a(n) = 2*sigma(n) - 1.

Terms

a(0) =1a(1) =5a(2) =7a(3) =13a(4) =11a(5) =23a(6) =15a(7) =29a(8) =25a(9) =35a(10) =23a(11) =55a(12) =27a(13) =47a(14) =47a(15) =61a(16) =35a(17) =77a(18) =39a(19) =83a(20) =63a(21) =71a(22) =47a(23) =119a(24) =61a(25) =83a(26) =79a(27) =111a(28) =59a(29) =143