Product_{k>=1} 1/(1 - x^k)^a(k) = 1 + 4x.

Terms

a(0) =4a(1) =-10a(2) =20a(3) =-60a(4) =204a(5) =-690a(6) =2340a(7) =-8160a(8) =29120a(9) =-104958a(10) =381300a(11) =-1397740a(12) =5162220a(13) =-19175130a(14) =71582716a(15) =-268431360a(16) =1010580540a(17) =-3817763040a(18) =14467258260a(19) =-54975528948a(20) =209430785460a(21) =-799645010850