# Nonlinear system

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In mathematics, a nonlinear system is a system which is not linear, that is, a system which does not satisfy the superposition principle, or whose output is not directly proportional to its input. Less technically, a nonlinear system is any problem where the variable(s) to be solved for cannot be written as a linear combination of independent components. A nonhomogeneous system, which is linear apart from the presence of a function of the independent variables, is nonlinear according to a strict definition, but such systems are usually studied alongside linear systems, because they can be transformed to a linear system of multiple variables.

Nonlinear problems are of interest to engineers, physicists and mathematicians because most physical systems are inherently nonlinear in nature. Nonlinear equations are difficult to solve and give rise to interesting phenomena such as chaos. The weather is famously chaotic, where simple changes in one part of the system produce complex effects throughout.

## Contents

### Definition

In mathematics, a linear function (or map) f(x) is one which satisfies both of the following properties:

• additivity, $\textstyle f(x + y)\ = f(x)\ + f(y);$
• homogeneity, $\textstyle f(\alpha x)\ = \alpha f(x).$

(Additivity implies homogeneity for any rational α, and, for continuous functions, for any real α. For a complex α, homogeneity does not follow from additivity; for example, an antilinear map is additive but not homogeneous.)