OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 695
FORMULA
D-finite with recurrence: a(1)=0; a(2)=2; a(3)=24; a(4)=288; (-4*n^2+4)*a(n) +(-4*n-2)*a(n+1) +a(n+2)=0.
a(n) = A052727(n), n>2.- R. J. Mathar, Jan 26 2020
MAPLE
spec := [S, {B=Prod(Z, Z), S=Prod(C, C), C=Union(B, S, Z)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[(1-Sqrt[1-4x-4x^2])/2-x(1+x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Jun 19 2018 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved