1. Vector spaces over rings;Everett, C. J.;Bull. Amer. Math. Soc.,1942
2. Modules over rings of words;Leavitt, William G.;Proc. Amer. Math. Soc.,1956
3. Modules without invariant basis number;Leavitt, W. G.;Proc. Amer. Math. Soc.,1957
4. American Mathematical Society Mathematical Surveys, Vol. II;Jacobson, Nathan,1943
5. Sur le nombre de dimensions d’un module;Dieudonné, Jean;C. R. Acad. Sci. Paris,1942