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

Terms

a(0) =5a(1) =-15a(2) =40a(3) =-150a(4) =624a(5) =-2620a(6) =11160a(7) =-48750a(8) =217000a(9) =-976872a(10) =4438920a(11) =-20343700a(12) =93900240a(13) =-435970980a(14) =2034504992a(15) =-9536718750a(16) =44878791360a(17) =-211927733500