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2. Charles Misner, Kip Thorne and John Archibald Wheeler: Gravitation, Freeman and Co., San Francisco 1973, Sec. 2.5.
3. Theodore Frankel, Gravitational Curvature. An Introduction to Einstein’s Theory, Freeman and Co., San Francisco 1979.
4. Walther Thirring: Course in Mathematical Physics, vol. 2: Classical Field Theory, Springer Verlag, New York 1979.
5. G.A. Deschamps: “Electromagnetics and differential forms”, Proc. IEEE
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