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

Terms

a(0) =0a(1) =3a(2) =7a(3) =21a(4) =54a(5) =144a(6) =376a(7) =987a(8) =2583a(9) =6765a(10) =17710a(11) =46368a(12) =121392a(13) =317811a(14) =832039a(15) =2178309a(16) =5702886a(17) =14930352a(18) =39088168a(19) =102334155a(20) =267914295a(21) =701408733a(22) =1836311902a(23) =4807526976a(24) =12586269024a(25) =32951280099a(26) =86267571271a(27) =225851433717a(28) =591286729878