a(n) = Sum_{d|n} (n/d)^(n-d+1).

Terms

a(0) =1a(1) =5a(2) =28a(3) =265a(4) =3126a(5) =46916a(6) =823544a(7) =16793633a(8) =387422677a(9) =10001953190a(10) =285311670612