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 A097685 #25 May 03 2024 12:38:42
%S A097685 2,5,6,8,17,50,684,720,1452,1679,3146,72821,111902,146063,180689,
%T A097685 330906
%N A097685 Numbers k such that R_k + 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A097685 Also numbers k such that (10^k + 71)/9 is prime.
%C A097685 The corresponding values R_k + 8 are primes of the form "(k-1) ones followed by a nine". Related to the base-10 repunit primes.
%C A097685 a(13) > 10^5. - _Robert Price_, Nov 01 2014
%H A097685 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11119.htm#prime">Prime numbers of the form 11...119</a>.
%H A097685 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A097685 a(n) = A056659(n) + 1.
%t A097685 Do[ If[ PrimeQ[(10^n - 1)/9 + 8], Print[n]], {n, 0, 5000}] (* _Robert G. Wilson v_, Oct 14 2004 *)
%Y A097685 Cf. A004023, A056659, A097683, A097684.
%K A097685 nonn,more,hard
%O A097685 1,1
%A A097685 _Carl R. White_, Aug 19 2004
%E A097685 a(12) from _Robert Price_, Nov 01 2014
%E A097685 a(13)-a(16) from Kamada data by _Tyler Busby_, May 03 2024