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

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =6a(5) =24a(6) =81a(7) =250a(8) =732a(9) =2073a(10) =5742a(11) =15664a(12) =42273a(13) =113202a(14) =301428a(15) =799273a(16) =2112774a(17) =5571816a(18) =14668209a(19) =38563882a(20) =101285580a(21) =265817145a(22) =697214430a(23) =1827923296a(24) =4790749761a(25) =12552714594a(26) =32884171236a(27) =86133353545a(28) =225582998262a(29) =590749858968