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 A056716 #24 Mar 01 2020 09:45:21
%S A056716 0,1,2,4,5,7,10,13,22,23,28,34,40,61,73,361,490,613,1624,2000,2994,
%T A056716 4301,4332,18668,32544,34936,267598,270658,293134,319889,414508
%N A056716 Numbers n such that 6*10^n-1 is prime.
%C A056716 Also numbers n such that 5*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%H A056716 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/59999.htm#prime">Prime numbers of the form 599...99</a>.
%H A056716 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056716 Do[ If[ PrimeQ[ 6*10^n - 1], Print[n]], {n, 0, 5000}]
%o A056716 (PARI) is(n)=ispseudoprime(6*10^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017
%Y A056716 Cf. A056805 (6*10^n+1 is prime).
%K A056716 hard,nonn,more
%O A056716 1,3
%A A056716 _Robert G. Wilson v_, Aug 11 2000
%E A056716 More terms from _Hugo Pfoertner_, Feb 12 2004
%E A056716 3 more terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 30 2007
%E A056716 a(27)-a(31) from Kamada data by _Robert Price_, Oct 19 2014
%E A056716 Outdated comment deleted by _Harvey P. Dale_, Mar 01 2020