1. Bongaarts, P. J. M. and Pijls, H. G. J.: Almost commutative algebra and differential calculus on the quantum hyperplane, Preprint Inst. Lorentz, 1992.
2. Borowiec, A., Marcinek, W., and Oziewicz, Z.: On multigraded differential calculus, in R. Gielerak (ed.),Quantum Groups and Related Topics, Kluwer Acad. Publ., Dordrecht, 1992, pp. 103?114.
3. Connes, A.:Géométrie non commutative, Inter. Editions, Paris, 1990.
4. Dade, E. C.: Group graded rings and modules,Math. Z. 174 (1980), 241?262.
5. Drinfeld, V. G.: Quantum groups, inProc. ICM, Amer. Math. Soc., Berkeley, 1986, pp. 798?820.