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 A127104 #21 May 25 2024 09:10:44
%S A127104 1,3,21,903,2667,7077,113799,114681,304311,389193,898779,932799,
%T A127104 4893357,6099429,8131683,8776257,14452473,38350263,38647497,40647747,
%U A127104 49427511,99583113,118465473,128794323,131158041,152643813,262275447,300510651,314353263,335873559,349662369
%N A127104 Numbers k such that k^2 divides 4^k-1.
%C A127104 From _Alexander Adamchuk_, Jan 11 2007: (Start)
%C A127104 3 divides a(n) for n > 1.
%C A127104 7 divides a(n) for n > 2.
%C A127104 43 divides a(n) for n = {4, 8, 9, 10, 12, 13, 16, ...}.
%C A127104 127 divides a(n) for n = {5, 8, 11, 14, 15, 17, ...}.
%C A127104 Prime factors of a(n) in order of their appearance in {a(n)} are {3, 7, 43, 127, 337, 5419, 431, 1033, 5419, 2287, 3049, 9719, ...}. (End)
%H A127104 Amiram Eldar, <a href="/A127104/b127104.txt">Table of n, a(n) for n = 1..56</a>
%t A127104 Select[Range[30000], IntegerQ[(PowerMod[4, #, #^2 ]-1)/#^2 ]&]
%o A127104 (PARI) is(k) = Mod(4, k^2)^k == 1; \\ _Amiram Eldar_, May 25 2024
%Y A127104 Cf. A127100, A127101, A127102, A127103, A127105, A127106, A127107, A127092.
%Y A127104 Subset of A014945 (numbers k such that k divides 4^nk-1).
%K A127104 nonn
%O A127104 1,2
%A A127104 _Alexander Adamchuk_, Jan 05 2007
%E A127104 More terms from _Ryan Propper_ and _Alexander Adamchuk_, Jan 05 2007
%E A127104 a(27)-a(31) from _Amiram Eldar_, May 25 2024