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 A056681 #18 Jul 04 2021 22:08:56
%S A056681 1,3,10,27,54,93,474,2079,4834,5844,12337,20193,49528,56988,71545,
%T A056681 77308
%N A056681 Numbers k such that 40*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056681 Also numbers k such that (4*10^(k+1)-31)/9 is prime.
%H A056681 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44441.htm#prime">Prime numbers of the form 44...441</a>.
%H A056681 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056681 a(n) = A099412(n+1) - 1. - _Robert Price_, Jan 30 2015
%t A056681 Do[ If[ PrimeQ[40*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
%Y A056681 Cf. A002275, A099412.
%K A056681 hard,nonn
%O A056681 1,2
%A A056681 _Robert G. Wilson v_, Aug 10 2000
%E A056681 a(11)-a(16) derived from A099412 by _Robert Price_, Jan 30 2015