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 A056685 #34 Sep 08 2022 08:45:01
%S A056685 0,1,7,25,65,73,232,472,539,2773,64714
%N A056685 Numbers k such that 50*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056685 Also numbers k such that (5*10^(k+1)-23)/9 is prime.
%C A056685 a(12) > 10^5. - _Robert Price_, Nov 13 2014
%H A056685 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55553.htm#prime">Prime numbers of the form 55...553</a>.
%H A056685 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056685 a(n) = A099416(n) - 1. - _Robert Price_, Nov 13 2014 [adapted by _Georg Fischer_, Jan 04 2021]
%t A056685 Do[ If[ PrimeQ[50*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]
%t A056685 Select[Range[0, 3000], PrimeQ[(5 10^(# + 1) - 23) / 9] &] (* _Vincenzo Librandi_, Nov 14 2014 *)
%o A056685 (Magma) [n: n in [0..400]| IsPrime((5*10^(n+1)-23)div 9)]; // _Vincenzo Librandi_, Nov 14 2014
%Y A056685 Cf. A002275, A099416.
%K A056685 hard,nonn
%O A056685 1,3
%A A056685 _Robert G. Wilson v_, Aug 10 2000
%E A056685 a(11) derived from A099416 by _Robert Price_, Nov 13 2014