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 A056712 #24 Sep 08 2022 08:45:01
%S A056712 2,3,4,6,14,54,210,390,594,3460,5028,5219,5332,8072,15796,16131,21456,
%T A056712 29282,78790,85142,179972,211029,445773,464843,511056
%N A056712 Numbers n such that 5*10^n-1 is prime.
%C A056712 Also numbers n such that 4*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%H A056712 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/49999.htm#prime">Prime numbers of the form 499...99</a>.
%H A056712 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056712 Do[ If[ PrimeQ[ 4*10^n + (10^n-1)], Print[n]], {n, 0, 4660}]
%o A056712 (Magma) [n: n in [0..2000] |IsPrime(5*10^n-1)]; // _Vincenzo Librandi_, Oct 20 2014
%o A056712 (PARI) is(n)=ispseudoprime(5*10^n-1) \\ _Charles R Greathouse IV_, Mar 01 2017
%K A056712 hard,nonn,more
%O A056712 1,1
%A A056712 _Robert G. Wilson v_, Aug 11 2000
%E A056712 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056712 a(21)-a(25) from Kamada data by _Robert Price_, Oct 19 2014