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 A099414 #27 Apr 29 2024 09:31:06
%S A099414 0,3,5,6,48,108,245,1044,20208,52740,89189,130080,183657,197061,348453
%N A099414 Numbers k such that 4*R_k + 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099414 Also numbers k such that (4*10^k + 41)/9 is prime.
%C A099414 a(12) > 10^5. - _Robert Price_, Nov 09 2014
%H A099414 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/44449.htm#prime">Prime numbers of the form 44...449</a>.
%H A099414 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099414 a(n) = A056683(n-1) + 1.
%t A099414 Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 5], Print[n]], {n, 5000}]
%o A099414 (Magma) [n: n in [0..300] | IsPrime((4*10^n+41) div 9)]; // _Vincenzo Librandi_ Nov 10 2014
%Y A099414 Cf. A002275, A056683.
%K A099414 nonn,more
%O A099414 1,2
%A A099414 _Robert G. Wilson v_, Oct 14 2004
%E A099414 a(9)-a(11) from _Robert Price_, Nov 09 2014
%E A099414 a(12)-a(15) from Kamada data by _Tyler Busby_, Apr 29 2024