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 A213970 #6 Jun 30 2012 09:29:30
%S A213970 1,3,13,31,113,131,133,311,313,331,1113,1131,1133,1311,1331,1333,3111,
%T A213970 3113,3133,3311,3313,3331,11113,11131,11133,11311,11313,11331,11333,
%U A213970 13111,13113,13131,13133,13311,13313,13331,13333,31111,31113,31131,31133,31311,31313,31331,31333,33111,33113,33131,33133,33311,33313,33331
%N A213970 List of primitive words over the alphabet {1,3}.
%C A213970 A word w is primitive if it cannot be written as u^k with k>1; otherwise it is imprimitive.
%C A213970 The {0,1} version of this sequence is
%C A213970 0, 1, 01, 10, 001, 010, 011, 100, 101, 110, 0001, 0010, 0011, 0100, 0110, 0111, 1000, 1001, 1011, 1100, 1101, 1110, 00001, 00010, 00011, 00100, 00101, 00110, 00111, 01000, 01001, 01010, 01011, 01100, 01101, 01110, 01111, 10000, 10001, 10010, 10011, 10100, 10101, 10110, 10111, 11000, 11001, 11010, 11011, 11100, 11101, 11110, ...,
%C A213970 but this cannot be included as a sequence in the OEIS since it contains nonzero "numbers" beginning with 0.
%D A213970 A. de Luca and S. Varricchio, Finiteness and Regularity in Semigroups and Formal Languages, Monographs in Theoretical Computer Science, Springer-Verlag, Berlin, 1999. See p. 10.
%Y A213970 Cf. A213969-A213974.
%K A213970 nonn,base
%O A213970 1,2
%A A213970 _N. J. A. Sloane_, Jun 30 2012