Product_{k>=1} 1/(1 - x^k)^a(k) = 1 + 3x.

Terms

a(0) =3a(1) =-6a(2) =8a(3) =-18a(4) =48a(5) =-124a(6) =312a(7) =-810a(8) =2184a(9) =-5928a(10) =16104a(11) =-44220a(12) =122640a(13) =-341796a(14) =956576a(15) =-2690010a(16) =7596480a(17) =-21524412a(18) =61171656a(19) =-174336264a(20) =498111952a(21) =-1426419852a(22) =4093181688a(23) =-11767874940a(24) =33891544368a(25) =-97764131640