a(n) = Sum_{ k, k|n } 2^(k-1).

Terms

a(0) =1a(1) =3a(2) =5a(3) =11a(4) =17a(5) =39a(6) =65a(7) =139a(8) =261a(9) =531a(10) =1025a(11) =2095a(12) =4097a(13) =8259a(14) =16405a(15) =32907a(16) =65537a(17) =131367a(18) =262145a(19) =524827a(20) =1048645a(21) =2098179a(22) =4194305a(23) =8390831a(24) =16777233a(25) =33558531a(26) =67109125a(27) =134225995a(28) =268435457a(29) =536887863