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 A056700 #30 Jul 04 2021 22:10:48
%S A056700 0,2,3,12,18,23,57,128,543,584,833,2450,2810,2873,3671,6384,10296,
%T A056700 16704,53049,56544,74253
%N A056700 Numbers k such that 2*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056700 Also numbers k such that (19*10^k-1)/9 is prime.
%H A056700 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/21111.htm#prime">Prime numbers of the form 211...11</a>.
%H A056700 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056700 Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y A056700 Cf. A002275, A068814.
%K A056700 nonn,hard,more
%O A056700 1,2
%A A056700 _Robert G. Wilson v_, Aug 10 2000
%E A056700 6384 from _Hugo Pfoertner_, Oct 16 2004
%E A056700 10296 and 16704 from Ray Chandler, Sep 2010
%E A056700 0 added by _Arkadiusz Wesolowski_, Mar 10 2011
%E A056700 53049 and 56544 from Serge Batalov, Mar 2009 confirmed as next terms by _Ray Chandler_, Feb 03 2012
%E A056700 74253 from _Ray Chandler_, Feb 03 2012