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 A123568 #19 Dec 05 2018 16:35:27
%S A123568 31,331,3331,33331,333331,3333331,33333331,333333333333333331,
%T A123568 3333333333333333333333333333333333333331,
%U A123568 33333333333333333333333333333333333333333333333331
%N A123568 Prime numbers of the form (10^n - 7)/3.
%C A123568 The number of initial 3s is n - 1.
%C A123568 Note that each n from 2 to 8 gives primes, but after that the n that correspond to primes are progressively further apart. Singh (1997) gives this as an example of why mathematicians don't trust a preponderance of evidence as proof: in the 17th century, when factoring numbers with as few as eight digits wasn't as easy as it is today, the pattern suggested that all numbers of this form are prime. - _Alonso del Arte_, Nov 11 2012
%D A123568 Simon Singh, Fermat's Enigma. New York: Walker & Company (1997) p. 159.
%H A123568 Alois P. Heinz, <a href="/A123568/b123568.txt">Table of n, a(n) for n = 1..17</a>
%e A123568 a(7) = 33333331 because that is the seventh number of the specified form to be prime.
%e A123568 333333331 is not in the sequence because it is composite, being the product of 17 and 19607843.
%t A123568 Do[If[PrimeQ[(10^n - 7)/3], Print[(10^n - 7)/3]], {n, 1, 100}] (* Jasinski *)
%t A123568 Select[(10^Range[50] - 7)/3, PrimeQ[#] &] (* _Alonso del Arte_, Nov 11 2012 *)
%t A123568 Select[Table[FromDigits[PadLeft[{1},n,3]],{n,50}],PrimeQ] (* _Harvey P. Dale_, Dec 05 2018 *)
%o A123568 (PARI) select(ispseudoprime, vector(20, n, (10^n-7)/3)) \\ _Charles R Greathouse IV_, Nov 12 2012
%Y A123568 Cf. A055557, A051200, A033175, A065586, A068104.
%K A123568 nonn
%O A123568 1,1
%A A123568 _Artur Jasinski_, Nov 12 2006