a(n) = round((2^n - n - 1)/4).

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =7a(5) =14a(6) =30a(7) =62a(8) =126a(9) =253a(10) =509a(11) =1021a(12) =2045a(13) =4092a(14) =8188a(15) =16380a(16) =32764a(17) =65531a(18) =131067a(19) =262139a(20) =524283a(21) =1048570a(22) =2097146a(23) =4194298a(24) =8388602a(25) =16777209a(26) =33554425a(27) =67108857a(28) =134217721a(29) =268435448