Duffinian numbers: composite numbers k relatively prime to sigma(k).

Terms

a(0) =4a(1) =8a(2) =9a(3) =16a(4) =21a(5) =25a(6) =27a(7) =32a(8) =35a(9) =36a(10) =39a(11) =49a(12) =50a(13) =55a(14) =57a(15) =63a(16) =64a(17) =65a(18) =75a(19) =77a(20) =81a(21) =85a(22) =93a(23) =98a(24) =100a(25) =111a(26) =115a(27) =119a(28) =121a(29) =125