6. LaSalle, J. P., Stability by Lyapunov direct
method , New York: Academic Press, 1961.
7. Hahn, W., Stability of motion , New York, Springer
-Verlag, 1967.
8. Desoer, C.A. and Vidyasagar, M., Feedback
systems: Input-output properties , New York:
Academic Press, 1975.
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