OFFSET
0,1
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 574
FORMULA
E.g.f.: (-2+x^4-x^3)/(-1+x).
Recurrence: {(-1-n)*a(n)+a(n+1)=0, a(2)=4, a(1)=2, a(0)=2, a(4)=48, a(3)=18}
2*n!, n != 3.
MAPLE
spec := [S, {S=Union(Sequence(Z), Sequence(Z), Prod(Z, Z, Z))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[(2+x^3-x^4)/(1-x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Feb 03 2015 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved