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

Terms

a(0) =1a(1) =10a(2) =88a(3) =736a(4) =6016a(5) =48640a(6) =391168a(7) =3137536a(8) =25133056a(9) =201195520a(10) =1610088448a(11) =12882804736a(12) =103070826496a(13) =824600166400