J. Stecki, and D. Davis. Proceedings of the Institution of Mechanical Engineers, Part A: Power and Process Engineering, 200 (41):
215--228(Jun 1, 1986)
B. Maschke, A. van der Schaft, and P. Breedveld. IEEE Transactions on Circuits and Systems I: Fundamental Theory and Applications, 42 (2):
73-82(February 1995)