OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..311
Wikipedia, Partition of a set
FORMULA
a(n) = Sum_{i=n..2*n} Stirling2(2*n,i)*Stirling2(i,n).
a(n) = ((2*n)!/n!) * [x^(2*n)] (exp(exp(x) - 1) - 1)^n. - Ilya Gutkovskiy, Feb 15 2021
From Vaclav Kotesovec, Feb 17 2021: (Start)
a(n) ~ c * d^n * (n-1)!, where
d = -4/(p^2*q*(1 + q + r)) = 14.158467948361614323478778011058425244554144983745335637776404207122781371002...
p = LambertW(-2/((1+r)*exp(2/(1+r))))
q = LambertW(-(1+r)/exp(1+r))
r = 0.49039351286814033601311908705923238442641817550970055325385921966197159992...
is the root of the equation p*(1+r)*(1+q+r) + (2 + p + p*r) = 0
and c = 0.1809999195056310772963776575864895285358912769365095026676184958683437... (End)
MAPLE
b:= proc(n, m, k) option remember; `if`(n=0, 1, add(
b(n-1, max(j, m), k)*`if`(j>m, k, 1) , j=1..m+1))
end:
a:= n-> add(b(2*n, 0, n-i)*(-1)^i*binomial(n, i), i=0..n)/n!:
seq(a(n), n=0..15);
MATHEMATICA
b[n_, m_, k_] := b[n, m, k] = If[n == 0, 1, Sum[b[n - 1, Max[j, m], k] If[j > m, k, 1] , {j, 1, m + 1}]];
a[n_] := Sum[b[2n, 0, n - i] (-1)^i Binomial[n, i], {i, 0, n}]/n!;
a /@ Range[0, 15] (* Jean-François Alcover, Dec 08 2020, after Alois P. Heinz *)
Table[Sum[StirlingS2[2*n, k] * StirlingS2[k, n], {k, n, 2*n}], {n, 0, 20}] (* Vaclav Kotesovec, Feb 17 2021 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Aug 27 2019
STATUS
approved