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 A099421 #34 Apr 30 2024 05:27:17
%S A099421 0,3,19,79,139,223,463,544,1096,1419,3247,3877,4417,9507,11091,14602,
%T A099421 27811,29188,106729,188308
%N A099421 0 together with numbers k such that 8*R_k - 7 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099421 Also numbers k such that abs(8*10^k - 71)/9 is a prime.
%C A099421 a(19) > 10^5. - _Robert Price_, Sep 06 2014
%H A099421 Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88881.htm#prime">Prime numbers of the form 88...881</a>.
%H A099421 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099421 a(n) = A056664(n-1) + 1.
%t A099421 Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 7], Print[n]], {n, 0, 15000}]
%o A099421 (PARI)
%o A099421 for(n=0,10^4,if(ispseudoprime(abs(8*(10^n-1)/9-7)),print1(n,", "))) \\ _Derek Orr_, Sep 06 2014
%Y A099421 Cf. A002275, A056664.
%K A099421 nonn
%O A099421 1,2
%A A099421 _Robert G. Wilson v_, Oct 14 2004
%E A099421 a(17)-a(18) from Kamada data by _Robert Price_, Sep 06 2014
%E A099421 a(19)-a(20) from Kamada data by _Tyler Busby_, Apr 30 2024