OFFSET
0,4
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 560
FORMULA
E.g.f.: x^3*(-2*x+2*x^2-1)/(-1+x)
Recurrence: {a(1)=0, a(0)=0, a(2)=0, (-1-n)*a(n)+a(n+1)=0, a(3)=6, a(4)=72, a(5)=120}
n!, n>5.
MAPLE
spec := [S, {S=Prod(Z, Z, Z, Union(Z, Z, Sequence(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