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

Terms

a(0) =0a(1) =2a(2) =7a(3) =20a(4) =54a(5) =143a(6) =376a(7) =986a(8) =2583a(9) =6764a(10) =17710a(11) =46367a(12) =121392a(13) =317810a(14) =832039a(15) =2178308a(16) =5702886a(17) =14930351a(18) =39088168a(19) =102334154a(20) =267914295a(21) =701408732a(22) =1836311902a(23) =4807526975a(24) =12586269024a(25) =32951280098a(26) =86267571271a(27) =225851433716