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 A056687 #26 Jul 04 2021 22:09:39
%S A056687 1,7,11,17,25,31,137,187,221,337,1001,2743,6529,38689,39463
%N A056687 Numbers k such that 50*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056687 Also numbers k such that (5*10^(k+1)+31)/9 is prime.
%H A056687 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55559.htm#prime">Prime numbers of the form 55...559</a>.
%H A056687 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056687 a(n) = A099418(n) - 1.
%t A056687 Do[ If[ PrimeQ[50*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
%Y A056687 Cf. A002275, A093403, A099418.
%K A056687 nonn
%O A056687 1,2
%A A056687 _Robert G. Wilson v_, Aug 10 2000
%E A056687 6529 from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056687 38689 and 39463 from Serge Batalov, Jan 06 2009 confirmed as next terms by _Ray Chandler_, Feb 20 2012