G.f.: exp( Sum_{n>=1} x^n/n * exp( Sum_{k>=1} sigma(n*k) * x^(n*k)/k ) ).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =16a(6) =35a(7) =61a(8) =124a(9) =222a(10) =427a(11) =749a(12) =1434a(13) =2493a(14) =4585a(15) =8032a(16) =14511a(17) =25096a(18) =44791a(19) =77019a(20) =135435a(21) =232002a(22) =402957a(23) =685582a(24) =1181399a(25) =1998168a(26) =3410288a(27) =5741978a(28) =9726821a(29) =16286497