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 A096846 #42 Apr 23 2024 08:28:03
%S A096846 1,3,4,6,9,12,72,118,124,190,244,304,357,1422,2691,5538,7581,21906,
%T A096846 32176,44358,120552,137073,152260
%N A096846 Numbers n for which 8*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C A096846 Also numbers n such that (8*10^n-17)/9 is prime.
%C A096846 The numbers corresponding to a(1)-a(15) are certified prime, the numbers corresponding to a(16)-a(20) are probable primes. a(21) > 10^5. - _Robert Price_, May 20 2014
%H A096846 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88887.htm#prime">Prime numbers of the form 88...887</a>.
%H A096846 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A096846 a(n) = A056695(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%e A096846 n=6: a(4)=888887 which is prime.
%t A096846 Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 1], Print[n]], {n, 0, 5000}] (* _Robert G. Wilson v_, Oct 15 2004; corrected by _Derek Orr_, Sep 06 2014 *)
%o A096846 (PARI)
%o A096846 for(n=1,10^4,if(ispseudoprime(8*(10^n-1)/9-1),print1(n,", "))) \\ _Derek Orr_, Sep 06 2014
%Y A096846 Cf. A002275, A056695, A093171, A096506, A096507, A096508, A096845.
%K A096846 more,nonn
%O A096846 1,2
%A A096846 _Labos Elemer_, Jul 15 2004
%E A096846 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A096846 a(18)-a(20) discovered and reported to Makoto Kamada by Erik Branger; added to OEIS by _Robert Price_, May 20 2014
%E A096846 a(21)-a(23) from Kamada data by _Tyler Busby_, Apr 23 2024