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 A055687 #9 Jul 11 2021 15:37:03
%S A055687 2,18,4098,7362,85074,658530,3999138,72281282,285143058,628944690,
%T A055687 854112114,1423081170,2561019282,3111576930,4298117634,5921265042,
%U A055687 14224884930,21336998130,34317377234,50723421714,63797137890,144269032050,163834314354,187397322210,212565453282,231286579794,232992827298,266198715090,314834584482,348160787730,580045717650,706501722498
%N A055687 Numbers k such that 4^k == -1 (mod k-1).
%t A055687 Do[If[PowerMod[4, n, n-1]==n-2, Print[n]], {n, 2, 10^8}]
%K A055687 nonn
%O A055687 1,1
%A A055687 _Robert G. Wilson v_, Jun 09 2000
%E A055687 Terms a(9) onward from _Max Alekseyev_, Oct 12 2012