Prime factors of numbers in A006521 (numbers k that divide 2^k + 1).

Terms

a(0) =3a(1) =19a(2) =163a(3) =571a(4) =1459a(5) =8803a(6) =9137a(7) =17497a(8) =41113a(9) =52489a(10) =78787a(11) =87211a(12) =135433a(13) =139483a(14) =144667a(15) =164617a(16) =174763a(17) =196579a(18) =274081a(19) =370009a(20) =370387a(21) =478243a(22) =760267a(23) =941489a(24) =944803a(25) =1041619a(26) =1220347a(27) =1236787a(28) =1319323a(29) =1465129