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 A055691 #9 Jul 11 2021 15:31:25
%S A055691 2,6,14,66,186,378,546,1190,1470,2406,4470,5250,7086,18578,20166,
%T A055691 25806,42602,58590,69006,85566,134946,137346,170586,187838,206646,
%U A055691 262146,272610,285390,289394,420210,500570,538734,592410,595686,602250,618450
%N A055691 Numbers k such that 8^k == -1 (mod k-1).
%t A055691 Do[If[PowerMod[8, n, n-1]==n-2, Print[n]], {n, 2, 10^7}]
%K A055691 nonn
%O A055691 1,1
%A A055691 _Robert G. Wilson v_, Jun 09 2000