OFFSET
0,4
FORMULA
a(n) = n! * Sum_{k=0..floor(3*n/7)} binomial(3*n-6*k,k)/(n-2*k)!.
a(n) == 1 (mod 18).
a(n) = a(n-1) + 9*(n-1)*(n-2)*a(n-3) + 15*(n-1)*(n-2)*(n-3)*(n-4)*a(n-5) + 7*(n-1)*(n-2)*(n-3)*(n-4)*(n-5)*(n-6)*a(n-7).
PROG
(PARI) a(n) = n!*sum(k=0, 3*n\7, binomial(3*n-6*k, k)/(n-2*k)!);
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jun 15 2024
STATUS
approved