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 A120729 #7 Jul 11 2015 11:21:07
%S A120729 3,2,2,5,1,1,1,1,1,2,4,2,3,7,4,3,6,6,4,1,2,4,13,2,4,3,7,21,6,9,3,1,5,
%T A120729 4,16,19,28,19,9,3
%N A120729 Smallest integer k>0 such that k*10^n + 1 is a semiprime.
%C A120729 The corresponding semiprimes are 4, 21, 201, 5001, 10001, 100001, 100001, 10000001, 2000000001, 40000000001, ... Semiprime analog of A121172.
%F A120729 Smallest integer k>0 such that k*10^n + 1 is in A001358.
%e A120729 a(0) = 3 because 3*10^0 + 1 = 4 = 2^2 is a semiprime.
%e A120729 a(1) = 2 because 2*10^1 + 1 = 21 = 3*7 is a semiprime.
%e A120729 a(2) = 2 because 2*10^2 + 1 = 201 = 3*67 is a semiprime.
%e A120729 a(3) = 5 because 5*10^3 + 1 = 5001 = 3*1667 is a semiprime.
%e A120729 a(4) = 1 because 1*10^4 + 1 = 10001 = 73*137 is a semiprime.
%e A120729 a(5) = 1 because 1*10^5 + 1 = 100001 = 11*9091 is a semiprime.
%t A120729 sik[n_]:=Module[{k=1,c=10^n},While[PrimeOmega[k*c+1]!=2,k++];k]; Array[sik,40,0] (* _Harvey P. Dale_, Aug 20 2012 *)
%Y A120729 Cf. A001358, A030430, A037071, A062800, A065582, A121172.
%K A120729 easy,nonn
%O A120729 0,1
%A A120729 _Jonathan Vos Post_, Aug 17 2006
%E A120729 More terms from _Harvey P. Dale_, Aug 20 2012