1. A. O. Gel'fond, Calculus of Finite Differences [in Russian], Fizmatgiz, Moscow (1967).
2. V. A. Dorodnitsyn, ?Taylor group and transformations preserving difference derivatives,? Preprint Inst. Prikl. Mat. (IPM) AN SSSR, Moscow (1987), No. 67.
3. V. A. Dorodnitsyn, ?Newton group and commutation properties of Lie-Bäcklund operators in net spaces,? Preprint, Inst. Prikl. Mat. (IPM) AN SSSR, Moscow (1988), No. 175.
4. N. Kh. Ibragimov, Transformation Groups in Mathematical Physics [in Russian], Nauka, Moscow (1983).
5. A. A. Markov, Calculus of Finite Differences [in Russian], Odessa (1910).