Hermann Bondi et F. A. E. Pirani, « Gravitational Waves in General Relativity. XIII. Caustic Property of Plane Waves », Proc. Roy. Soc. A, vol. 251, no 1227, , p. 395 (DOI10.1098/rspa.1989.0016, Bibcode1989RSPSA.421..395B)
Hermann Bondi et F. A. E. Pirani, « Gravitational Waves in General Relativity. XIII. Caustic Property of Plane Waves », Proc. Roy. Soc. A, vol. 251, no 1227, , p. 395 (DOI10.1098/rspa.1989.0016, Bibcode1989RSPSA.421..395B)