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

Terms

a(0) =1a(1) =3a(2) =9a(3) =25a(4) =67a(5) =177a(6) =465a(7) =1219a(8) =3193a(9) =8361a(10) =21891a(11) =57313a(12) =150049a(13) =392835a(14) =1028457a(15) =2692537a(16) =7049155a(17) =18454929a(18) =48315633a(19) =126491971a(20) =331160281a(21) =866988873a(22) =2269806339a(23) =5942430145a(24) =15557484097a(25) =40730022147a(26) =106632582345a(27) =279167724889a(28) =730870592323