a(1) = 4, a(2) = 12, for n>1: a(n) = 3*4^(n-1).

Terms

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