OFFSET
0,7
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..445
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets.
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 88.
FORMULA
E.g.f.: x^6/(1-x)^2.
(n-5)*a(n+1) + (4 + 3*n - n^2)*a(n) = 0, with a(0) = a(1) = a(2) = a(3) = a(4) = a(5) = 0, a(6) = 720.
a(n) = (n-5)*n!.
From Amiram Eldar, Jan 14 2021: (Start)
MAPLE
spec := [S, {B=Sequence(Z, 3 <= card), S=Prod(B, B)}, labeled]: # Pairs spec
seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
Table[If[n<6, 0, (n-5)*n!], {n, 0, 20}] (* G. C. Greubel, May 13 2019 *)
PROG
(PARI) {a(n) = if(n<6, 0, (n-5)*n!)}; \\ G. C. Greubel, May 13 2019
(Magma) [n le 5 select 0 else (n-5)*Factorial(n): n in [0..20]]; // G. C. Greubel, May 13 2019
(Sage) [0, 0, 0, 0, 0, 0]+[(n-5)*factorial(n) for n in (6..20)] # G. C. Greubel, May 13 2019
(GAP) Concatenation([0, 0, 0, 0, 0, 0], List([6..20], n-> (n-5)*Factorial(n))) # G. C. Greubel, May 13 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved