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

Terms

a(0) =0a(1) =1a(2) =4a(3) =12a(4) =33a(5) =88a(6) =232a(7) =609a(8) =1596a(9) =4180a(10) =10945a(11) =28656a(12) =75024a(13) =196417a(14) =514228a(15) =1346268a(16) =3524577a(17) =9227464a(18) =24157816a(19) =63245985a(20) =165580140a(21) =433494436a(22) =1134903169a(23) =2971215072a(24) =7778742048a(25) =20365011073a(26) =53316291172a(27) =139583862444a(28) =365435296161a(29) =956722026040