OFFSET
0,2
COMMENTS
We can prove this using generating functions. a(n) is given also by 4*binomial(4n,n-2), for n>1.
LINKS
Indranil Ghosh, Table of n, a(n) for n = 0..500
N. Combe, V. Jugé, Counting bi-colored A'Campo forests, arXiv:1702.07672 [math.AG], 2017.
FORMULA
a(n) = 4*binomial(4n,n-2), for n>1.
EXAMPLE
For n=2 the a(2)=4 solutions are the number of A'Campo forests with co-dimension 1 and degree 2.
MATHEMATICA
Table[4*Binomial[4n, n-2], {n, 1, 23}] (* Indranil Ghosh, Feb 28 2017 *)
PROG
(PARI) a(n) = 4*binomial(4*n, n-2) \\ Indranil Ghosh, Feb 28 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Noemie Combe, Feb 27 2017
STATUS
approved