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 A090968 #21 Apr 03 2023 10:36:10
%S A090968 3,7,13,43,137,63061489
%N A090968 Primes p such that p^2 divides 19^(p-1) - 1.
%C A090968 Primes p such that p divides the Fermat quotient of p (with base 19). The Fermat quotient of p with base a denotes the integer q_p(a) = ( a^(p-1) - 1) / p, where p is a prime which does not divide the integer a. - C. Ronaldo (aga_new_ac(AT)hotmail.com), Jan 20 2005
%C A090968 No further terms up to 3.127*10^13.
%D A090968 J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 43, p. 17, Ellipses, Paris 2008.
%D A090968 Paulo Ribenboim, The Little Book Of Big Primes, Springer-Verlag, NY 1991, page 170.
%D A090968 Roozbeh Hazrat, Mathematica: A Problem-Centered Approach, Springer 2010, pp. 39, 171. [_Harvey P. Dale_, Oct 17 2011]
%H A090968 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 A090968 C. Caldwell, <a href="https://t5k.org/glossary/page.php?sort=FermatQuotient">Fermat quotient</a>
%H A090968 W. Keller and J. Richstein <a href="http://www1.uni-hamburg.de/RRZ/W.Keller/FermatQuotient.html">Fermat quotients q_p(a) that are divisible by p</a>
%t A090968 NextPrim[n_] := Block[{k = n + 1}, While[ !PrimeQ[k], k++ ]; k]; p = 1; Do[ p = NextPrim[p]; If[PowerMod[19, p - 1, p^2] == 1, Print[p]], {n, 1, 2*10^8}]
%t A090968 Select[Prime[Range[4*10^6]],PowerMod[19,#-1,#^2]==1&] (* _Harvey P. Dale_, Nov 08 2017 *)
%Y A090968 Cf. A001220, A014127, A123692, A123693, A128667, A128668, A128669, A039951, A096082
%K A090968 nonn,hard,more
%O A090968 1,1
%A A090968 _Robert G. Wilson v_, Feb 27 2004