4177436domain: NAppears in sequencesNumber of different hierarchical orderings that can be formed from n unlabeled elements with no repetition of subhierarchies.at n=19A098407G.f. satisfies: A(x)^(1/2) = x + A(x) + A(A(x)) + A(A(A(x))) + A(A(A(A(x)))) +...at n=12A141201