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 A056664 #27 Jan 17 2019 13:44:05
%S A056664 2,18,78,138,222,462,543,1095,1418,3246,3876,4416,9506,11090,14601,
%T A056664 27810,29187
%N A056664 Numbers k such that 80*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056664 Also numbers k such that (8*10^(k+1) - 71)/9 is prime.
%C A056664 There are no other terms <= 2500. - _Rick L. Shepherd_, Mar 02 2004
%C A056664 a(18) > 10^5. - _Robert Price_, Nov 01 2014
%H A056664 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88881.htm#prime">Prime numbers of the form 88...881</a>.
%H A056664 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056664 a(n) = A099421(n+1) - 1. - _Robert Price_, Nov 01 2014
%t A056664 Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}]
%Y A056664 Cf. A002275, A092675 (corresponding primes), A099421.
%K A056664 hard,nonn
%O A056664 1,1
%A A056664 _Robert G. Wilson v_, Aug 09 2000
%E A056664 a(9) (giving a probable prime) from _Rick L. Shepherd_, Mar 02 2004
%E A056664 a(10)-a(15) from _N. J. A. Sloane_, Feb 20 2005
%E A056664 a(16)-a(17) derived from A099421 by _Robert Price_, Nov 01 2014