OFFSET
0,3
FORMULA
a(n) = n! * Sum_{k=0..floor(n/3)} binomial(n,3*k)/k!.
From Vaclav Kotesovec, Mar 17 2023: (Start)
a(n) = (4*n - 3)*a(n-1) - 3*(n-1)*(2*n - 3)*a(n-2) + 2*(n-2)*(n-1)*(2*n - 3)*a(n-3) - (n-3)^2*(n-2)*(n-1)*a(n-4).
a(n) ~ 3^(-1/8) * exp(-1/4 + 5*3^(-1/4)*n^(1/4)/8 - sqrt(3*n)/2 + 4*3^(-3/4) * n^(3/4) - n) * n^(n + 1/8) / 2 * (1 + (1511/2560)*3^(1/4)/n^(1/4)). (End)
MATHEMATICA
Table[n! * Sum[Binomial[n, 3*k]/k!, {k, 0, n/3}], {n, 0, 20}] (* Vaclav Kotesovec, Mar 17 2023 *)
PROG
(PARI) my(N=20, x='x+O('x^N)); Vec(serlaplace(exp((x/(1-x))^3)/(1-x)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Mar 16 2023
STATUS
approved