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 A099419 #18 Jul 08 2021 03:12:24
%S A099419 2,13,20,23,31,100,241,275,925,1067,1369,2065,7163,37963,91856,111706,
%T A099419 260198,271757,314564,348724
%N A099419 Numbers k such that 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A099419 Also numbers k such that (7*10^k - 61)/9 is prime.
%H A099419 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77771.htm#prime">Prime numbers of the form 77...771</a>.
%H A099419 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A099419 a(n) = A056688(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%t A099419 Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 6], Print[n]], {n, 0, 7000}]
%Y A099419 Cf. A002275, A056688, A093176.
%K A099419 nonn
%O A099419 1,1
%A A099419 _Robert G. Wilson v_, Oct 14 2004
%E A099419 a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A099419 a(14)-a(15) from Erik Branger, Dec 19 2009
%E A099419 a(16) from Erik Branger, Jan 29 2011
%E A099419 a(17) from Erik Branger, Nov 26 2011
%E A099419 a(18) from Erik Branger, Mar 05 2012
%E A099419 a(19)-a(20) from Erik Branger, Sep 23 2016