%I
%S 1,3,13,159,33337,2147648859,9223372049740171909,
%T 170141183460469231796250908018965844535,
%U 57896044618658097711785492504343953927996121800504035873840544850835832773873
%N Number of collections F of subsets of {1,2,...,n} whose union is itself an element of F.
%C Equivalently, the number of partial orders (on some subset of the powerset of {1,2,...,n} ordered by set inclusion) that contain a greatest element.
%H Alois P. Heinz, <a href="/A246418/b246418.txt">Table of n, a(n) for n = 0..11</a>
%F a(n) = Sum_{k=0..n} C(n,k)*2^(2^k1).
%F a(n) = 2^(2^n)  A246537(n).
%e a(2) = 13 because there are 16 families of subsets of {1,2}. All of these contain their union except: {}, {{1},{2}}, {{},{1},{2}}. 163=13.
%t nn = 9; Table[Sum[Binomial[n, i] 2^(2^i  1), {i, 0, n}], {n, 0, nn}]
%o (PARI) a(n)=sum(k=0,n, binomial(n,k)*2^(2^k1));
%Y Cf. A246537.
%K nonn
%O 0,2
%A _Geoffrey Critzer_, Aug 25 2014
