OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 544
FORMULA
E.g.f.: -1/(-1+2*x+x^3)
Recurrence: {a(0)=1, a(1)=2, a(2)=8, (-11*n-6-n^3-6*n^2)*a(n) +(-2*n-6)*a(n+2) +a(n+3)=0}
Sum(1/59*(16+12*_alpha^2+9*_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^3))*n!
a(n) = n!*A008998(n). - R. J. Mathar, Nov 27 2011
MAPLE
spec := [S, {S=Sequence(Union(Z, Z, Prod(Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[1/(1-2x-x^3), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Apr 10 2022 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
EXTENSIONS
Definition clarified by Harvey P. Dale, Apr 10 2022
STATUS
approved