Another approximation to A000084(n).

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =24a(5) =63a(6) =177a(7) =514a(8) =1527a(9) =4625a(10) =14230a(11) =44358a(12) =139779a(13) =444558a(14) =1425151a(15) =4600339a(16) =14939849a(17) =48778197a(18) =160019885a(19) =527200711a(20) =1743607826a(21) =5786756470a(22) =19266336882a(23) =64331275266a(24) =215377539119a(25) =722840378042