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

Terms

a(0) =1a(1) =3a(2) =5a(3) =13a(4) =17a(5) =55a(6) =65a(7) =201a(8) =293a(9) =779a(10) =1025a(11) =3365a(12) =4097a(13) =12303a(14) =17781a(15) =49681a(16) =65537a(17) =204547a(18) =262145a(19) =791549a(20) =1095429a(21) =3145751a(22) =4194305a(23) =12897625a(24) =16787217a(25) =50331675a(26) =68788805a(27) =201591509a(28) =268435457a(29) =815505231