* Williams, J.R. and Pentland, A.P., "Superquadric and Modal Dynamics for Discrete Elements in Concurrent Design," National Science Foundation Sponsored 1st U.S. Conference of Discrete Element Methods, Golden, CO, October 19–20, 1989.
* Williams, J.R. and Pentland, A.P., "Superquadric and Modal Dynamics for Discrete Elements in Concurrent Design, " National Science Foundation Sponsored 1st U.S. Conference of Discrete Element Methods, Golden, CO, October 19-20, 1989.