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 A319505 #12 Sep 21 2018 11:06:44 %S A319505 1,389993,476729,2927297,3048569,3862529,3914049,4755729,5077209, %T A319505 7460409,9009489,9314009,11066649,11981409,12928289,13342769,15371609, %U A319505 15574449,22975249,34429449,44876649,57046209,66358417,66963369,79552209,83006769,96783929,99127313,118462737,126209609,128566689,139745969 %N A319505 Numbers k such that 2^(k*(k+1)/2) == -1 (mod k^2). %o A319505 (PARI) isok(n) = Mod(2, n^2)^(n*(n+1)/2)==-1; %Y A319505 Cf. A006125, A077816, A126884. %K A319505 nonn %O A319505 1,2 %A A319505 _Altug Alkan_, Sep 21 2018