OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 569
FORMULA
E.g.f.: x*(-1+x)^2/(1-3*x+x^2)
Recurrence: {a(1)=1, a(0)=0, a(2)=2, (n^2+3*n+2)*a(n)+(-6-3*n)*a(n+1)+a(n+2)=0, a(3)=18}
Sum(-1/5*(-3+7*_alpha)*_alpha^(-1-n), _alpha=RootOf(_Z^2-3*_Z+1))*n!
a(n)=n!*A088305(n-1). - R. J. Mathar, Jun 03 2022
MAPLE
spec := [S, {S=Prod(Z, Sequence(Prod(Z, Sequence(Z), Sequence(Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[x (1-x)^2/(1-3x+x^2), {x, 0, nn}], x] Range[ 0, nn]!] (* Harvey P. Dale, May 30 2021 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved