a(n) = 3*n! - 1.

Terms

a(0) =2a(1) =2a(2) =5a(3) =17a(4) =71a(5) =359a(6) =2159a(7) =15119a(8) =120959a(9) =1088639a(10) =10886399a(11) =119750399a(12) =1437004799a(13) =18681062399a(14) =261534873599