Expansion of g.f. (1+x)/(1-2*x).

Terms

a(0) =1a(1) =3a(2) =6a(3) =12a(4) =24a(5) =48a(6) =96a(7) =192a(8) =384a(9) =768a(10) =1536a(11) =3072a(12) =6144a(13) =12288a(14) =24576a(15) =49152a(16) =98304a(17) =196608a(18) =393216a(19) =786432a(20) =1572864a(21) =3145728a(22) =6291456a(23) =12582912a(24) =25165824a(25) =50331648a(26) =100663296a(27) =201326592a(28) =402653184a(29) =805306368