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 A128361 #9 Oct 22 2016 09:45:06
%S A128361 2,19,6,17,218,15,14,13,12,11,86,9249,214,133,69,4084085,106,39,422,
%T A128361 581831,23,5053,38,9237,26,775,46,1253,206,51,82,671,34,617741981,58,
%U A128361 45,202,289,87,6401,185,217,341,3485351,66,2718013,394,111,56,8064317,75
%N A128361 a(n) = least k such that the remainder when 21^k is divided by k is n.
%H A128361 Robert G. Wilson v, <a href="/A128361/a128361.txt">Table of n, a(n) for n = 1..10000 with -1 for large entries where a(n) has not yet been found</a>
%t A128361 t = Table[0, {10000}]; k = 1; While[k < 3000000000, a = PowerMod[21, k, k]; If[a < 10001 && t[[a]] == 0, t[[a]] = k]; k++ ]; t (* _Robert G. Wilson v_, Jun 25 2009 *)
%t A128361 lk[n_]:=Module[{k=1},While[PowerMod[21,k,k]!=n,k++];k]; Array[lk,60] (* The program takes a long time to run *) (* _Harvey P. Dale_, Oct 22 2016 *)
%Y A128361 Cf. A128362, A128363, A128364, A128365, A128366, A128367, A128368, A128369, A129370, A128371, A128372.
%Y A128361 Cf. A036236, A078457, A119678, A119679, A127816, A119715, A119714, A127817, A127818, A127819, A127820, A127821, A128154, A128155, A128156, A128157, A128158, A128159, A128160.
%Y A128361 Cf. A128149, A128150, A128172.
%K A128361 hard,nonn
%O A128361 1,1
%A A128361 _Alexander Adamchuk_, Feb 27 2007
%E A128361 a(16) - a(51) from _Robert G. Wilson v_, Jun 25 2009