1. M. Billaud, A problem with words, Letter in Newsgroup Comp.Theory (1993) https://groups.google.com/d/topic/comp.theory/V_xDDtoR9a4/discussion.
2. N.G. De Bruijn, A combinatorial problem, Proc. Koninklijke Nederlandse Akademie v. Wetenschappen, vol. 49, 1946, pp. 758–764.
3. Equations in words;Choffrut,1997
4. Weakly unambiguous morphisms;Freydenberger;Theoretical Computer Science,2012
5. The unambiguity of segmented morphisms;Freydenberger;Discrete Applied Mathematics,2009