OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 694
FORMULA
E.g.f.: RootOf(-_Z+_Z^4+_Z^2+x)^2
Recurrence: a(1)=0; a(2)=2; a(3)=12; (9216*n^3-2496*n-576+1152*n^2+6144*n^4)*a(n) +(-9096-30320*n-28032*n^2-7936*n^3)*a(n+1) +(15488*n+6272*n^2+10404)*a(n+2) +(-3784*n-7600)*a(n+3) +589*a(n+4)=0; a(4)=120.
a(n) = n!*A052708(n). - R. J. Mathar, Oct 16 2013
MAPLE
spec := [S, {B=Union(S, Z, C), S=Prod(B, B), C=Prod(S, S)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved