OFFSET
1,1
COMMENTS
Equivalently, the graph can be described as the graph on 3*n + 2 vertices with labels 0..3*n+1 and with i and j adjacent iff i+j> 0 mod 3.
These graphs are cographs.
LINKS
H-Y. Ching, R. Florez, and A. Mukherjee, Families of Integral Cographs within a Triangular Arrays, arXiv:2009.02770 [math.CO], 2020.
Eric Weisstein's World of Mathematics, Spanning Tree
FORMULA
a(n) = (n + 1)*(2 n + 2)^n*(2 n + 1)^(2 n - 1).
EXAMPLE
The adjacency matrix of the graph associated with n = 2 is: (compare A204437)
[0, 1, 0, 0, 0, 1, 1, 1]
[1, 0, 0, 0, 0, 1, 1, 1]
[0, 0, 0, 1, 1, 1, 1, 1]
[0, 0, 1, 0, 1, 1, 1, 1]
[0, 0, 1, 1, 0, 1, 1, 1]
[1, 1, 1, 1, 1, 0, 0, 0]
[1, 1, 1, 1, 1, 0, 0, 0]
[1, 1, 1, 1, 1, 0, 0, 0]
a(2) = 13500 because the graph has 13500 spanning trees.
MATHEMATICA
Table[(n + 1)*(2 n + 2)^n*(2 n + 1)^(2 n - 1), {n, 1, 10}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Rigoberto Florez, Nov 07 2020
EXTENSIONS
Offset changed by Georg Fischer, Nov 03 2023
STATUS
approved