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 A099420 #19 Jul 08 2021 03:12:28
%S A099420 1,2,3,5,9,12,15,21,264,383,2720,4494,21167,45825,55851,64568,70727
%N A099420 Numbers k such that 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099420 Also numbers k such that (7*10^k - 43)/9 is prime.
%C A099420 a(18) > 10^5. - _Robert Price_, Oct 19 2014
%H A099420 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77773.htm#prime">Prime numbers of the form 77...773</a>.
%H A099420 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099420 a(n) = A056689(n) + 1.
%t A099420 Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}]
%Y A099420 Cf. A002275, A056689, A093165.
%K A099420 nonn,more
%O A099420 1,2
%A A099420 _Robert G. Wilson v_, Oct 14 2004
%E A099420 a(13)-a(17) from _Robert Price_, Oct 19 2014