Product_{k>=1} (1+x^k)^a(k) = 1 + 5x.

Terms

a(0) =5a(1) =-10a(2) =40a(3) =-160a(4) =624a(5) =-2580a(6) =11160a(7) =-48910a(8) =217000a(9) =-976248a(10) =4438920a(11) =-20346280a(12) =93900240a(13) =-435959820a(14) =2034504992a(15) =-9536767660a(16) =44878791360a(17) =-211927516500