a(n) = 3*4^(n-1), n>0; a(0)=1.

Terms

a(0) =1a(1) =3a(2) =12a(3) =48a(4) =192a(5) =768a(6) =3072a(7) =12288a(8) =49152a(9) =196608a(10) =786432a(11) =3145728a(12) =12582912a(13) =50331648a(14) =201326592a(15) =805306368a(16) =3221225472a(17) =12884901888a(18) =51539607552a(19) =206158430208a(20) =824633720832