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 A120642 #10 Mar 20 2015 18:10:48
%S A120642 2,2,2,5,2,3,2,8,11,6,11,35,6,5,15,14,11,21,3,21,14,6,6,80,8,2,2,6,9,
%T A120642 48,48,21,15,6,44,11,9,15,18,6,33,30,3,278,74,92,89,33,8,71,59,11,2,5,
%U A120642 3,24,108,47,39,41,6,14,53,173,26,26,51,114,23,17,246,44,6,131,56,8,26,77,74
%N A120642 Smallest integer k>0 such that k*10^n - 1 is a prime.
%H A120642 Pierre CAMI, <a href="/A120642/b120642.txt">Table of n,a(n) for n=1,...,3000</a>
%e A120642 The primes are 19, 199, 1999, 49999, 199999, 2999999,
%e A120642 19999999, 799999999, 10999999999, 59999999999, ...,.
%t A120642 f[n_] := Block[{k = 1}, While[ !PrimeQ[k*10^n - 1], k++ ]; k]; Array[f, 79] (* _Robert G. Wilson v_ *)
%Y A120642 Cf. A030430, A037071, A062800, A065582, A121172.
%K A120642 easy,nonn
%O A120642 1,1
%A A120642 _Jonathan Vos Post_, Aug 17 2006
%E A120642 a(11) onwards from _Robert G. Wilson v_, Aug 20 2006