a(n) = 3*2^n - Fibonacci(n+3) - 1.

Terms

a(0) =0a(1) =2a(2) =6a(3) =15a(4) =34a(5) =74a(6) =157a(7) =328a(8) =678a(9) =1391a(10) =2838a(11) =5766a(12) =11677a(13) =23588a(14) =47554a(15) =95719a(16) =192426a(17) =386450a(18) =775485a(19) =1555152a(20) =3117070a(21) =6245087a(22) =12507886a(23) =25044430a(24) =50135229a(25) =100345484a(26) =200812362a(27) =401821143a(28) =803960098a(29) =1608434426