OFFSET
1,5
COMMENTS
Any tree with 2n vertices is a bipartite graph with s vertices in one part and t vertices in the other part, where s <= t and s + t = 2n. We count trees with s = t = n, and which are asymmetric, that is, their only automorphism is the identity automorphism. These are also called identity trees.
REFERENCES
R. C. Read and R. J. Wilson, Atlas of Graphs, Oxford Science Publications, Clarendon Press, OUP, 2004.
LINKS
F. Hüffner, tinygraph, software for generating integer sequences based on graph properties.
Austin Mohr, Unlabeled Trees.
EXAMPLE
a(3) = 0 because there are six trees with 6 vertices, but only three of these have s = t = n = 3, and none of these three is asymmetric. The fourth term a(4) = 1 because there are nine trees with 8 vertices with s = t = n = 4 but only 1 is asymmetric, namely tree T46. See "Atlas of Graphs", page 65.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
John P. McSorley, Aug 08 2017
EXTENSIONS
a(10)-a(13) added using tinygraph by Falk Hüffner, Jul 25 2019
STATUS
approved