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 A237251 #26 Jan 04 2020 15:05:00 %S A237251 2,3,5,17,257,16487 %N A237251 Primes p such that p*2^(p-1)-1 is prime. %C A237251 The fifth Fermat prime, 65537, is not in the sequence: 65537*2^65536-1 is composite (per PFGW). - _Michael B. Porter_, Feb 11 2014 %C A237251 Also 65537*2^65536-1 is divisible by 16267 and 2058772459. - _Jeppe Stig Nielsen_, Jan 04 2020 %o A237251 (PARI) isok(p) = isprime(p) && isprime(p*2^(p-1) - 1); \\ _Michel Marcus_, Feb 06 2014 %Y A237251 Cf. A019434, A092506, A230769, A236752. %K A237251 nonn,more,hard %O A237251 1,1 %A A237251 _Gerasimov Sergey_, Feb 05 2014 %E A237251 a(5) from _Ralf Stephan_, Feb 03 2014 %E A237251 a(6) = A230769(26)+1 appended by _Jeppe Stig Nielsen_, Jan 04 2020