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 A128399 #15 Jun 11 2021 04:59:00
%S A128399 1,2,3,4,6,9,10,12,18,20,30,36,42,60,84,90,110,126,156,180,210,220,
%T A128399 252,294,330,381,420,468,588,630,660,724,762,780,882,930,990,1092,
%U A128399 1143,1260,1332,1470,1510,1524,1764,1806,1830,1860,1980,2028,2058,2172,2286,2310
%N A128399 Numbers k such that k^2 divides 19^k-1.
%H A128399 Seiichi Manyama, <a href="/A128399/b128399.txt">Table of n, a(n) for n = 1..5000</a>
%t A128399 Join[{1},Select[Range[3000],PowerMod[19,#,#^2]==1&]] (* _Harvey P. Dale_, Oct 24 2017 *)
%o A128399 (PARI) for(k=1, 1e4, if(Mod(19, k^2)^k==1, print1(k", "))) \\ _Seiichi Manyama_, Mar 25 2020
%Y A128399 Columns k=19 of A333500.
%Y A128399 Cf. A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128400, A128401, A128402, A128403, A128404.
%K A128399 nonn
%O A128399 1,2
%A A128399 _Alexander Adamchuk_, Mar 01 2007