a(n) = Product_{i=2..n} (tau(i)+1)/(tau(i)-1), where tau(.)=A000005(.).

Terms

a(0) =1a(1) =3a(2) =9a(3) =18a(4) =54a(5) =90a(6) =270a(7) =450a(8) =900a(9) =1500a(10) =4500a(11) =6300a(12) =18900a(13) =31500a(14) =52500a(15) =78750a(16) =236250a(17) =330750a(18) =992250a(19) =1389150a(20) =2315250a(21) =3858750a(22) =11576250a(23) =14883750a(24) =29767500a(25) =49612500a(26) =82687500a(27) =115762500a(28) =347287500a(29) =446512500