a(n) = 3*2^n.

Terms

a(0) =3a(1) =6a(2) =12a(3) =24a(4) =48a(5) =96a(6) =192a(7) =384a(8) =768a(9) =1536a(10) =3072a(11) =6144a(12) =12288a(13) =24576a(14) =49152a(15) =98304a(16) =196608a(17) =393216a(18) =786432a(19) =1572864a(20) =3145728a(21) =6291456a(22) =12582912a(23) =25165824a(24) =50331648a(25) =100663296a(26) =201326592a(27) =402653184a(28) =805306368a(29) =1610612736