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 A135479 #10 Mar 30 2012 16:50:47 %S A135479 123,12123,12323,123123,1212123,1212323,1232323,12123123,12312123, %T A135479 12312323,12313123,12323123,121212123,121212323,121232123,121232323, %U A135479 123123123,123212323,123232323,1212123123,1212312123,1212312323,1212313123,1212323123,1231212123 %N A135479 Sorted list of primitive strings that can be obtained by starting with 123 and repeatedly doubling any substring in place. %C A135479 The list contains A135017(n) strings of length n. %C A135479 The subset of strings in A135475 that contain no repeated symbol. %H A135479 Alois P. Heinz, <a href="/A135479/b135479.txt">Table of n, a(n) for n = 1..11498</a> %H A135479 <a href="/index/Do#repeat">Index entries for doubling substrings</a> %K A135479 nonn,easy %O A135479 1,1 %A A135479 _N. J. A. Sloane_, Feb 22 2008