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A342795
a(n) is the number of core triads with n nodes up to isomorphism and transposition.
0
1, 3, 9, 24, 62, 159, 381, 927, 2150, 5051, 11483, 26332, 58986, 132796, 294046, 652772, 1431813, 3143879, 6841673
OFFSET
8,2
COMMENTS
A triad is an orientation of a tree that has a single vertex of degree 3 and otherwise only vertices of degree 2 and 1. A triad T is called a core triad if every endomorphism of T is an automorphism.
EXAMPLE
For n = 9 the a(9) = 3 solutions are:
1. 0111,000,1
2. 01000,00,1
3. 01111,00,1
Here 0 stands for forward edge, 1 stands for backward edge, and the three words stand for the three paths that leave the vertex of degree 3 of the triad.
CROSSREFS
Sequence in context: A279978 A269601 A006684 * A267465 A018788 A098690
KEYWORD
nonn,more
AUTHOR
Michael Wernthaler, Mar 21 2021
STATUS
approved