a(n) = Sum_{d|n} 4^(d-1).

Terms

a(0) =1a(1) =5a(2) =17a(3) =69a(4) =257a(5) =1045a(6) =4097a(7) =16453a(8) =65553a(9) =262405a(10) =1048577a(11) =4195413a(12) =16777217a(13) =67112965a(14) =268435729a(15) =1073758277a(16) =4294967297a(17) =17179935765a(18) =68719476737a(19) =274878169413