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 A130838 #6 Mar 30 2012 16:50:41
%S A130838 12,112,122,1112,1122,1212,1222,11112,11122,11212,11222,12112,12122,
%T A130838 12212,12222,111112,111122,111212,111222,112112,112122,112212,112222,
%U A130838 121112,121122,121212,121222,122112,122122,122212,122222
%N A130838 Sorted list of strings that can be obtained by starting with 12 and repeatedly doubling any substring in place.
%C A130838 The list contains 2^(n-2) strings of length n, namely 1{1,2}^(n-2)2, for n >= 2.
%H A130838 <a href="/index/Do#repeat">Index entries for doubling substrings</a>
%K A130838 nonn
%O A130838 1,1
%A A130838 _N. J. A. Sloane_, Feb 22 2008