OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 591
FORMULA
E.g.f.: 2*x^2*(-x+x^2-1)/(-1+x)
Recurrence: {a(1)=0, a(0)=0, (-1-n)*a(n)+a(n+1)=0, a(2)=4, a(3)=24, a(4)=48}
2*n!, n>3.
MAPLE
spec := [S, {S=Prod(Z, Union(Z, Z), Union(Z, Sequence(Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[2x^2 (1+x-x^2)/(1-x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, May 14 2017 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved