OFFSET
0,5
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..400
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets.
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 87.
FORMULA
E.g.f.: x^4/(1-x)^2.
(n-3)*a(n+1) + (2+n-n^2)*a(n) = 0, with a(0) = a(1) = a(2) = a(3) = 0, a(4) = 24.
a(n) = (n-3)*n!, n>2.
a(n) = (n+1)!*(n-3)/(n+1), n>2. - Gary Detlefs, Oct 02 2011
From Amiram Eldar, Jan 14 2021: (Start)
MAPLE
Pairs spec := [S, {B=Sequence(Z, 2 <= card), S=Prod(B, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
Table[Sum[n!, {i, 4, n}], {n, 0, 19}] (* Zerinvary Lajos, Jul 12 2009 *)
With[{nn=20}, CoefficientList[Series[x^4/(x-1)^2, {x, 0, nn}], x] Range[0, nn]!] (* Harvey P. Dale, Jun 03 2016 *)
PROG
(PARI) {a(n) = if(n<4, 0, (n-3)*n!)}; \\ G. C. Greubel, May 13 2019
(Magma) [n le 3 select 0 else (n-3)*Factorial(n): n in [0..20]]; // G. C. Greubel, May 13 2019
(Sage) [0, 0, 0, 0]+[(n-3)*factorial(n) for n in (4..20)] # G. C. Greubel, May 13 2019
(GAP) Concatenation([0, 0, 0, 0], List([4..20], n-> (n-3)*Factorial(n))) # G. C. Greubel, May 13 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved