OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 592
FORMULA
E.g.f.: 1/(-1+x)/(-1+x+x^2)
Recurrence: {a(0)=1, a(1)=2, a(2)=8, (n^3+6*n^2+11*n+6)*a(n)+(-2*n-6)*a(n+2)+a(n+3)=0}
(-1+Sum(1/5*(4+3*_alpha)*_alpha^(-1-n), _alpha =RootOf(-1+_Z+_Z^2)))*n!
n!*Sum(k=0, n, A000045(n+1)).
a(n) = n!*A000071(n+3). - R. J. Mathar, Nov 27 2011
MAPLE
spec := [S, {S=Prod(Sequence(Z), Sequence(Union(Z, Prod(Z, Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[1/((1-x)(1-x-x^2)), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Dec 01 2018 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved