OFFSET
0,3
LINKS
J. East, R. D. Gray, Idempotent generators in finite partition monoids and related semigroups, arXiv preprint arXiv:1404.2359 [math.GR], 2014-2016.
FORMULA
An explicit formula is given in Th. 7.13 of East-Gray.
MAPLE
A038205 := proc(n)
option remember ;
if n = 0 then
1;
elif n <=2 then
0 ;
else
(n-1)*procname(n-1)+(n-1)*(n-2)*procname(n-3) ;
end if;
end proc:
A244490 := proc(n, k)
add((-1)^i*binomial(k, 2*i)*doublefactorial(2*i-1)*n^(k-2*i), i=0..floor(k/2)) ;
end proc:
A244491 := proc(n)
end proc:
seq(A244491(n), n=0..30) ; # R. J. Mathar, Aug 26 2014
MATHEMATICA
a05[n_] := SeriesCoefficient[Exp[-x - x^2/2]/(1 - x), {x, 0, n}]*n!;
a90[n_, k_] := Sum[(-1)^i*Binomial[k, 2i]*(2i-1)!!*n^(k-2*i), {i, 0, k/2}];
a[n_] := Sum[Binomial[n, k]*a05[k]*a90[n, n - k], {k, 0, n}];
Table[a[n], {n, 0, 18}] (* Jean-François Alcover, Dec 01 2017, after R. J. Mathar *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 05 2014
STATUS
approved