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 A099411 #21 May 03 2024 12:41:10
%S A099411 1,2,3,6,46,394,978,2586,2811,2968,3642,4827,4918,5592,5706,10683,
%T A099411 12891,14118,74350,88680,162138,279978
%N A099411 Numbers k such that 3*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099411 Also numbers k such that (10^k + 11)/3 is prime.
%C A099411 a(21) > 10^5. - _Robert Price_, Nov 02 2014
%H A099411 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33337.htm#prime">Prime numbers of the form 33...337</a>.
%H A099411 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099411 a(n) = A056680(n) + 1.
%t A099411 Do[ If[ PrimeQ[ 3(10^n - 1)/9 + 4], Print[n]], {n, 10000}]
%Y A099411 Cf. A002275, A056680, A093168.
%K A099411 hard,nonn
%O A099411 1,2
%A A099411 _Robert G. Wilson v_, Oct 14 2004
%E A099411 a(16)-a(20) from _Robert Price_, Nov 02 2014
%E A099411 a(21)-a(22) from Kamada data by _Tyler Busby_, May 03 2024