OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..445
Wikipedia, Partition of a set
FORMULA
a(n) = Sum_{j=0..n} binomial(2n,j) * Bell(j).
a(n) = A283424(2n,n).
a(n) ~ 2^(2*n) * exp(n/LambertW(n) - n - 1) * n^(n - 1/2) / (sqrt(Pi*(1 + LambertW(n))) * LambertW(n)^n). - Vaclav Kotesovec, Jul 23 2021
EXAMPLE
a(2) = 17: 1234, 123|4, 124|3, 12|34, 12|3|4, 134|2, 13|24, 13|2|4, 14|23, 1|234, 1|23|4, 14|2|3, 1|24|3, 1|2|34. Here three set partitions contain 2 blocks of size 2.
MAPLE
b:= proc(n, k) option remember; `if`(k>n, 0,
binomial(n, k)*combinat[bell](n-k)+b(n, k+1))
end:
a:= n-> b(2*n, n):
seq(a(n), n=0..25);
MATHEMATICA
a[n_] := Sum[Binomial[2 n, j] BellB[j], {j, 0, n}];
Table[a[n], {n, 0, 25}] (* Jean-François Alcover, May 28 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 15 2017
STATUS
approved