OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 584
FORMULA
E.g.f.: x^2*(-x+2*x^2-1)/(-1+2*x)
Recurrence: {a(1)=0, a(0)=0, a(2)=2, (-2*n-2)*a(n)+a(n+1)=0, a(3)=18, a(4)=96}
1/4*2^n*n!
MAPLE
spec := [S, {S=Prod(Z, Z, Union(Z, Sequence(Union(Z, Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[x^2 (1+x-2x^2)/(1-2x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Feb 29 2012 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved