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

Terms

a(0) =1a(1) =3a(2) =4a(3) =21a(4) =6a(5) =367a(6) =8a(7) =5129a(8) =19693a(9) =106411a(10) =12a(11) =9590989a(12) =14a(13) =105614223a(14) =2439477016a(15) =8590983185a(16) =18a(18) =20a(22) =24