1. On extendability of unavoidable sets;Choffrut;Discr. Appl. Math.,1984
2. An optimal test on finite unavoidable sets of words;Crochemore;Inform. Proc. Lett.,1983
3. Lothaire, M. 1983, “Combinatorics on Words”, Encyclopedia of Mathematics and Its Applications, Vol, 17, Addison-Wesley, Reading, MA. Reprinted in the, Cambridge Mathematical Library, Cambridge University Press, Cambridge, UK, 1997.
4. Lothaire, M. 2000, Algebraic Combinatorics on Words. Available at, http://www-igm.univ-mlv.fr/~berstel/Lothaire/.
5. Niven, I, Zuckerman, H. S, and, Montgomery, H. L. 1960, An Introduction to the Theory of Numbers, Wiley, New York.