OFFSET
0,3
COMMENTS
Essentially the same as A051578. - R. J. Mathar, Feb 22 2008
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 532
FORMULA
E.g.f.: x^2*(-1+x)/(-1+2*x)
Recurrence: {a(1)=0, a(0)=0, a(3)=6, a(2)=2, (-2*n-2)*a(n)+a(n+1)=0}
1/8*2^n*n!, n>2.
MAPLE
spec := [S, {S=Prod(Z, Z, Sequence(Prod(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