a(n) = floor(n^(n/2)/n!!).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =4a(6) =8a(7) =10a(8) =20a(9) =26a(10) =51a(11) =64a(12) =128a(13) =163a(14) =326a(15) =416a(16) =834a(17) =1067a(18) =2148a(19) =2755a(20) =5559a(21) =7147a(22) =14449a(23) =18613a(24) =37696a(25) =48638a(26) =98650a(27) =127463a(28) =258857a(29) =334864