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 A099416 #24 May 03 2024 14:07:52
%S A099416 1,2,8,26,66,74,233,473,540,2774,64715,102492,141594
%N A099416 Numbers k such that 5*R_k - 2 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099416 Also numbers k such that (5*10^k-23)/9 is prime.
%C A099416 a(12) > 10^5. - _Robert Price_, Nov 13 2014
%H A099416 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/55553.htm#prime">Prime numbers of the form 55...553</a>.
%H A099416 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099416 a(n) = A056685(n) + 1.
%t A099416 Do[ If[ PrimeQ[ 5(10^k - 1)/9 - 2], Print[k]], {k, 1, 5000}]
%Y A099416 Cf. A002275, A056685.
%K A099416 nonn
%O A099416 1,2
%A A099416 _Robert G. Wilson v_, Oct 14 2004
%E A099416 a(11) from _Robert Price_, Nov 13 2014
%E A099416 0 removed by _Georg Fischer_, Jan 03 2021
%E A099416 a(12)-a(13) from Kamada data by _Tyler Busby_, May 03 2024