OFFSET
0,5
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 697
FORMULA
E.g.f.: (1/2-x-1/2*(1-4*x)^(1/2))*(1/2-1/2*(1-4*x)^(1/2))*x
Recurrence: {a(1)=0, a(2)=0, a(3)=0, a(4)=24, a(5)=360, (-22*n+2*n^2-20+4*n^3)*a(n) +(-5*n^2+6-7*n)*a(n+1) +(n+1)*a(n+2) =0.
a(n) = n!*A000245(n-3). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {B=Union(Z, C), C=Prod(B, B), S=Prod(B, Z, C)}, 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