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 A242982 #20 May 17 2019 14:43:17 %S A242982 281,46457,9377747,122959073 %N A242982 Primes p such that p^2 divides 20^(p-1) - 1. %C A242982 Base 20 Wieferich primes. According to Richard Fischer, there is no other term up to approximately 5*10^13. %H A242982 Amir Akbary and Sahar Siavashi, <a href="http://math.colgate.edu/~integers/s3/s3.Abstract.html">The Largest Known Wieferich Numbers</a>, INTEGERS, 18(2018), A3. See Table 1 p. 5. %H A242982 R. Fischer, <a href="http://www.fermatquotient.com/FermatQuotienten/FermQ_Sort">Thema: Fermatquotient B^(P-1) == 1 (mod P^2)</a> %t A242982 Select[Prime[Range[1000000]], PowerMod[20, # - 1, #^2] == 1 &] (* _Robert Price_, May 17 2019 *) %o A242982 (PARI) forprime(n=2, 10^9, if(Mod(20, n^2)^(n-1)==1, print1(n, ", "))); %Y A242982 Cf. A001220, A014127, A123692, A212583, A123693, A045616, A111027, A039951, A096082. %K A242982 nonn,hard,bref,more %O A242982 1,1 %A A242982 _Felix Fröhlich_, May 28 2014