OFFSET
0,4
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 686
FORMULA
E.g.f.: RootOf(-_Z+_Z^2+_Z^3+x)-RootOf(-_Z+_Z^2+_Z^3+x)^2-x
D-finite Recurrence: {a(1)=0, a(2)=0, a(3)=6, a(4)=72, (-105*n+135*n^3+30)*a(n) +(-130-107*n+29*n^2)*a(n+1) +(-99-91*n)*a(n+2) +15*a(n+3)=0.
a(n) = n!*A052703(n). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {C=Prod(B, B), B=Union(S, Z, C), S=Prod(B, 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