OFFSET
0,4
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 687
FORMULA
D-finite with recurrence: a(1)=0; a(2)=0; a(3)=6; (-15*n+15*n^3+15*n^2+3*n^4-18)*a(n) +(-n^3-7*n^2-16*n-12)*a(n+1) +(-3*n^2-16*n-21)*a(n+2) +(n+4)*a(n+3)=0; a(4)=48; a(5)=600.
Conjecture: a(n) = n!*A002026(n-2). - R. J. Mathar, Oct 16 2013
a(n) ~ sqrt(2) * 3^(n - 1/2) * n^(n-1) / exp(n). - Vaclav Kotesovec, Nov 19 2021
MAPLE
spec := [S, {C=Prod(B, Z), S=Prod(B, C), B=Union(S, 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