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3. J. Lambek, Lectures on Rings and Modules (Blaisdell, 1966; Factorial Press, Moscow, 2005).
4. C. Nastasescu and F. van Oystaeyen, Graded Ring Theory (North-Holland, Amsterdam, 2004).
5. Liu Shaoxue, M. Beattie, and Fang Hongjin, “Graded Division Rings and the Jacobson Density Theorem,” J. Boijing Normal Univ. (Natural Science) 27(2), 129 (1991).