a(n) = 4*8^n - 3*2^n.

Terms

a(0) =1a(1) =26a(2) =244a(3) =2024a(4) =16336a(5) =130976a(6) =1048384a(7) =8388224a(8) =67108096a(9) =536869376a(10) =4294964224a(11) =34359732224a(12) =274877894656