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 A128400 #28 Jun 06 2021 09:05:39 %S A128400 1,19,1432001198261,3661225986659,275941052631578947368421, %T A128400 43838606653900416716028459121,26357241068661238901986943659181 %N A128400 Numbers k such that k^2 divides 20^k - 1. %C A128400 19 divides a(n) for n > 1. %C A128400 Subsequence of A128360. %o A128400 (PARI) for( n=1, 10^6, Mod(20,n^2)^n - 1 || print1(n",")) \\ _M. F. Hasler_, Oct 20 2008 %Y A128400 Cf. A128360, A127103, A127104, A127105, A127106, A127107, A127102, A127101, A127100, A127092, A128405, A128393, A128394, A128395, A128396, A128397, A128398, A128399, A128400, A128401, A128402, A128403, A128404. %K A128400 nonn,hard,more %O A128400 1,2 %A A128400 _Alexander Adamchuk_, Mar 08 2007 %E A128400 a(4) from _Max Alekseyev_, Oct 18 2008 %E A128400 a(5)-a(7) from _Max Alekseyev_, May 06 2010