OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 583
FORMULA
E.g.f.: 3*(-x+x^2-1)*x/(-1+x)
Recurrence: {a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(1)=3, a(3)=18, a(2)=12}
3*n!, n>2.
MAPLE
spec := [S, {S=Prod(Union(Z, Sequence(Z)), 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