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R.K. Williams (2004). «Collimated escaping vortical polar e−e+ jets intrinsically produced by rotating black holes and Penrose processes». The Astrophysical Journal611 (2): 952–963. doi:10.1086/422304. Bibcode: 2004ApJ...611..952W.
R.K. Williams (2001). «Collimated energy–momentum extraction from rotating black holes in quasars and microquasars using the Penrose mechanism». 586, pp. 448–453. doi:10.1063/1.1419591. Bibcode: 2001AIPC..586..448W.