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 A056683 #29 Sep 08 2022 08:45:01
%S A056683 2,4,5,47,107,244,1043,20207,52739,89188
%N A056683 Numbers k such that 40*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056683 Also numbers k such that (4*10^(k+1) + 41)/9 is prime.
%C A056683 a(11) > 10^5. - _Robert Price_, Nov 09 2014
%H A056683 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44449.htm#prime">Prime numbers of the form 44...449</a>.
%H A056683 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056683 a(n) = A099414(n+1) - 1. - _Robert Price_, Nov 09 2014
%t A056683 Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
%o A056683 (Magma) [n: n in [0..300] | IsPrime((4*10^(n+1)+41)div 9)]; _Vincenzo Librandi_, Nov 10 2014
%Y A056683 Cf. A002275, A099414.
%K A056683 hard,nonn
%O A056683 1,1
%A A056683 _Robert G. Wilson v_, Aug 10 2000
%E A056683 a(8)-a(10) derived from A099414 by _Robert Price_, Nov 09 2014