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 A056703 #17 Jan 17 2019 13:44:05
%S A056703 0,1,3,6,7,19,27,43,55,207,1311,3204,7050,9439,26044,33058,34507,
%T A056703 49314,119292
%N A056703 Numbers n such that 3*10^n - 1 is prime.
%C A056703 Also numbers n such that 2*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%H A056703 Makoto Kamada, <a href="https://stdkmd.net/nrr/2/29999.htm#prime">Prime numbers of the form 299...99</a>.
%H A056703 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056703 Do[ If[ PrimeQ[ 2*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]
%t A056703 Select[Range[200000],PrimeQ[3*10^#-1]&] (* _Harvey P. Dale_, Sep 29 2012 *)
%o A056703 (PARI) is(n)=ispseudoprime(3*10^n-1) \\ _Charles R Greathouse IV_, Jun 12 2017
%K A056703 hard,nonn,more
%O A056703 1,3
%A A056703 _Robert G. Wilson v_, Aug 10 2000
%E A056703 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008