a(n) = 4^n + 3^(n + 1) - 2.

Terms

a(0) =11a(1) =41a(2) =143a(3) =497a(4) =1751a(5) =6281a(6) =22943a(7) =85217a(8) =321191a(9) =1225721a(10) =4725743a(11) =18371537a(12) =71891831a(13) =282784361a(14) =1116788543a(15) =4424107457a(16) =17567289671a(17) =69881738201a(18) =278364691343