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

Terms

a(0) =1a(1) =3a(2) =10a(3) =69a(4) =626a(5) =7866a(6) =117650a(7) =2101265a(8) =43047451a(9) =1000390658a(10) =25937424602a(11) =743069105634