OFFSET
0,5
COMMENTS
Previous name was: A simple grammar.
LINKS
Amiram Eldar, Table of n, a(n) for n = 0..450
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 716.
FORMULA
E.g.f.: x^3*log(-1/(-1+x)).
Recurrence: a(0)=0, a(1)=0, a(2)=0, a(3)=0, a(4)=24, (-n^2+2*n+3)*a(n)+(-2+n)*a(n+1) = 0.
a(n) = n! / (n-3) (n > 3). - Olivier Gérard, Jun 13 2001
Sum_{n>=4} 1/a(n) = 11/2 - 2*e. - Amiram Eldar, Oct 07 2020
Sum_{n>=4} (-1)^n/a(n) = 3/2 - 4/e. - Amiram Eldar, Aug 20 2022
MAPLE
spec := [S, {B=Cycle(Z), S=Prod(B, Z, Z, Z)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
Join[{0, 0, 0, 0}, Table[n!/(n - 3), {n, 4, 30}]] (* Vincenzo Librandi, Jul 08 2015 *)
PROG
(PARI) a(n) = if (n <= 3, 0, n!/(n-3)); \\ Michel Marcus, Jul 08 2015
(Magma) [n le 3 select 0 else Factorial(n)/(n-3): n in [0..30]]; // Vincenzo Librandi, Jul 08 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved