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 A056702 #26 Jul 08 2021 00:48:01
%S A056702 0,2,3,9,15,18,36,63,114,225,405,482,1241,2018,9830,15795,22535,27752,
%T A056702 41033,95574,131144,156221
%N A056702 Numbers k such that 2*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056702 Also numbers k such that (25*10^k - 7)/9 is prime.
%C A056702 a(21) > 10^5. - _Robert Price_, Dec 17 2014
%C A056702 a(23) > 2*10^5. - _Robert Price_, May 19 2018
%H A056702 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/27777.htm#prime">Prime numbers of the form 277...77</a>.
%H A056702 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056702 Do[ If[ PrimeQ[ 2*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]
%Y A056702 Cf. A002275, A093938.
%K A056702 hard,nonn
%O A056702 1,2
%A A056702 _Robert G. Wilson v_, Aug 10 2000
%E A056702 a(15) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056702 a(16)-a(20) from _Robert Price_, Dec 17 2014
%E A056702 a(21)-a(22) from _Robert Price_, May 19 2018