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 A255207 #8 Mar 07 2015 15:33:37 %S A255207 487,1163,2203,2731,3373,4391,5261,6343,6451,6569,7753,8377,8863,9041, %T A255207 9397,9463,9941,10079,10321,11783,12703,13121,13151,13807,13903,14419, %U A255207 15061,15263,15313,15601,16631,16883,16943,17477,17519,18253,18773,20173,22279,23291 %N A255207 Primes p for which exactly four bases b with 1 < b < p exist such that p is a base b Wieferich prime. %C A255207 p = prime(n) such that A242830(n) = 4. %o A255207 (PARI) forprime(p=1, , b=2; i=0; while(b < p, if(Mod(b, p^2)^(p-1)==1, i++); b++); if(i==4, print1(p, ", "))) %Y A255207 Cf. A255203, A255204, A255205, A255206, A255208, A255209, A255210. %K A255207 nonn %O A255207 1,1 %A A255207 _Felix Fröhlich_, Feb 17 2015