OFFSET
0,1
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 607
FORMULA
E.g.f.: -(-2+x^2+2*x)/(-1+x)/(-1+x+x^2)
Recurrence: {a(1)=2, a(2)=6, a(0)=2, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0}
(1+Sum(1/5*(1+2*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+_Z+_Z^2)))*n!
a(n) = n!*A001611(n+1). - R. J. Mathar, Nov 27 2011
MAPLE
spec := [S, {S=Union(Sequence(Z), Sequence(Union(Z, Prod(Z, 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