a(n) = Fibonacci((prime(n)+3)/2) - 1.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =20a(5) =54a(6) =88a(7) =232a(8) =986a(9) =1596a(10) =6764a(11) =17710a(12) =28656a(13) =75024a(14) =317810a(15) =1346268a(16) =2178308a(17) =9227464a(18) =24157816a(19) =39088168a(20) =165580140a(21) =433494436a(22) =1836311902a(23) =12586269024a(24) =32951280098a(25) =53316291172a(26) =139583862444a(27) =225851433716