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 A138931 #4 Dec 18 2018 16:33:57
%S A138931 13,139,14251,1428571428571428571364245156301286091
%N A138931 Primes of the form (10^n-3^n)/7.
%C A138931 The next term is too large to include here.
%t A138931 a={};Do[p=(10^n-3^n)/7;If[PrimeQ[p],AppendTo[a,p]],{n,1,25^2}];a
%t A138931 Select[Table[(10^n-3^n)/7,{n,50}],PrimeQ] (* _Harvey P. Dale_, Dec 18 2018 *)
%Y A138931 Cf. A128026.
%K A138931 nonn
%O A138931 1,1
%A A138931 _Vladimir Joseph Stephan Orlovsky_, May 16 2008