1. The semigroup of ideals of a fir is (usually) free;Bergman;J. London Math. Soc.,1975
2. On the laws of a metabelian variety;Cohen;J. Algebra,1967
3. On subsemigroups of free semigroups;Cohn,1962
4. Krull Dimension;Gordon;Mem. Amer. Math. Soc.,1973
5. Ordering by divisibility in abstract algebras;Higman,1952