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 A130421 #24 Feb 26 2022 16:34:54
%S A130421 1,2,14,1022,20066,80519,107663,485918,1284113,1510313,2531678,
%T A130421 3677198,3933023,4557713,8277458,8893262,21122318,24849833,26358638,
%U A130421 39852014,42448478,71871113,76712318,80646143,98058097,104832833,106694033,131492498,144322478,146987033,164360606,168204191,175126478,176647378,188997463,196705598
%N A130421 Numbers k such that 4^k == 2 (mod k).
%C A130421 Some terms above 10^15: 26435035805519327, 158975398896178078, 64044049390757098943, 1063423126446412987081943, 1091220919655042176978844783, 81074850280355100090334498663, 6317483763950169936179578094903, 5672799393875320397186007124651847, 170923900137537174138295268515194974, 195746953975871672436191077726091399305155458, 325665752547333314939363628501536024940097079718953, 953533053776414279913696071891872697927468471633033, 85791212788381063775490416118630897060666265030605503, 334519297382630382793758729321508383611586565722054114034741260213364710519401967713. - _Max Alekseyev_, Jun 18 2014
%H A130421 Max Alekseyev, <a href="/A130421/b130421.txt">Table of n, a(n) for n = 1..2617</a> (all terms below 10^15)
%t A130421 Join[{1,2},Select[Range[107000000],PowerMod[4,#,#]==2&]] (* _Harvey P. Dale_, Jun 13 2013 *)
%Y A130421 Cf. A006935 (odd terms times 2), A130422, A347906 (odd terms), A347908 (even terms).
%K A130421 nonn
%O A130421 1,2
%A A130421 _Jon E. Schoenfield_, May 26 2007
%E A130421 Terms a(28) onward from _Max Alekseyev_, Jun 18 2014
%E A130421 b-file corrected by _Max Alekseyev_, Oct 09 2016