Product_{n>=1} (1 + x^n)^a(n) = 1 + Sum_{n>=1} sigma(n) * x^n, where sigma = A000203.

Terms

a(0) =1a(1) =3a(2) =1a(3) =3a(4) =-3a(5) =2a(6) =-1a(7) =4a(8) =3a(9) =-8a(10) =-1a(11) =6a(12) =3a(13) =-4a(14) =-7a(15) =12a(16) =1a(17) =-6a(18) =7a(19) =0a(20) =-13a(21) =-13a(22) =27a(23) =13a(24) =-19a(25) =-11a(26) =11a(27) =-21a(28) =-25a(29) =191