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 A056698 #31 Jul 04 2021 22:10:41
%S A056698 1,15,41,83,95,341,551,669,989,1223,6923,103703
%N A056698 Numbers k such that 10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056698 Also numbers k such that (4*10^k-1)/3 is prime.
%C A056698 a(13) > 850000 (from Kamada data). - _Robert Price_, Oct 19 2014
%H A056698 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/13333.htm#prime">Prime numbers of the form 133...33</a>.
%H A056698 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056698 Do[ If[ PrimeQ[ 10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 30470}]
%o A056698 (PARI) for(k=1,1500,if(ispseudoprime(4*(10^k-1)/3+1),print1(k, ", "))) \\ _Hugo Pfoertner_, Jul 22 2020
%Y A056698 Cf. A002275, A093671, A097166, A259050.
%K A056698 nonn,hard,more
%O A056698 1,2
%A A056698 _Robert G. Wilson v_, Aug 10 2000
%E A056698 a(12) from Kamada data by _Robert Price_, Oct 19 2014