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

Terms

a(0) =1a(1) =5a(2) =9a(3) =19a(4) =37a(5) =73a(6) =143a(7) =281a(8) =553a(9) =1091a(10) =2157a(11) =4273a(12) =8479a(13) =16849a(14) =33521a(15) =66755a(16) =133045a(17) =265337a(18) =529455a(19) =1056937a(20) =2110681a(21) =4216195a(22) =8424029a(23) =16834529a(24) =33647167a(25) =67258913a(26) =134460513a(27) =268828291a(28) =537506533a(29) =1074770281