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