34578domain: NAppears in sequencesNumber of proper T_1-hypergraphs with 3 labeled nodes and n hyperedges.at n=18A056078A089450 indexed by A000040.at n=20A089525G.f. A(x) satisfies: A(x) = 1 / (1 - x - x^2 * A(2*x)).at n=8A348878