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 A056689 #28 Jul 04 2021 22:09:52
%S A056689 0,1,2,4,8,11,14,20,263,382,2719,4493,21166,45824,55850,64567,70726
%N A056689 Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056689 Also numbers k such that (7*10^(k+1)-43)/9 is prime.
%C A056689 a(18) > 10^5. - _Robert Price_, Nov 01 2014
%H A056689 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77773.htm#prime">Prime numbers of the form 77...773</a>.
%H A056689 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056689 a(n) = A099420(n) - 1. - _Robert Price_, Nov 01 2014
%t A056689 Do[ If[ PrimeQ[70*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]
%Y A056689 Cf. A002275, A093165, A099420.
%K A056689 hard,nonn
%O A056689 1,3
%A A056689 _Robert G. Wilson v_, Aug 10 2000
%E A056689 a(13)-a(17) derived from A099420 by _Robert Price_, Nov 01 2014