Expansion of Product_{k>=1} ((1 + x^k) * (1 + 3*x^k)).

Terms

a(0) =1a(1) =4a(2) =7a(3) =20a(4) =35a(5) =60a(6) =124a(7) =200a(8) =324a(9) =524a(10) =865a(11) =1320a(12) =2016a(13) =3036a(14) =4453a(15) =6684a(16) =9668a(17) =13856a(18) =19792a(19) =27876a(20) =38956a(21) =54640a(22) =75320a(23) =103268a(24) =141191a(25) =191320a(26) =257892a(27) =346164a(28) =463284a(29) =615292