a(n) = floor(exp(1/2)*n!).

Terms

a(0) =1a(1) =3a(2) =9a(3) =39a(4) =197a(5) =1187a(6) =8309a(7) =66476a(8) =598287a(9) =5982879a(10) =65811677a(11) =789740126a(12) =10266621646a(13) =143732703044