An introduction to control systems I am looking for an introduction on control systems in the context of engineering, but treated from a more mathematical point of view. Does anybody have a good reference?
 A: You might want to peruse these online.
Linear Control(mostly):


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*Modern Control Theory (3rd Edition), William L. Brogan

*Mathematical Control Theory: Deterministic Finite Dimensional Systems. Eduardo D. Sontag, Second Edition, Springer, New York, 1998 (you can review entire book online)


Update 
Nonlinear Control:


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*Nonlinear Optimal Control Theory, L. D. Berkovitz, N. G. Medhin 

*Nonlinear Control Systems II, (v. 2), Alberto Isidori 

A: Feedback systems: An introduction to scientists and engineers is an excellent book and is available for free. The site also contains lot of supplemental material. 
A: For linear control systems, "Linear System Theory" by Wilson Rugh offers a pretty good treatment of things. For nonlinear systems, I like "Nonlinear Systems" by Hassan Khalil because it covers certain aspects of nonlinear dynamical systems such as bifurcations, limit cycles, and other features not found in linear systems in addition to discussing various techniques of how control of nonlinear systems.
For optimal control, Daniel Liberzon has a book called "Calculus of Variations and Optimal Control Theory" which has a preliminary version available here for free. In it, he builds the important tools of optimal control theory from the ground up in a clear and concise way. The book also proves Pontryagin's Maximum Principle (which is among the most important results in optimal control and not proven in every book) and later he covers a few more advanced topics such as hybrid systems. 
