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 A056663 #21 Jan 17 2019 13:44:05
%S A056663 1,13,16,34,4174,4471,9811,12259,12340,13759,14575,53410,144682,148327
%N A056663 Numbers k such that 80*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056663 Also numbers k such that (8*10^(k+1) + 1)/9 is prime.
%C A056663 a(15) > 2*10^5. - _Robert Price_, Nov 01 2014
%H A056663 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88889.htm#prime">Prime numbers of the form 88...889</a>.
%H A056663 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056663 a(n) = A096508(n) - 1. - _Robert Price_, Nov 01 2014
%t A056663 Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 9 ], Print[n]], {n, 0, 30000}]
%Y A056663 Cf. A002275, A096508.
%K A056663 hard,nonn
%O A056663 1,2
%A A056663 _Robert G. Wilson v_, Aug 09 2000
%E A056663 Intermediate missing terms 9811 (from M. Kamada), 12259, 12340, 13759 added by _Serge Batalov_, Dec 14 2008
%E A056663 a(12)-a(14) derived from A096508 by _Robert Price_, Nov 01 2014