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2. See H.-J. Schmidt, Int. J. Geom. Meth. Phys. 4, (2007) 209 for an historical overview.
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4. R. Utiyama, B. DeWitt, J. Math. Phys 3, 608 (1962); A. Strominger, Phys. Rev. D. 30, 2257 (1984)
5. I.L. Buchbinder, S.D. Odintsov, I.L. Shapiro, Effective Action in Quantum Gravity (IOP, Bristol, 1992)