1. On subsemigroups of free semigroups;Cohn, P. M.;Proc. Amer. Math. Soc.,1962
2. 2. M. W. Curtis, On some commutative sets in a free monoid, Abstract 64T-203, Notices Amer. Math. Soc. 11 (1964), 250.
3. Translations of Mathematical Monographs, Vol. 3;Ljapin, E. S.,1963
4. The equation 𝑎^{𝑀}=𝑏^{𝑁}𝑐^{𝑃} in a free group;Lyndon, R. C.;Michigan Math. J.,1962