a(n) = 0^n + 4^n - 1.

Terms

a(0) =1a(1) =3a(2) =15a(3) =63a(4) =255a(5) =1023a(6) =4095a(7) =16383a(8) =65535a(9) =262143a(10) =1048575a(11) =4194303a(12) =16777215a(13) =67108863a(14) =268435455a(15) =1073741823a(16) =4294967295a(17) =17179869183a(18) =68719476735a(19) =274877906943