a(n) = n!*(1 + Sum_{i=1..n} 1/i).

Terms

a(0) =1a(1) =2a(2) =5a(3) =17a(4) =74a(5) =394a(6) =2484a(7) =18108a(8) =149904a(9) =1389456a(10) =14257440a(11) =160460640a(12) =1965444480a(13) =26029779840a(14) =370643938560