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Kovner has observed that Fermat's principle can be used to describe the motion of light rays in arbitrary gravitational fields, not just stationary ones. We give a simple demonstration of this fact.Received 20 November 1991DOI:https://doi.org/10.1103/PhysRevD.45.3862©1992 American Physical Society
Nityananda et al. (Fri,) studied this question.