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 A319538 #11 Sep 25 2018 04:01:10 %S A319538 1,7,217,1057,3937,10447,24601,32767,91657,145337,279527,666967, %T A319538 1412113,2484247,2874847,3124327,4169137,4472167,9526207,12021439, %U A319538 16539337,16646017,19384207,20139367,24639727,28127137,28940887,30583087,66131279,68068777,70694167,72299857,72903847,73498471,87507049 %N A319538 Numbers k such that k divides 2^(2*k+1) - 1. %C A319538 Sequence is infinite because 2^A187787(t) - 1 is a term for all t >= 1. %t A319538 Select[Range[10^5], Mod[2^(2 # + 1) - 1, #] == 0 &] (* _Michael De Vlieger_, Sep 24 2018 *) %o A319538 (PARI) isok(n) = Mod(2, n)^(2*n+1)==1; %Y A319538 Cf. A187787, A296369, A319222. %K A319538 nonn %O A319538 1,2 %A A319538 _Altug Alkan_, Sep 22 2018