Table of n, a(n) for n=1..12.
Andries E. Brouwer, Numbers of characteristic polynomials and cospectral graphs for A
A. E. Brouwer and E. Spence, Cospectral graphs on 12 vertices, Electr. J. Combin. 16 (2009) N20. (p. 199).
C. Godsil and B. D. McKay, Some computational results on the spectra of graphs, pp. 7392 of Combinatorial Mathematics IV (Adelaide 1975), Lect. Notes Math., 560 (1976).
Andreas Holmstrom, A first step towards automated conjecturemaking in higher arithmetic geometry, Workinprogress paper presented at the Conference on Intelligent Computer Mathematics, July 2016. Published in the CEUR Workshop Proceedings.
Jürgen Jost, Raffaella Mulas, and Leo Torres, Spectral theory of the nonbacktracking Laplacian for graphs, arXiv:2203.10824 [math.SP], 2022.
P. W. Mills, R. P. Rundle, J. H. Samson, Simon J. Devitt, Todd Tilma, V. M. Dwyer and Mark J. Everitt, Quantum invariants and the graph isomorphism problem, Phys. Rev. A 100, 052317 (2019).
E. Spence, Numbers of characteristic polynomials and cospectral graphs for A
Eric Weisstein's World of Mathematics, Determined by Spectrum
Eric Weisstein's World of Mathematics, Isospectral Graphs
