a(n) = Fibonacci(n) + Fibonacci(2n+1).

Terms

a(0) =1a(1) =3a(2) =6a(3) =15a(4) =37a(5) =94a(6) =241a(7) =623a(8) =1618a(9) =4215a(10) =11001a(11) =28746a(12) =75169a(13) =196651a(14) =514606a(15) =1346879a(16) =3525565a(17) =9229062a(18) =24160401a(19) =63250167a(20) =165586906a(21) =433505383a(22) =1134920881a(23) =2971243730a(24) =7778788417a(25) =20365086099a(26) =53316412566a(27) =139584058863