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 A099417 #21 Jul 08 2021 03:12:17
%S A099417 0,1,3,4,6,10,15,22,88,207,528,960,2100,18522,36783,38517
%N A099417 Numbers k such that 5*R_k + 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099417 Also numbers k such that (5*10^k + 13)/9 is prime.
%C A099417 a(17) > 10^5. - _Robert Price_, Oct 18 2014
%H A099417 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55557.htm#prime">Prime numbers of the form 55...557</a>.
%H A099417 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099417 a(n) = A056686(n-1) + 1.
%t A099417 Do[ If[ PrimeQ[ 5(10^n - 1)/9 + 2], Print[n]], {n, 0, 5000}]
%Y A099417 Cf. A002275, A056686.
%K A099417 nonn
%O A099417 1,3
%A A099417 _Robert G. Wilson v_, Oct 14 2004
%E A099417 a(14)-a(16) from _Robert Price_, Oct 18 2014