OFFSET
0,1
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 516
FORMULA
E.g.f.: -2*(-1+2*x)/(-1+x)/(-1+3*x)
D-finite Recurrence: {a(1)=4, a(0)=2, (3*n^2+9*n+6)*a(n)+(-4*n-8)*a(n+1)+a(n+2)=0}
(1+3^n)*n!
MAPLE
spec := [S, {S=Union(Sequence(Z), Sequence(Union(Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[-2(-1+2x)/(-1+x)/(-1+3x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Jul 04 2021 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved