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 A068813 #11 Jan 17 2019 13:44:05
%S A068813 31,311,311111,31111111111,311111111111,31111111111111,
%T A068813 31111111111111111111111111111111111,
%U A068813 311111111111111111111111111111111111111111111111,31111111111111111111111111111111111111111111111111111
%N A068813 Primes with a 3 followed by 1's.
%H A068813 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/31111.htm#prime">Prime numbers of the form 311...11</a>.
%H A068813 H. C. Williams, <a href="http://www.jstor.org/stable/2006356">Some primes with interesting digit patterns</a>, Math. Comp. vol 32 no 144 (1978) pp 1306-1310, Table 2, r=3.
%H A068813 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%e A068813 Digit 3 followed by rep-units of length 1,2,5,10,11,13,34,47,52,77,88,...
%t A068813 Select[Table[FromDigits[PadRight[{3},n,1]],{n,2,100}],PrimeQ] (* _Harvey P. Dale_, Nov 18 2012 *)
%o A068813 (PARI) for(n=1,200, if(isprime(3*10^n+(10^(n)-1)/9)==1,print1(3*10^n+(10^(n)-1)/9,",")))
%Y A068813 Cf. A002275, A056704.
%K A068813 nonn,base
%O A068813 1,1
%A A068813 _Amarnath Murthy_, Mar 07 2002
%E A068813 More terms from _Benoit Cloitre_, Mar 09 2002
%E A068813 One additional term from _Harvey P. Dale_, Nov 18 2012