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 A056659 #23 Jul 04 2021 19:04:31
%S A056659 1,4,5,7,16,49,683,719,1451,1678,3145,72820
%N A056659 Numbers k such that 10*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056659 Also numbers k such that (10^(k+1)+71)/9 is prime.
%C A056659 a(13) > 10^5. - _Robert Price_, Nov 01 2014
%H A056659 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.
%H A056659 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056659 a(n) = A097685(n) - 1. - _Robert Price_, Nov 01 2014
%t A056659 Do[ If[ PrimeQ[10*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
%Y A056659 Cf. A002275, A097685.
%K A056659 hard,nonn
%O A056659 1,2
%A A056659 _Robert G. Wilson v_, Aug 09 2000
%E A056659 a(12) derived from A097685 by _Robert Price_, Nov 01 2014