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

Terms

a(0) =1a(1) =5a(2) =34a(3) =260a(4) =2056a(5) =16400a(6) =131104a(7) =1048640a(8) =8388736a(9) =67109120a(10) =536871424a(11) =4294968320a(12) =34359740416a(13) =274877911040