1. Abstract and Concrete Categories;Adámek,1990
2. Rees matrix rings;Ánh;J. Algebra,1983
3. Functors into categories of M-sets;Banaschewski;Hbg. Math. Abh.,1972
4. Pullback-flat acts are strongly flat;Bulman-Fleming;Canad. Math. Bull.,1991
5. Absolutely flat semigroups;Bulman-Fleming;Pacific J. Math.,1983