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 A056678 #25 Jul 05 2021 02:16:39
%S A056678 1,2,4,13,175,415,2504,2758,7924,9400,10390,12104,19615
%N A056678 Numbers k such that 20*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056678 Also numbers k such that (2*10^(k+1)+61)/9 is prime.
%C A056678 a(14) > 10^5. - _Robert Price_, Nov 01 2014
%H A056678 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22229.htm#prime">Prime numbers of the form 22...229</a>.
%H A056678 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056678 a(n) = A099410(n+1) - 1. - _Robert Price_, Nov 01 2014
%t A056678 Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
%Y A056678 Cf. A002275, A093401 (corresponding primes), A099410.
%K A056678 hard,nonn
%O A056678 1,2
%A A056678 _Robert G. Wilson v_, Aug 10 2000
%E A056678 Two more terms from _Rick L. Shepherd_, Mar 30 2004
%E A056678 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056678 a(11)-a(13) derived from A099410 by _Robert Price_, Nov 01 2014