OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..n} (-9)^k * binomial(-(n-k)/3,k) = Sum_{k=0..n} 9^(n-k) * binomial(n-2*k/3-1,n-k).
a(n) ~ (1-9*r)^(1/3) * (6 - 18*r + r^2) / (109 * r^n), where r = 0.1109593191262346... is the root of the equation r*(9 + r^2) = 1. - Vaclav Kotesovec, Feb 19 2024
MATHEMATICA
Table[Sum[9^(n - k)*Binomial[n - 2*k/3 - 1, n - k], {k, 0, n}], {n, 0, 25}] (* Vaclav Kotesovec, Feb 19 2024 *)
PROG
(PARI) my(N=30, x='x+O('x^N)); Vec(1/(1-x/(1-9*x)^(1/3)))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Apr 11 2023
STATUS
approved