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 A135156 #6 Jan 21 2014 12:30:21 %S A135156 1,0,2,0,3,1,4,0,5,2,6,1,7,3,8,9,9,7,10,34,11,36,12,136,13,190,14,567, %T A135156 15,1018,16,2445,17,5474,18,11371,19,28233,20,57961,21,143391,22, %U A135156 308793,23,740519,24,1668316 %N A135156 a(n) = number of strings of length n that can be obtained by starting with abc and repeatedly doubling any substring of length >= 2 in place. %C A135156 A "weakly primitive" version of A135473. Cf. A135017. %C A135156 Differs from A135017 in that the strings may contain repeated letters. %H A135156 <a href="/index/Do#repeat">Index entries for doubling substrings</a> %K A135156 nonn %O A135156 0,3 %A A135156 _David Applegate_ and _N. J. A. Sloane_, Feb 15 2008