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

Terms

a(0) =1a(1) =4a(2) =11a(3) =56a(4) =127a(5) =1100a(6) =1717a(7) =19300a(8) =64406a(9) =383010a(10) =352717a(11) =23214660a(12) =5200301a(13) =191172406a(14) =3465549077a(15) =20859527460a(16) =1166803111a(18) =17672631901