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 A056715 #24 Jul 08 2021 00:45:37
%S A056715 0,2,8,14,17,18,33,35,126,183,324,344,866,992,1226,2355,13344,75027,
%T A056715 77322
%N A056715 Numbers k such that 5*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056715 Also numbers n such that (52*10^k - 7)/9 is prime.
%C A056715 a(20) > 10^5. - _Robert Price_, Dec 30 2014
%H A056715 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/57777.htm#prime">Prime numbers of the form 577...77</a>.
%H A056715 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056715 Do[ If[ PrimeQ[ 5*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 5000}]
%Y A056715 Cf. A002275, A093941.
%K A056715 hard,nonn
%O A056715 1,2
%A A056715 _Robert G. Wilson v_, Aug 11 2000
%E A056715 a(17)-a(19) from _Robert Price_, Dec 30 2014