Witt vector *2!/2!.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =25a(5) =72a(6) =245a(7) =772a(8) =2692a(9) =8925a(10) =32065a(11) =109890a(12) =400023a(13) =1402723a(14) =5165327a(15) =18484746a(16) =68635477a(17) =248339122a(18) =930138521a(19) =3406231198a(20) =12810761323a(21) =47306348881a(22) =178987624513a(23) =665627041157