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 A345402 #22 Sep 29 2021 08:32:38
%S A345402 2,1319,337081
%N A345402 Numbers k such that (42^k-1)/41 is prime.
%C A345402 These are the repunit primes in base 42.
%H A345402 P. Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>
%e A345402 2 is a member since (42^2-1)/41 = 43 is prime.
%t A345402 Do[ If[ PrimeQ[ (42^n-1)/41], Print[n]], {n, 2, 400000}]
%o A345402 (PARI) is(n)=isprime((42^n-1)/41)
%Y A345402 Cf. A218745.
%K A345402 nonn,hard,more,bref
%O A345402 1,1
%A A345402 _Paul Bourdelais_, Sep 28 2021