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

Terms

a(0) =-2a(1) =-1a(2) =1a(3) =6a(4) =19a(5) =53a(6) =142a(7) =375a(8) =985a(9) =2582a(10) =6763a(11) =17709a(12) =46366a(13) =121391a(14) =317809a(15) =832038a(16) =2178307a(17) =5702885a(18) =14930350a(19) =39088167a(20) =102334153a(21) =267914294a(22) =701408731a(23) =1836311901a(24) =4807526974a(25) =12586269023a(26) =32951280097a(27) =86267571270a(28) =225851433715a(29) =591286729877