OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 520
FORMULA
E.g.f.: 1/(-1+x)/(-1+3*x).
Recurrence: {a(0)=1, a(1)=4, (3*n^2+9*n+6)*a(n)+(-4*n-8)*a(n+1)+a(n+2)=0.}.
a(n) = (-1/2+1/2*3^(n+1))*n!.
a(n)=n!*A003462(n+1). - R. J. Mathar, Jun 03 2022
MAPLE
spec := [S, {S=Prod(Sequence(Z), Sequence(Union(Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved