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 A056660 #28 Jul 04 2021 19:04:19
%S A056660 3,17,99,120,243,545,630,1493,2565,8087
%N A056660 Numbers k such that 20*R_k + 1 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056660 Also numbers k such that (2*10^(k+1)-11)/9 is a prime.
%H A056660 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22221.htm#prime">Prime numbers of the form 22...221</a>.
%H A056660 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056660 a(n) = A084832(n) - 1. - _Robert Price_, Jan 30 2015
%t A056660 Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]
%Y A056660 Cf. A002275, A084832, A091189 (corresponding primes).
%K A056660 nonn
%O A056660 1,1
%A A056660 _Robert G. Wilson v_, Aug 09 2000
%E A056660 a(8) and a(9) from _Rick L. Shepherd_, Feb 22 2004
%E A056660 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008