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 A138861 #6 Jan 15 2017 13:32:14
%S A138861 31,1021,1000000021,100000000000000021,
%T A138861 10000000000000000000000000000000000000000000000000000021,
%U A138861 100000000000000000000000000000000000000000000000000000000000000000000000000021
%N A138861 Primes of form 10^n + 21.
%C A138861 The next term has 134 digits. - _Harvey P. Dale_, Jan 15 2017
%t A138861 a={};Do[p=10^n+21;If[PrimeQ[p],AppendTo[a,p]],{n,0,10^2}];Print[a];
%t A138861 Join[{31},Select[Table[100FromDigits[PadRight[{1},n,0]]+21,{n,150}],PrimeQ]] (* _Harvey P. Dale_, Jan 15 2017 *)
%o A138861 (PARI) for(n=1,1e3,if(ispseudoprime(t=10^n+21),print1(t", "))) \\ _Charles R Greathouse IV_, May 15 2013
%Y A138861 Cf. A108050.
%K A138861 nonn
%O A138861 1,1
%A A138861 _Vladimir Joseph Stephan Orlovsky_, May 16 2008