a(n) = (6^n + (-4)^n)/2.

Terms

a(0) =1a(1) =1a(2) =26a(3) =76a(4) =776a(5) =3376a(6) =25376a(7) =131776a(8) =872576a(9) =4907776a(10) =30757376a(11) =179301376a(12) =1096779776a(13) =6496792576a(14) =39316299776a(15) =234555621376