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

Terms

a(0) =3a(1) =-3a(2) =8a(3) =-21a(4) =48a(5) =-116a(6) =312a(7) =-831a(8) =2184a(9) =-5880a(10) =16104a(11) =-44336a(12) =122640a(13) =-341484a(14) =956576a(15) =-2690841a(16) =7596480a(17) =-21522228a(18) =61171656a(19) =-174342144a(20) =498111952a(21) =-1426403748a(22) =4093181688a(23) =-11767919276a(24) =33891544368a(25) =-97764009000