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R.K. Williams (2001). "Collimated energy–momentum extraction from rotating black holes in quasars and microquasars using the Penrose mechanism". AIP Conference Proceedings. Vol. 586. pp. 448–453. arXiv:astro-ph/0111161. Bibcode:2001AIPC..586..448W. doi:10.1063/1.1419591.
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R.K. Williams (2001). "Collimated energy–momentum extraction from rotating black holes in quasars and microquasars using the Penrose mechanism". AIP Conference Proceedings. Vol. 586. pp. 448–453. arXiv:astro-ph/0111161. Bibcode:2001AIPC..586..448W. doi:10.1063/1.1419591.
B. Mashhoon (2007). "Gravitoelectromagnetism: A Brief Review". In Iorio (ed.). The Measurement of Gravitomagnetism: A Challenging Enterprise. Nova Science Publishers. arXiv:gr-qc/0311030. Bibcode:2003gr.qc....11030M.