a(n) = 3^n - Fibonacci(n).

Terms

a(0) =1a(1) =2a(2) =8a(3) =25a(4) =78a(5) =238a(6) =721a(7) =2174a(8) =6540a(9) =19649a(10) =58994a(11) =177058a(12) =531297a(13) =1594090a(14) =4782592a(15) =14348297a(16) =43045734a(17) =129138566a(18) =387417905a(19) =1162257286a(20) =3486777636a(21) =10460342257a(22) =31381041898a(23) =94143150170a(24) =282429490113a(25) =847288534418