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 A056701 #37 May 31 2023 18:54:18
%S A056701 0,1,2,3,4,10,16,22,53,91,94,106,138,210,282,522,597,1049,2227,6459,
%T A056701 10582,18895,41269,50702,53185,59796,101395,116514,137551,153116
%N A056701 Numbers k such that 2*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056701 Also numbers k such that (7*10^k - 1)/3 is prime.
%C A056701 a(31) > 3*10^5. - _Robert Price_, Oct 19 2014
%H A056701 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/23333.htm#prime">Prime numbers of the form 233...33</a>.
%H A056701 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056701 Do[ If[ PrimeQ[ 2*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 15001}]
%Y A056701 Cf. A002275, A093672.
%K A056701 hard,nonn,more
%O A056701 1,3
%A A056701 _Robert G. Wilson v_, Aug 10 2000
%E A056701 a(22)-a(26) from Kamada data by _Robert Price_, Oct 19 2014
%E A056701 a(27)-a(30) from _Robert Price_, May 31 2023