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

Terms

a(0) =1a(1) =3a(2) =4a(3) =13a(4) =6a(5) =109a(6) =8a(7) =777a(8) =2197a(9) =7541a(10) =12a(11) =374809a(12) =14a(13) =1675773a(14) =31954096a(15) =100794385a(16) =18a(17) =7391871271a(18) =20a(19) =163547770441a(21) =570634875581a(22) =24a(28) =30