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4. Pirani, F. A. E.: Bull. Acad. Polonica, Math.-Astr.-Phys. Series,13, 239–242 (1965).
5. Pirani, F. A. E., Schild, A.: Bull. Acad. Polonica, Math.-Astr.-Phys. Series,9, 543–547 (1961); Perspectives in geometry and relativity, essays in honor of Václav Hlavatý (ed. B. Hoffmann) 291–309. Bloomington: Indiana University Press 1966.