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通常，非线性系统的行为在数学上是由一组非线性联立方程来描述的，其中未知数（或微分方程中的未知函数）作为一个高于一次的多项式的变量出现，或者作为一个不是一次多项式的函数的论元出现。__[[用户:Dorr|Dorr]]（[[用户讨论:Dorr|讨论]]）后半句不知如何翻译
通常，非线性系统的行为在数学上是由一组非线性联立方程来描述的，其中未知数（或微分方程中的未知函数）作为一个高于一次的多项式的变量出现，或者作为一个不是一次多项式的函数的论元出现。__[[用户:Dorr|Dorr]]（[[用户讨论:Dorr|讨论]]）后半句不知如何翻译
<font color='blue'>通常，非线性系统的行为在数学上被描述为一组非线性的联立方程组，其中未知数（或微分方程中的未知函数）作为一个高于一次的多项式变量出现，或者作为一个非一次多项式函数的参数出现。</font>
In other words, in a nonlinear system of equations, the equation(s) to be solved cannot be written as a [[linear combination]] of the unknown [[variable (mathematics)|variables]] or [[function (mathematics)|functions]] that appear in them. Systems can be defined as nonlinear, regardless of whether known linear functions appear in the equations. In particular, a differential equation is ''linear'' if it is linear in terms of the unknown function and its derivatives, even if nonlinear in terms of the other variables appearing in it.
In other words, in a nonlinear system of equations, the equation(s) to be solved cannot be written as a [[linear combination]] of the unknown [[variable (mathematics)|variables]] or [[function (mathematics)|functions]] that appear in them. Systems can be defined as nonlinear, regardless of whether known linear functions appear in the equations. In particular, a differential equation is ''linear'' if it is linear in terms of the unknown function and its derivatives, even if nonlinear in terms of the other variables appearing in it.
In other words, in a nonlinear system of equations, the equation(s) to be solved cannot be written as a linear combination of the unknown variables or functions that appear in them. Systems can be defined as nonlinear, regardless of whether known linear functions appear in the equations. In particular, a differential equation is linear if it is linear in terms of the unknown function and its derivatives, even if nonlinear in terms of the other variables appearing in it.
In other words, in a nonlinear system of equations, the equation(s) to be solved cannot be written as a linear combination of the unknown variables or functions that appear in them. Systems can be defined as nonlinear, regardless of whether known linear functions appear in the equations. In particular, a differential equation is linear if it is linear in terms of the unknown function and its derivatives, even if nonlinear in terms of the other variables appearing in it.
换句话说，在非线性方程系统中，<font color='red'>要求解</font> <font color='blue'>待解</font>的方程不能被写成未知变量或函数的线性组合。无论方程中是否有已知的线性函数，系统都可以被定义为非线性。特别是当一个微分方程的未知函数及其导数是线性的，即使其他变量是非线性的，也称该方程是线性的。