L.g.f.: Sum_{n>=1} x^n/n * Product_{d|n} (1 + d*x^(n/d))^d.

Terms

a(0) =1a(1) =3a(2) =7a(3) =23a(4) =76a(5) =249a(6) =974a(7) =4151a(8) =16558a(9) =70308a(10) =342937a(11) =1680725a(12) =8012252a(13) =40903572a(14) =222539812a(15) =1202060807a(16) =6608077855a(17) =38523427818a(18) =228629565951