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 A331424 #33 Apr 22 2025 12:20:28 %S A331424 7,79,6451,2806861 %N A331424 Prime numbers p such that p^2 divides 31^(p-1) - 1. %D A331424 Paulo Ribenboim, The Little Book of Bigger Primes, Springer-Verlag NY 2004. See p. 233. %H A331424 Richard Fischer, <a href="http://www.fermatquotient.com/FermatQuotienten/FermQ_Sort.txt">Fermatquotienten von 2 bis 1052</a>, Dec 19 2019. %H A331424 Wikipedia, <a href="https://en.wikipedia.org/wiki/Wieferich_prime">Wieferich prime</a> %t A331424 Select[Range[3*10^6], PrimeQ[#] && PowerMod[31, # - 1, #^2] == 1 &] (* _Amiram Eldar_, May 05 2021 *) %o A331424 (PARI) forprime(p=2, 1e8, if(Mod(31, p^2)^(p-1)==1, print1(p", "))) %Y A331424 Wieferich primes to base b: A001220 (b=2), A014127 (b=3), A123692 (b=5), A123693 (b=7), A128667 (b=13), A128668 (b=17), A090968 (b=19), A128669 (b=23), this sequence (b=31), A331426 (b=37), A331427 (b=41). %Y A331424 Cf. A039951. %K A331424 nonn,more %O A331424 1,1 %A A331424 _Seiichi Manyama_, Jan 16 2020