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 A056680 #27 Jul 04 2021 22:08:49
%S A056680 0,1,2,5,45,393,977,2585,2810,2967,3641,4826,4917,5591,5705,10682,
%T A056680 12890,14117,74349,88679
%N A056680 Numbers k such that 30*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056680 Also numbers k such that (10^(k+1)+11)/3 is prime.
%C A056680 a(21) > 10^5. - _Robert Price_, Nov 02 2014
%H A056680 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33337.htm#prime">Prime numbers of the form 33...337</a>.
%H A056680 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056680 a(n) = A099411(n) - 1. - _Robert Price_, Nov 02 2014
%t A056680 Do[ If[ PrimeQ[30*(10^n - 1)/9 + 7], Print[n]], {n, 0, 10000}]
%Y A056680 Cf. A002275, A093168, A099411.
%K A056680 hard,nonn
%O A056680 1,3
%A A056680 _Robert G. Wilson v_, Aug 10 2000
%E A056680 a(16)-a(20) derived from A099411 by _Robert Price_, Nov 02 2014