OFFSET
0,3
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..350
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 618
FORMULA
E.g.f.: x^2/(1 - 4*x)
Recurrence: a(0)=0, a(1)=0, a(2)=2, a(n) = 4*n*a(n-1).
a(n) = 4^(n-2) * n!, n>1.
G.f.: 2*x^2*Hypergeometric2F0([3,1], [], 4*x). - G. C. Greubel, Jun 13 2022
MAPLE
spec := [S, {S=Prod(Z, Z, Sequence(Union(Z, Z, Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
With[{nn=20}, CoefficientList[Series[x^2/(1-4x), {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Sep 27 2015 *)
PROG
(Magma) [0, 0] cat [4^(n-2)*Factorial(n): n in [2..30]]; // G. C. Greubel, Jun 13 2022
(SageMath) [(4^(n-2) - 4^(n-2)*bool(n<2))*factorial(n) for n in (0..30)] # G. C. Greubel, Jun 13 2022
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved