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%I #34 Mar 02 2020 12:29:38
%S 1,1,1,1,2,3,6,11,22,42,102,204,488,1078,2723,6403,16479,40313,106135,
%T 271295,724455
%N Number of distinct characteristic polynomials of trees with n nodes.
%D D. M. Cvetković, M. Doob and H. Sachs, Spectra of graphs. Theory and applications, Johann Ambrosius Barth, Heidelberg, 1995.
%H M. Fürer, <a href="https://doi.org/10.1007/978-3-642-04128-0_2">Efficient computation of the characteristic polynomial of a tree and related tasks</a>. Algorithms-ESA 2009, p. 11-22.
%o (Sage) [len(Set([t.charpoly() for t in graphs.trees(n)])) for n in range(14)]
%Y Cf. A000055.
%K nonn,more
%O 0,5
%A _F. Chapoton_, Jun 13 2013
%E a(0) = 1 added by _Peter Luschny_, Mar 01 2020
%E a(17)-a(20) from _F. Chapoton_, Mar 02 2020
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