Characterization of a graph by its spectrum
On the flags of the 11 point biplane 2-(11,5,2) we
construct a graph on 55 points by declaring two flags
(P,l) and (Q,m) adjacent if l and m intersect in the
pair P,Q, so P and Q are different, l and m are
different, and P is on m and Q is on l.
We show that this graph is determined by the spectrum
of its 0/1 adjacency matrix.