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 A093938 #14 Jul 08 2021 06:24:12
%S A093938 2,277,2777,2777777777,2777777777777777,2777777777777777777,
%T A093938 2777777777777777777777777777777777777,
%U A093938 2777777777777777777777777777777777777777777777777777777777777777
%N A093938 Primes of the form 2*10^k + 7*R_k, where R_k is the repunit (A002275) of length k.
%H A093938 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/27777.htm#prime">Prime numbers of the form 277...77</a>.
%H A093938 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A093938 Select[FromDigits/@Table[PadRight[{2},n,7],{n,200}], PrimeQ] (* _Harvey P. Dale_, May 14 2011 *)
%Y A093938 Cf. A002275, A056702 (corresponding k).
%K A093938 nonn
%O A093938 1,1
%A A093938 _Rick L. Shepherd_, Apr 17 2004