1. On the endomorphism problem for free groups;Edmunds;Comm. Algebra,1975
2. On the endomorphism problem for free groups, II;Edmunds,1979
3. A note on commutators in free products;Griffiths,1954
4. A note on commutators in free products, II;Griffiths,1955
5. Equations in free groups;Lyndon;Trans. Amer. Math. Soc.,1960