a(n) = (3*8^n + 0^n)/4.

Terms

a(0) =1a(1) =6a(2) =48a(3) =384a(4) =3072a(5) =24576a(6) =196608a(7) =1572864a(8) =12582912a(9) =100663296a(10) =805306368a(11) =6442450944a(12) =51539607552a(13) =412316860416