I don't know how far this list is known to be complete. As of May 2007, the known values were 5,13,16,17,27,50,56,77,81,100,105,112,120,126,162,176,275
Table of n, a(n) for n=1..10.
Edwin R. van Dam and Willem H. Haemers, Developments on spectral characterizations of graphs, Discrete Mathematics 309.3 (2009): 576-586.
Sequence in context: A297862 A298129 A158334 * A088908 A327638 A092218
Adjacent sequences: A282744 A282745 A282746 * A282748 A282749 A282750
N. J. A. Sloane, Mar 05 2017