An approximation to A000084(n).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =23a(6) =63a(7) =177a(8) =514a(9) =1526a(10) =4624a(11) =14230a(12) =44357a(13) =139779a(14) =444557a(15) =1425151a(16) =4600338a(17) =14939849a(18) =48778197a(19) =160019884a(20) =527200711a(21) =1743607825a(22) =5786756469a(23) =19266336882a(24) =64331275266a(25) =215377539119a(26) =722840378041