a(n) = 9*Fibonacci(2n+1) - 1.

Terms

a(0) =8a(1) =17a(2) =44a(3) =116a(4) =305a(5) =800a(6) =2096a(7) =5489a(8) =14372a(9) =37628a(10) =98513a(11) =257912a(12) =675224a(13) =1767761a(14) =4628060a(15) =12116420a(16) =31721201a(17) =83047184a(18) =217420352a(19) =569213873a(20) =1490221268a(21) =3901449932a(22) =10214128529a(23) =26740935656a(24) =70008678440a(25) =183285099665a(26) =479846620556