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

Terms

a(0) =0a(1) =0a(2) =2a(3) =5a(4) =12a(5) =26a(6) =55a(7) =114a(8) =234a(9) =477a(10) =968a(11) =1958a(12) =3951a(13) =7958a(14) =16006a(15) =32157a(16) =64548a(17) =129474a(18) =259559a(19) =520106a(20) =1041810a(21) =2086205a(22) =4176592a(23) =8359950a(24) =16730847a(25) =33479406a(26) =66987470a(27) =134021309a(28) =268117644a(29) =536356682