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 A255206 #8 Mar 07 2015 15:33:28 %S A255206 269,331,571,863,883,907,1097,1103,1291,1579,1697,1741,2179,2213,2221, %T A255206 2281,2309,2311,2551,2671,2677,2693,2707,2789,2791,3191,3253,3571, %U A255206 3617,3877,3931,4049,4787,4813,4987,5021,5153,5197,5227,5347,5519,5669,5689,5693 %N A255206 Primes p for which exactly three bases b with 1 < b < p exist such that p is a base b Wieferich prime. %C A255206 p = prime(n) such that A242830(n) = 3. %o A255206 (PARI) forprime(p=1, , b=2; i=0; while(b < p, if(Mod(b, p^2)^(p-1)==1, i++); b++); if(i==3, print1(p, ", "))) %Y A255206 Cf. A255203, A255204, A255205, A255207, A255208, A255209, A255210. %K A255206 nonn %O A255206 1,1 %A A255206 _Felix Fröhlich_, Feb 17 2015