OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 684
FORMULA
E.g.f.: (1/2)/(x^2+1+2*x)*(1-2*x-2*x^2-(1-4*x-4*x^2)^(1/2))
Recurrence: {a(1)=0, a(2)=2, a(3)=12, (-4*n^3-16*n^2-20*n-8)*a(n) +(-8*n^2-26*n-20)*a(n+1) +(-2-3*n)*a(n+2) +a(n+3) =0.
a(n) = n!*A052705(n). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {B=Union(S, Z, C), S=Prod(B, B), C=Prod(S, 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