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3. J.M. Rodriguez-Ortiz, thesis, Universidad Politecnico de Madrid, Spain (1974) in Spanish.
4. T. Kawai, International Journal for Numerical Methods in Engineering 16 (1980) 81–120.
5. P.A. Cundall, BALL-A program to model granular media using the distinct element method. Technical note, Dames and Moore, London (1978).