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

Terms

a(0) =1a(1) =5a(2) =9a(3) =69a(4) =33a(5) =2061a(6) =129a(7) =16453a(8) =4105a(9) =131109a(10) =2049a(11) =134219853a(12) =8193a(13) =8388741a(14) =8388649a(15) =1073758277a(16) =131073a(17) =274877913101a(18) =524289a(20) =2147483785a(21) =34359740421a(22) =8388609a(24) =1073741857a(26) =549755817993a(28) =536870913