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 A099415 #26 Sep 08 2022 08:45:15
%S A099415 12,13,609
%N A099415 Numbers k such that 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099415 Also numbers k such that (5*10^k - 41)/9 is prime.
%C A099415 a(4) > 10^5. - _Robert Price_, Nov 16 2014
%H A099415 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55551.htm#prime">Prime numbers of the form 55...551</a>.
%H A099415 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099415 a(n) = A056684(n) + 1. - _Robert Price_, Nov 17 2014
%p A099415 A099415:=n->`if`(isprime((5*10^n-41)/9), n, NULL): seq(A099415(n), n=1..10^3); # _Wesley Ivan Hurt_, Nov 16 2014
%t A099415 Do[ If[ PrimeQ[ 5(10^n - 1)/9 - 4], Print[n]], {n, 15000}]
%t A099415 Select[Range[10000], PrimeQ[(5 10^# - 41) / 9] &] (* _Vincenzo Librandi_, Nov 17 2014 *)
%o A099415 (Magma) [n: n in [0..500] | IsPrime((5*10^n-41) div 9)]; // _Vincenzo Librandi_, Nov 17 2014
%Y A099415 Cf. A002275, A056684.
%K A099415 nonn,bref,more
%O A099415 1,1
%A A099415 _Robert G. Wilson v_, Oct 14 2004