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 A055685 #23 Jul 11 2021 20:15:42
%S A055685 2,6,66,378,1190,1470,25806,58590,134946,137346,170586,272610,285390,
%T A055685 420210,538734,592410,618450,680706,778806,1163066,1520442,1700946,
%U A055685 2099202,2831010,4020030,4174170,4516110,5059890,5215770
%N A055685 Numbers k such that 2^k == -1 (mod k-1).
%C A055685 The sequence is infinite. In fact, even its intersection with A006517 (given by A219037) is infinite.
%H A055685 Chai Wah Wu, <a href="/A055685/b055685.txt">Table of n, a(n) for n = 1..1192</a>
%H A055685 Kin Y. Li et al., <a href="http://www.math.ust.hk/excalibur/v14_n2.pdf">Solution to Problem 323</a>, Mathematical Excalibur 14(2), 2009, p. 3.
%F A055685 a(n) = A296369(n) + 1.
%t A055685 Do[If[PowerMod[2, n, n-1]==n-2, Print[n]], {n, 2, 12900000}]
%o A055685 (Python)
%o A055685 A055685_list = [n for n in range(2,10**6) if pow(2,n,n-1) == n-2] # _Chai Wah Wu_, Nov 04 2019
%Y A055685 Cf. A064935, A006517, A219037, A296369.
%K A055685 nonn
%O A055685 1,1
%A A055685 _Robert G. Wilson v_, Jun 09 2000
%E A055685 Edited by _Max Alekseyev_, Oct 11 2012
%E A055685 Incorrect term 4285390 removed by _Chai Wah Wu_, Nov 04 2019