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 A055686 #14 Jul 23 2021 05:25:04
%S A055686 2,3,5,6,15,245,366,435,855,2295,3795,5967,7875,10878,26475,33915,
%T A055686 117615,188775,231435,284355,487635,501039,589155,593775,621675,
%U A055686 755595,1255815,1306935,1642095,1911195,2193125,2434755,2484675,2507835
%N A055686 Numbers k such that 3^k == -1 (mod k-1).
%H A055686 Amiram Eldar, <a href="/A055686/b055686.txt">Table of n, a(n) for n = 1..1000</a>
%t A055686 Do[If[PowerMod[3, n, n-1]==n-2, Print[n]], {n, 2, 10^7}]
%Y A055686 Cf. A055685, A055687, A055688, A055689, A055690, A055691, A055692, A055693, A055694, A055695.
%K A055686 nonn
%O A055686 1,1
%A A055686 _Robert G. Wilson v_, Jun 09 2000