1. Coproducts and some universal ring constructions;Bergman;Trans. Am. Math. Soc.,1974
2. Free Rings and Their Relations;Cohn,1985
3. Localization in general rings, a historical survey;Cohn,2006
4. Free Ideal Rings and Localization in General Rings;Cohn,2006
5. Rational Homotopy Theory and Differential Forms;Griffiths,2013