Expansion of Product_{k>=1} (1 + 4^(k-1)*x^k).

Terms

a(0) =1a(1) =1a(2) =4a(3) =20a(4) =80a(5) =384a(6) =1600a(7) =7424a(8) =30720a(9) =143360a(10) =593920a(11) =2703360a(12) =11403264a(13) =51118080a(14) =214958080a(15) =965738496a(16) =4047503360a(17) =17951621120a(18) =76168560640a(19) =334202142720