a(n) = Sum_{d|n} (-1)^(d-1) * 2^(n/d-1).

Terms

a(0) =1a(1) =1a(2) =5a(3) =5a(4) =17a(5) =29a(6) =65a(7) =117a(8) =261a(9) =497a(10) =1025a(11) =2017a(12) =4097a(13) =8129a(14) =16405a(15) =32629a(16) =65537a(17) =130845a(18) =262145a(19) =523765a(20) =1048645a(21) =2096129a(22) =4194305a(23) =8386641a(24) =16777233a(25) =33550337a(26) =67109125a(27) =134209477a(28) =268435457a(29) =536855053