Problem of the Week
Hilton Memorial Lecture
Equiangular lines are lines that all make the same angle with each other. Take a unit vector in each line. The matrix of inner products of these vectors is (except for the units in its diagonal) a fixed multiple of a signed-graph adjacency matrix. Examples are the classical root systems An and Dn. I will ramble on about these topics and their connection with signed graphs and regular two-graphs for an hour or so.