G.f.: exp( Sum_{n>=1} sigma(3n)*x^n/n ).

Terms

a(0) =1a(1) =4a(2) =14a(3) =39a(4) =101a(5) =238a(6) =533a(7) =1131a(8) =2314a(9) =4566a(10) =8763a(11) =16376a(12) =29939a(13) =53612a(14) =94302a(15) =163112a(16) =277953a(17) =467064a(18) =774943a(19) =1270528a(20) =2060331a(21) =3306771a(22) =5256579a(23) =8280649a(24) =12934125a(25) =20040761a(26) =30817437a(27) =47048638a(28) =71339593a(29) =107469716