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 A056682 #23 Jul 04 2021 22:09:12
%S A056682 0,1,3,9,19,25,721,1309,3169,28933,66283,67795
%N A056682 Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056682 Also numbers k such that (4*10^(k+1)+23)/9 is prime.
%C A056682 a(13) > 10^5. - _Robert Price_, Nov 30 2014
%H A056682 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44447.htm#prime">Prime numbers of the form 44...447</a>.
%H A056682 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056682 a(n) = A099413(n+1)-1. - _Robert Price_, Nov 30 2014
%t A056682 Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
%Y A056682 Cf. A002275, A099413.
%K A056682 hard,nonn
%O A056682 1,3
%A A056682 _Robert G. Wilson v_, Aug 10 2000
%E A056682 a(10)-a(12) derived from A099413 by _Robert Price_, Nov 30 2014