OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 588
FORMULA
E.g.f.: x^2*(-x+x^2-2)/(-1+x).
Recurrence: {a(1)=0, a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(2)=4, a(4)=48, a(3)=18}
2*n!
a(n) = A052628(n), n>1. - R. J. Mathar, Jan 30 2009
MAPLE
spec := [S, {S=Prod(Z, Z, Union(Z, Sequence(Z), Sequence(Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[x^2 (2+x-x^2)/(1-x), {x, 0, nn}], x] Range[ 0, nn]!] (* Harvey P. Dale, Sep 04 2020 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved