1. G. Prater, Jr. and R. SinghJournal of Sound and Vibration. Quantification of the extent of non-proportional viscous damping in discrete vibratory systems.
2. Classical normal modes in damped linear dynamic systems;Caughey;American Society of Mechanical Engineers, Journal of Applied Mechanics,1965
3. Analysis of proportional damping approximations in discrete vibratory systems;Nair,1984