a(n) = floor( exp(11/18)*n! ).

Terms

a(0) =1a(1) =3a(2) =11a(3) =44a(4) =221a(5) =1326a(6) =9286a(7) =74288a(8) =668598a(9) =6685982a(10) =73545804a(11) =882549650a(12) =11473145461a(13) =160624036454