a(n) = Sum_{d|n} d^(n-d+1) * binomial(d+n/d-2, d-1).

Terms

a(0) =1a(1) =3a(2) =4a(3) =21a(4) =6a(5) =346a(6) =8a(7) =4617a(8) =13132a(9) =80696a(10) =12a(11) =4903847a(12) =14a(13) =40410966a(14) =756336736a(15) =2416181265a(16) =18a(17) =306560794753a(18) =20a(22) =24