a(n) = (8^n + 4^n)/2.

Terms

a(0) =1a(1) =6a(2) =40a(3) =288a(4) =2176a(5) =16896a(6) =133120a(7) =1056768a(8) =8421376a(9) =67239936a(10) =537395200a(11) =4297064448a(12) =34368126976a(13) =274911461376