G.f.: x * Product_{j>=1, k>=1} 1/(1 - a(j)*x^(j*k)).

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =19a(5) =47a(6) =134a(7) =357a(8) =1031a(9) =2912a(10) =8612a(11) =25007a(12) =75378a(13) =223884a(14) =683915a(15) =2067578a(16) =6376800a(17) =19503546a(18) =60749341a(19) =187592661a(20) =587938043a(21) =1831377952a(22) =5773159368a(23) =18092820941a(24) =57328904204a(25) =180657986051a(26) =574735018826