Product_{k>=1}1/(1 - x^k)^a(k) = 1 + 2x.

Terms

a(0) =2a(1) =-3a(2) =2a(3) =-3a(4) =6a(5) =-11a(6) =18a(7) =-30a(8) =56a(9) =-105a(10) =186a(11) =-335a(12) =630a(13) =-1179a(14) =2182a(15) =-4080a(16) =7710a(17) =-14588a(18) =27594a(19) =-52377a(20) =99858a(21) =-190743a(22) =364722a(23) =-698870a(24) =1342176a(25) =-2581425a(26) =4971008a(27) =-9586395a(28) =18512790a(29) =-35792449