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 A127103 #19 May 21 2024 05:27:11
%S A127103 1,2,4,20,220,1220,2420,5060,13420,14740,23620,55660,145420,147620,
%T A127103 162140,237820,259820,290620,308660,339020,447740,847220,899140,
%U A127103 1210220,1440820,1599620,1759340,2332660,2616020,2858020,3196820,3344660
%N A127103 Numbers k such that k^2 divides 3^k-1.
%C A127103 From _Alexander Adamchuk_, Jan 11 2007: (Start)
%C A127103 2 divides a(n) for n>1. 2^2 divides a(n) for n>2. 5 divides a(n) for n>3.
%C A127103 11 divides a(n) for n = {5,7,8,9,10,12,13,14,15,16,17,18,19,20,22,23,24,26,27, 28,29,30,31,31,33,34,35,...}.
%C A127103 11^2 divides a(n) for n = {7,12,14,15,26,27,29,30,31,33,34,...}.
%C A127103 Prime factors of a(n) in order of their appearance in a(n) are {2,5,11,61,23,67,1181,661,47,1321,367,3851,5501,727,461,269,...}. (End)
%H A127103 Amiram Eldar, <a href="/A127103/b127103.txt">Table of n, a(n) for n = 1..300</a>
%t A127103 Select[Range[30000], IntegerQ[(PowerMod[3, #, #^2 ]-1)/#^2 ]&]
%t A127103 Join[{1},Select[Range[335*10^4],PowerMod[3,#,#^2]==1&]] (* _Harvey P. Dale_, Oct 02 2019 *)
%o A127103 (PARI) is(k) = Mod(3, k^2)^k == 1; \\ _Amiram Eldar_, May 21 2024
%Y A127103 Subset of A067945 (numbers k that divide 3^k - 1).
%Y A127103 Cf. A127100, A127101, A127102, A127104, A127105, A127106, A127107, A127092.
%K A127103 nonn
%O A127103 1,2
%A A127103 _Alexander Adamchuk_, Jan 05 2007
%E A127103 More terms from _Ryan Propper_ and _Alexander Adamchuk_, Jan 05 2007