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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =8a(5) =19a(6) =43a(7) =94a(8) =201a(9) =423a(10) =880a(11) =1815a(12) =3719a(13) =7582a(14) =15397a(15) =31171a(16) =62952a(17) =126891a(18) =255379a(19) =513342a(20) =1030865a(21) =2068495a(22) =4147936a(23) =8313583a(24) =16655823a(25) =33358014a(26) =66791053a(27) =133703499a(28) =267603416a(29) =535524643