|
|
A052747
|
|
a(0) = a(1) = a(2) = 0; a(n) = n!/(n-2) for n > 2.
|
|
11
|
|
|
0, 0, 0, 6, 12, 40, 180, 1008, 6720, 51840, 453600, 4435200, 47900160, 566092800, 7264857600, 100590336000, 1494484992000, 23712495206400, 400148356608000, 7155594141696000, 135161222676480000, 2688996956405760000, 56200036388880384000, 1231048416137379840000
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,4
|
|
COMMENTS
|
A simple grammar.
|
|
LINKS
|
|
|
FORMULA
|
E.g.f.: log(-1/(-1+x))*x^2
Recurrence: {a(0)=0, a(1)=0, a(2)=0, a(3)=6, (n+2-n^2)*a(n)+(n-1)*a(n+1)}
Sum_{n>=3} (-1)^(n+1)/a(n) = 3/e - 1. - Amiram Eldar, Aug 20 2022
|
|
MAPLE
|
spec := [S, {B=Cycle(Z), S=Prod(Z, Z, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
|
|
MATHEMATICA
|
a[n_] := If[n < 3, 0, n!/(n-2)]; Array[a, 20, 0] (* Amiram Eldar, Oct 07 2020 *)
|
|
PROG
|
|
|
CROSSREFS
|
|
|
KEYWORD
|
easy,nonn
|
|
AUTHOR
|
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|