5^n + 4^n - 1.

Terms

a(0) =1a(1) =8a(2) =40a(3) =188a(4) =880a(5) =4148a(6) =19720a(7) =94508a(8) =456160a(9) =2215268a(10) =10814200a(11) =53022428a(12) =260917840a(13) =1287811988a(14) =6371951080a(15) =31591319948a(16) =156882857920a(17) =780119322308