Infinite dimensional systems can be used to describe many phenomena in
the real world. As is well known, heat conduction, properties of
elastic- plastic material, fluid dynamics, diffusion-reaction processes,
etc., all lie within this area. The object that we are studying
(temperature, displace- ment, concentration, velocity, etc.) is usually
referred to as the state. We are interested in the case where the state
satisfies proper differential equa- tions that are derived from certain
physical laws, such as Newton's law, Fourier's law etc. The space in
which the state exists is called the state space, and the equation that
the state satisfies is called the state equation. By an infinite
dimensional system we mean one whose corresponding state space is
infinite dimensional. In particular, we are interested in the case where
the state equation is one of the following types: partial differential
equation, functional differential equation, integro-differential
equation, or abstract evolution equation. The case in which the state
equation is being a stochastic differential equation is also an infinite
dimensional problem, but we will not discuss such a case in this book.