degree 5. We also provide several new infinite families of 6- and 8-regular orthogonal signed graphs.