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 A099413 #23 May 03 2024 15:02:25
%S A099413 0,1,2,4,10,20,26,722,1310,3170,28934,66284,67796,231254,338476
%N A099413 Numbers k such that 4*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099413 Also numbers k such that (4*10^k + 23)/9 is prime.
%C A099413 a(14) > 10^5. - _Robert Price_, Nov 26 2014
%H A099413 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44447.htm#prime">Prime numbers of the form 44...447</a>.
%H A099413 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099413 a(n) = A056682(n-1) + 1. - _Robert Price_, Nov 26 2014
%t A099413 Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 3], Print[n]], {n, 5000}]
%Y A099413 Cf. A002275, A056682.
%K A099413 nonn,hard,more
%O A099413 1,3
%A A099413 _Robert G. Wilson v_, Oct 14 2004
%E A099413 a(11)-a(13) from _Robert Price_, Nov 26 2014
%E A099413 a(14)-a(15) from Kamada data by _Tyler Busby_, May 03 2024