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C. O. Alley, C.O.: Introduction to some fundamental concepts of general relativity and to their required use in some modern timekeeping systems. In: Proceedings of the Precise Time And Time Interval systems and applications meeting. 13. Jahrgang, 1981, S.687–727 (pttimeeting.org (Memento des Originals vom 26. August 2012 im Internet Archive)).