%I A052506
%S A052506 1,0,2,3,16,65,336,1897,11824,80145,586000,4588001,38239224,
%T A052506 337611001,3144297352,30779387745,315689119456,3383159052833,
%U A052506 37790736663456,439036039824193,5294386116882280
%N A052506 E.g.f.: exp(x*exp(x)-x)
%C A052506 Maps with f^2=f; trees of height at most 1.
%H A052506 INRIA Algorithms Project, <a href="http://algo.inria.fr/bin/encyclopedia?Search=ECSnb&argsearch=39">
               Encyclopedia of Combinatorial Structures 39</a>
%F A052506 a(n) = Sum_{k=0..n} binomial(n, k)*(n-k-1)^k. - Vladeta Jovovic (vladeta(AT)eunet.rs), 
               Apr 12 2003
%F A052506 a(n) = Sum_{k=0..floor(n/2)} binomial(n, k)*k!*Stirling2(n-k, k). - Vladeta 
               Jovovic (vladeta(AT)eunet.rs), Dec 19 2004
%p A052506 Maps spec := [S,{S=Set(Tree), Tree=Prod(Z,Set(Z,0 < card))},labeled]: 
               seq(combstruct[count](spec, size=n), n=0..20);
%Y A052506 Cf. A000248.
%Y A052506 Sequence in context: A012700 A012705 A103331 this_sequence A052858 A073997 
               A007118
%Y A052506 Adjacent sequences: A052503 A052504 A052505 this_sequence A052507 A052508 
               A052509
%K A052506 easy,nonn
%O A052506 0,3
%A A052506 encyclopedia(AT)pommard.inria.fr, Jan 25 2000