a(n) = (2*8^(n+1) - 9) / 7.

Terms

a(0) =1a(1) =17a(2) =145a(3) =1169a(4) =9361a(5) =74897a(6) =599185a(7) =4793489a(8) =38347921a(9) =306783377a(10) =2454267025a(11) =19634136209a(12) =157073089681