This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A319222 #27 Jul 17 2021 06:43:46 %S A319222 1,3,129,2537,51889,101617,226873,270427,653467,945667,1740979, %T A319222 5819937,6520987,9828587,15452867,24950857,51377539,89519449, %U A319222 108627601,135776371,160126609,296338873,310026163,400431289,641706243,643359937,678257563,803419697,902661523,952431331,1004273987,1243893697,1796055907 %N A319222 Numbers k such that k divides 2^(2k+1) + 1. %C A319222 Also, numbers k such that 4^k == -1/2 (mod k) (cf. A296369). - _Max Alekseyev_, Sep 15 2018 %C A319222 If k is in the sequence, and m is another divisor of 2^(2*k+1)+1 and is coprime to k, then m*k is in the sequence. - _Robert Israel_, Sep 14 2018 %p A319222 filter:= n -> 2 &^ (2*n+1)+1 mod n = 0: %p A319222 select(filter, [$1..10^7]); # _Robert Israel_, Sep 14 2018 %o A319222 (PARI) is_A319222(n) = Mod(2, n)^(2*n+1)==-1; %Y A319222 Cf. A006517, A006521, A015950. %K A319222 nonn %O A319222 1,2 %A A319222 _Altug Alkan_, Sep 13 2018