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 A096845 #23 Feb 10 2022 12:39:37
%S A096845 1,2,3,6,9,12,30,32,183,297,492,41316
%N A096845 Numbers n for which 4*R_n - 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C A096845 Also numbers n such that (4*10^n-13)/9 is prime.
%C A096845 a(13) > 10^5. - _Robert Price_, Oct 25 2014
%H A096845 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44443.htm#prime">Prime numbers of the form 44...443</a>.
%H A096845 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A096845 a(n) = A056661(n) + 1.
%e A096845 n=30 means that 444444444444444444444444444443 is prime.
%t A096845 Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 1], Print[n]], {n, 5000}] (* _Robert G. Wilson v_, Oct 14 2004 *)
%t A096845 Select[Range[500],PrimeQ[FromDigits[PadLeft[{3},#,4]]]&] (* The program generates the first 11 terms of the sequence. *) (* _Harvey P. Dale_, Feb 10 2022 *)
%Y A096845 Cf. A096503-A096508, A096841-A096846, A000203, A056661.
%K A096845 hard,more,nonn
%O A096845 1,2
%A A096845 _Labos Elemer_, Jul 15 2004
%E A096845 a(12) from _Robert Price_, Oct 25 2014