OFFSET
0,5
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 599
FORMULA
E.g.f.: -x^4/(-1+2*x)
Recurrence: {a(1)=0, a(0)=0, a(2)=0, a(3)=0, (-2*n-2)*a(n)+a(n+1)=0, a(4)=24}
2^(n-4)*n!, n>3.
MAPLE
spec := [S, {S=Prod(Z, Z, Z, Z, Sequence(Union(Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[x^4/(1-2x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Mar 30 2017 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved