a(n) = 4^n + 5^n.

Terms

a(0) =2a(1) =9a(2) =41a(3) =189a(4) =881a(5) =4149a(6) =19721a(7) =94509a(8) =456161a(9) =2215269a(10) =10814201a(11) =53022429a(12) =260917841a(13) =1287811989a(14) =6371951081a(15) =31591319949a(16) =156882857921a(17) =780119322309