2^n + 1 - 2*Fibonacci(n+1).

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =7a(5) =17a(6) =39a(7) =87a(8) =189a(9) =403a(10) =847a(11) =1761a(12) =3631a(13) =7439a(14) =15165a(15) =30795a(16) =62343a(17) =125905a(18) =253783a(19) =510759a(20) =1026685a(21) =2061731a(22) =4136991a(23) =8295873a(24) =16627167a(25) =33311647a(26) =66716029a(27) =133582107a(28) =267406999a(29) =535206833