OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 570
FORMULA
E.g.f.: (1+x^2-2*x^3+x^4)/(-1+x)^2
Recurrence: {a(0)=1, a(1)=2, a(2)=8, a(3)=24, (-2-n)*a(n)+a(n+1)=0, a(4)=120}
(n+1)!, n>2.
MAPLE
spec := [S, {S=Union(Prod(Sequence(Z), Sequence(Z)), Prod(Z, Z))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[(1+x^2-2x^3+x^4)/(1-x)^2, {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Oct 31 2016 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved