Einstein, H.H. (1993). Modern Developments in Discontinuity Analysis – The Persistence–Connectivity Problem. In J.A. Hudson (ed.): Comprehensive Rock Engineering, Volume 3: Rock Testing and Site Characterization: Principles, Practice and Projects, Chapter 9, 193-213. Pergamon Press Ltd., 1993. ISBN: 978-0080420660 doi:10.1016/B978-0-08-042066-0.50016-1