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 A056684 #26 Jul 04 2021 22:09:20
%S A056684 11,12,608
%N A056684 Numbers k such that 50*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056684 Also numbers k such that (5*10^(k+1)-41)/9 is prime.
%C A056684 a(4) > 10^5. - _Robert Price_, Nov 16 2014
%H A056684 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55551.htm#prime">Prime numbers of the form 55...551</a>.
%H A056684 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056684 a(n) = A099415(n) - 1. - _Robert Price_, Nov 16 2014
%t A056684 Do[ If[ PrimeQ[50*(10^n - 1)/9 + 1], Print[n]], {n, 15000}]
%Y A056684 Cf. A002275, A099415.
%K A056684 hard,nonn,bref,more
%O A056684 1,1
%A A056684 _Robert G. Wilson v_, Aug 10 2000