a(n) = (8^n + 2*(-1)^n)/3.

Terms

a(0) =1a(1) =2a(2) =22a(3) =170a(4) =1366a(5) =10922a(6) =87382a(7) =699050a(8) =5592406a(9) =44739242a(10) =357913942a(11) =2863311530a(12) =22906492246a(13) =183251937962