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 A056723 #19 Jul 08 2021 05:05:48
%S A056723 1,7,23,29,133,173,367,1925,3707,5765,9709,19573,43753
%N A056723 Numbers k such that 8*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056723 Also numbers k such that (25*10^k - 1)/3 is prime.
%H A056723 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/83333.htm#prime">Prime numbers of the form 833...33</a>.
%H A056723 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056723 Do[ If[ PrimeQ[ 8*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y A056723 Cf. A002275, A093676.
%K A056723 nonn
%O A056723 1,2
%A A056723 _Robert G. Wilson v_, Aug 11 2000
%E A056723 Corrected by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056723 19573 and 43753 from Serge Batalov, Jan 2009 confirmed as next terms by _Ray Chandler_, Feb 13 2012