Linear system

In control engineering, a linear system may be thought of as a dynamical system which relates a certain set of signals (the input signals) to another set of signals (the output signals) in a linear fashion. Here an input signal is a signal that can be interpreted as entering the system while an output signal is one which can be interpreted as leaving the system. This is the definition of a linear system in an input output formalism in which signals can always be classified as being either an input or an output. Since it is not clear that this distinction between signals is generic to every system, it is debatable whether the input output formalism is always the most appropriate way of thinking about systems and other formalisms have been developed such as the behavioral appoach to systems theory. Nonetheless, there are similar definitions of a linear system within these other formalisms.

The linearity property of a linear system makes it more amenable to mathematical analysis. For instance, the linear equations describing the system can often be explicitly solved. Therefore, it is the most extensively studied type of system and this has led to the development of key system theoretic concepts, such as observability, controllability and passivity, which were subsequently generalized to other types of systems, such as nonlinear systems.

Formal definition

In this section a formal definition of a linear system will be given in an input output formalism.

Let $T\subset \mathbb {R}$, $U\subset \mathbb {R} ^{m}$ and $Y\subset \mathbb {R} ^{n}$, where $m,n$ are positive integers. Define ${\mathcal {U}}=\{u\mid u:T\rightarrow U\}$ while the set of output signals belong to the set ${\mathcal {Y}}=\{y\mid y:T\rightarrow Y\}$. Then a linear system L is simply a linear map $L:{\mathcal {U}}\rightarrow {\mathcal {Y}}$.

A map L from ${\mathcal {U}}$ to ${\mathcal {Y}}$ is said to be linear if $L(ax+by)=aL(x)+bL(y)$ for all $a,b\in \mathbb {R}$ and for all $x,y\in {\mathcal {U}}$

References

1. W. Brogan, Modern Control Theory (3 ed.), Saddle River, J: Prentice Hall, 1991.
2. K. Ogata, Modern Control Engineering (2 ed.), Lebanon, IN: Prentice Hall, 1990.
3. J. W. Polderman and J. C. Willems, Introduction to Mathematical Systems Theory: A Behavioral Approach (2 ed.), ser. Texts in Applied Mathematics Vol. 26, Springer, 2007.