1. The Burnside problem and identities in groups, Ergetnisse der Mathematik und ihrer Grenzgetiete, Vol. 95;Adian,1979
2. S.E. Arshon, A proof of the existence of infinite asymmetric sequences on n symbols, Mat. Sb. 2 (44) (4) (1937) 769–779 (in Russian).
3. Avoidable patterns in strings of symbols;Bean;Pac. J. Math.,1979
4. Combinatorics of words;Choffrut,1997
5. M. Crochemore, P. Goralcik, Mutually avoiding ternary words of small exponent, Int. J. Algebra Comput. 1 (4) (1991) 407–410 (for further reading).