1. N. Bourbaki, Algèbre, Chapter IV-V, Actualités Sci. Ind. 1102, Paris, Hermann et Cie, 1960.
2. W. Burnside, Theory of groups, 2nd ed., New York, Dover, 1950.
3. Manifolds of difference polynomials;Cohn, Richard M.;Trans. Amer. Math. Soc.,1948
4. Inversive difference fields;Cohn, Richard M.;Bull. Amer. Math. Soc.,1949
5. Extensions of difference fields;Cohn, Richard M.;Amer. J. Math.,1952