1. Algèbre commutative, Chap 3, Graduations, Filtrations et Topologies;Bourbaki,1985
2. Algèbre;Bourbaki,1981
3. Free Rings and their Relations;Cohn,1985
4. Difference Algebra;Cohn,1965
5. S. El Asmi, An algebraic approach to essentiallity, Proceeding of the First European Control Conference, Grenoble, 1991, Paris: Hermès, pp.468–473.