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 A111473 #8 Mar 04 2018 03:16:45
%S A111473 3,1,1,11,113,7,23,41,37,141,733,241,3,791,781,701,239,441,2019,189,
%T A111473 2071,401,851,463,4421,497,2267,213,1653,1683,1227,667,3261,6673,5799,
%U A111473 3579,1907,6483,7813,2443,1923,11439,6657,7861,1847,7521,8277,8459
%N A111473 a(1) = 3, a(n) = least k such that concatenation of n copies of k with all previous concatenation gives a prime.
%e A111473 3,311,311111,31111111111111 are all prime.
%e A111473 31111111111111 = one copy of 3, two copies of 1, three copies of 1, four copies of 11.
%o A111473 (PARI) { x=3; for(n=2,50, k=0; until(ispseudoprime(y), k++; y=eval(concat(Str(x),concat(vector(n,i,Str(k))))); ); print1(k,", "); x=y; ) } \\ _Max Alekseyev_, May 18 2009
%Y A111473 Cf. A111471, A111472, A111474.
%K A111473 base,hard,nonn
%O A111473 1,1
%A A111473 _Amarnath Murthy_, Aug 05 2005
%E A111473 More terms from _Max Alekseyev_, May 18 2009