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This is a pre-print. The definitive version: PODOLSKY, J. and GRIFFITHS, J.B., 1999. Expanding impulsive gravitational waves. Classical and Quantum Gravity, 16(9), pp. 2937-2946, is available at: http://www.iop.org/EJ/journal/CQG.
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Podolský et al. (1999) studied this question.
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