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 A056714 #30 Feb 23 2022 20:19:02
%S A056714 0,1,3,13,25,49,143,419,1705,13993,35753,40889
%N A056714 Numbers k such that 5*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056714 Also numbers k such that (16*10^k - 1)/3 is prime.
%C A056714 5*10^a(n) + 3*(10^a(n) - 1)/9 is a solution for part (b) of questions of puzzle 244 from www.primepuzzles.net. If a(n) is greater than 5812 then a(n) is an example that is asked for in this question. - _Farideh Firoozbakht_, Dec 02 2003
%H A056714 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/53333.htm#prime">Prime numbers of the form 533...33</a>.
%H A056714 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_244.htm">Puzzle 244. Null Conjunction</a>, The Prime Puzzles and Problems Connection.
%H A056714 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056714 Do[ If[ PrimeQ[ 5*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]
%Y A056714 Cf. A002275, A093674, A350995.
%K A056714 nonn
%O A056714 1,3
%A A056714 _Robert G. Wilson v_, Aug 11 2000
%E A056714 1705 from _Farideh Firoozbakht_, Dec 18 2003
%E A056714 13993, 35753 and 40889 from _Serge Batalov_, Jan 2009 confirmed as next terms by _Ray Chandler_, Feb 11 2012