%I
%S 1,1,2,7,42,376,4513,68090,1238968,26416729,646140364,17837852044,
%T 548713088399,18612963873492,690271321314292,27785827303491579,
%U 1206582732097720126,56224025231569020724,2798445211000659147033
%N Number of labeled trees of nonempty sets with n points. (Each node is a set of 1 or more points.)
%H T. D. Noe, <a href="/A038052/b038052.txt">Table of n, a(n) for n = 0..100</a>
%H <a href="/index/Tra#trees">Index entries for sequences related to trees</a>
%F E.g.f.: B(e^x1) where B is e.g.f. of A000272.
%F a(n) = Sum_{k=1..n} Stirling2(n, k)*k^(k2).  _Vladeta Jovovic_, Sep 20 2003
%F a(n) ~ (1+exp(1))^(3/2) * n^(n2) / (exp(n) * (log(1+exp(1)))^(n3/2)).  _Vaclav Kotesovec_, Feb 17 2017
%t a[0] = 1; a[n_] := Sum[StirlingS2[n, k]*k^(k  2), {k, 1, n}]; Table[a[n], {n, 0, 18}] (* _JeanFrançois Alcover_, Sep 09 2013, after _Vladeta Jovovic_ *)
%Y Cf. A036250, A048802.
%K nonn,nice,easy
%O 0,3
%A _Christian G. Bower_, Jan 04 1999
