高等数学笔记:多元抽象复合函数求二阶偏导数 |
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多元抽象复合函数求二阶偏导
一、多元复合函数
形如
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f(x+y+z,xyz)
f(x+y+z,xyz) 的函数
二、求导策略
一阶偏导求解的核心策略是:链式法则(最好用!)二阶偏导求解的核心策略是:矩阵公式法
三、雅可比矩阵与海森矩阵
01 雅可比矩阵与海森矩阵的概念
现有多元函数组 f 1 ( x 1 , x 2 , ⋯ , x n ) , f 2 ( x 1 , x 2 , ⋯ , x n ) , ⋯ , f m ( x 1 , x 2 , ⋯ , x n ) \mathrm{f_1(x_1,x_2,\cdots,x_n)\ ,\ f_2(x_1,x_2,\cdots,x_n)\ ,\cdots\ ,\ f_m(x_1,x_2,\cdots,x_n)} f1(x1,x2,⋯,xn) , f2(x1,x2,⋯,xn) ,⋯ , fm(x1,x2,⋯,xn), 那么其雅可比矩阵如下所示,它储存了该函数组所有一阶偏导数的信息 J = [ ∂ f 1 ∂ x 1 ∂ f 1 ∂ x 2 ⋯ ∂ f 1 ∂ x n ∂ f 2 ∂ x 1 ∂ f 2 ∂ x 2 ⋯ ∂ f 2 ∂ x n ⋮ ⋮ ⋱ ⋮ ∂ f m ∂ x 1 ∂ f m ∂ x 2 ⋯ ∂ f m ∂ x n ] \mathrm{ J= \left[\begin{array}{cccc} \displaystyle{ \frac{\partial f_{1}}{\partial x_{1}} } & \displaystyle{ \frac{\partial f_{1}}{\partial x_{2}} } & \cdots & \displaystyle{ \frac{\partial f_{1}}{\partial x_{n}} } \\ \displaystyle{ \frac{\partial f_{2}}{\partial x_{1}} } & \displaystyle{ \frac{\partial f_{2}}{\partial x_{2}} } & \cdots & \displaystyle{ \frac{\partial f_{2}}{\partial x_{n}} } \\ \vdots & \vdots & \ddots & \vdots \\ \displaystyle{ \frac{\partial f_{m}}{\partial x_{1}} } & \displaystyle{ \frac{\partial f_{m}}{\partial x_{2}} } & \cdots & \displaystyle{ \frac{\partial f_{m}}{\partial x_{n}} } \\ \end{array}\right] } J= ∂x1∂f1∂x1∂f2⋮∂x1∂fm∂x2∂f1∂x2∂f2⋮∂x2∂fm⋯⋯⋱⋯∂xn∂f1∂xn∂f2⋮∂xn∂fm 我们可以观察到,它满足同一行(hang)同一函数(han),同一列(lie)同一自变量(liang)的规律 当多元函数组退化为一个多元函数时,其表示成 J = [ ∂ f ∂ x 1 , ∂ f ∂ x 2 , ⋯ , ∂ f ∂ x n ] \mathrm{ J= \left[\begin{array}{cccc} \displaystyle{ \frac{\partial f}{\partial x_{1}} } \ ,\ \displaystyle{ \frac{\partial f}{\partial x_{2}} } \ ,\ \cdots \ ,\ \displaystyle{ \frac{\partial f}{\partial x_{n}} } \end{array}\right] } J=[∂x1∂f , ∂x2∂f , ⋯ , ∂xn∂f] 此时这个多元函数的海森矩阵如下所示,它储存了这个多元函数所有二阶偏导数的信息 H = [ ∂ 2 f ∂ x 1 2 ∂ 2 f ∂ x 1 ∂ x 2 ⋯ ∂ 2 f ∂ x 1 ∂ x n ∂ 2 f ∂ x 2 ∂ x 1 ∂ 2 f ∂ x 2 2 ⋯ ∂ 2 f ∂ x 2 ∂ x n ⋮ ⋮ ⋱ ⋮ ∂ 2 f ∂ x n ∂ x 1 ∂ 2 f ∂ x n ∂ x 2 ⋯ ∂ 2 f ∂ x n 2 ] \mathrm{ H= \left[\begin{array}{cccc} \displaystyle{ \frac{\partial^2 f}{\partial x_1^2} } & \displaystyle{ \frac{\partial^2 f}{\partial x_1\partial x_2} } & \cdots & \displaystyle{ \frac{\partial^2 f}{\partial x_1\partial x_n} } \\ \displaystyle{ \frac{\partial^2 f}{\partial x_2\partial x_1} } & \displaystyle{ \frac{\partial^2 f}{\partial x_2^2} } & \cdots & \displaystyle{ \frac{\partial^2 f}{\partial x_2\partial x_n} } \\ \vdots & \vdots & \ddots & \vdots \\ \displaystyle{ \frac{\partial^2 f}{\partial x_n\partial x_1} } & \displaystyle{ \frac{\partial^2 f}{\partial x_n\partial x_2} } & \cdots & \displaystyle{ \frac{\partial^2 f}{\partial x_n^2} } \\ \end{array}\right] } H= ∂x12∂2f∂x2∂x1∂2f⋮∂xn∂x1∂2f∂x1∂x2∂2f∂x22∂2f⋮∂xn∂x2∂2f⋯⋯⋱⋯∂x1∂xn∂2f∂x2∂xn∂2f⋮∂xn2∂2f 显然,当多元函数 f \mathrm{f} f 二阶偏导数连续时,海森矩阵为实对称矩阵 02 不同情况下雅可比行列式的形式当多元函数组由两个二元函数 { u ( x , y ) = 0 v ( x , y ) = 0 \mathrm{\begin{cases}\ u(x,y)=0 \\ \ v(x,y)=0 \end{cases}} { u(x,y)=0 v(x,y)=0 组成时,我们可以把它写成 J = [ ∂ u ∂ x ∂ u ∂ y ∂ v ∂ x ∂ v ∂ y ] \mathrm{ J= \left[\begin{array}{cccc} \displaystyle{ \frac{\partial u}{\partial x} } & \displaystyle{ \frac{\partial u}{\partial y} } \\ \displaystyle{ \frac{\partial v}{\partial x} } & \displaystyle{ \frac{\partial v}{\partial y} } \end{array}\right] } J= ∂x∂u∂x∂v∂y∂u∂y∂v 当多元函数组由三个二元函数 { u ( x , y ) = 0 v ( x , y ) = 0 w ( x , y ) = 0 \mathrm{\begin{cases}\ u(x,y)=0 \\ \ v(x,y)=0\\ \ w(x,y)=0 \end{cases}} ⎩ ⎨ ⎧ u(x,y)=0 v(x,y)=0 w(x,y)=0 组成时,我们可以把它写成 J = [ ∂ u ∂ x ∂ u ∂ y ∂ v ∂ x ∂ v ∂ y ∂ w ∂ x ∂ w ∂ y ] \mathrm{ J= \left[\begin{array}{cccc} \displaystyle{ \frac{\partial u}{\partial x} } & \displaystyle{ \frac{\partial u}{\partial y} } \\ \displaystyle{ \frac{\partial v}{\partial x} } & \displaystyle{ \frac{\partial v}{\partial y} } \\ \displaystyle{ \frac{\partial w}{\partial x} } & \displaystyle{ \frac{\partial w}{\partial y} } \end{array}\right] } J= ∂x∂u∂x∂v∂x∂w∂y∂u∂y∂v∂y∂w 03 不同情况下海森矩阵的形式当函数为 f ( x , y ) = 0 f(x,y)=0 f(x,y)=0 时,海森矩阵表达为: H = [ f x x ′ ′ f x y ′ ′ f y x ′ ′ f y y ′ ′ ] \mathrm{ H= \left[\begin{array}{cccc} f''_{xx} & f''_{xy}\\ f''_{yx} & f''_{yy} \end{array}\right] } H=[fxx′′fyx′′fxy′′fyy′′] 当函数为 F ( x , y , z ) = 0 F(x,y,z)=0 F(x,y,z)=0 时,海森矩阵表达为: H = [ F x x ′ ′ F x y ′ ′ F x z ′ ′ F y x ′ ′ F y y ′ ′ F y z ′ ′ F z x ′ ′ F z y ′ ′ F z z ′ ′ ] \mathrm{ H= \left[\begin{array}{cccc} F''_{xx} & F''_{xy} & F''_{xz}\\ F''_{yx} & F''_{yy} & F''_{yz}\\ F''_{zx} & F''_{zy} & F''_{zz} \end{array}\right] } H= Fxx′′Fyx′′Fzx′′Fxy′′Fyy′′Fzy′′Fxz′′Fyz′′Fzz′′ 四、矩阵公式法 四、矩阵公式法 01 定理内容 (1) 一般的偏导数求解 若函数 u = f ( x 1 , x 2 , ⋯ , x n ) u=f\left(x_{1}, x_{2}, \cdots, x_{n}\right) u=f(x1,x2,⋯,xn) 在 ( x 1 , x 2 , ⋯ , x n ) \left(x_{1}, x_{2}, \cdots, x_{n}\right) (x1,x2,⋯,xn) 可微,而 x i = φ i ( t 1 , t 2 , ⋯ , t m ) x_{i}=\varphi_{i}\left(t_{1}, t_{2}, \cdots, t_{m}\right) xi=φi(t1,t2,⋯,tm) 在点 ( t 1 , t 2 \left(t_{1}, t_{2}\right. (t1,t2, ⋯ , t m ) \left.\cdots, t_{m}\right) ⋯,tm) 处存在偏导数 ( i = 1 , 2 , ⋯ , n ) (i=1,2, \cdots, n) (i=1,2,⋯,n),则 u = f ( φ 1 ( t 1 , t 2 , ⋯ , t m ) , ⋯ , φ n ( t 1 , t 2 , ⋯ , t m ) ) u=f\left(\varphi_{1}\left(t_{1}, t_{2}, \cdots, t_{m}\right), \cdots, \varphi_{n}\left(t_{1}, t_{2}, \cdots, t_{m}\right)\right) u=f(φ1(t1,t2,⋯,tm),⋯,φn(t1,t2,⋯,tm)) 在 ( t 1 , t 2 , ⋯ , t m ) \left(t_{1}, t_{2}, \cdots, t_{m}\right) (t1,t2,⋯,tm) 处存在偏导数,且有: ∂ u ∂ t j = ∂ u ∂ x 1 ∂ φ 1 ∂ t j + ∂ u ∂ x 2 ∂ φ 2 ∂ t j + … + ∂ u ∂ x n ∂ φ n ∂ t j , ( j = 1 , ⋯ , m ) . \displaystyle{ \frac{\partial u}{\partial t_{j}}=\frac{\partial u}{\partial x_{1}} \frac{\partial \varphi_{1}}{\partial t_{j}}+\frac{\partial u}{\partial x_{2}} \frac{\partial \varphi_{2}}{\partial t_{j}}+\ldots+\frac{\partial u}{\partial x_{n}} \frac{\partial \varphi_{n}}{\partial t_{j}},(j=1, \cdots, m) . } ∂tj∂u=∂x1∂u∂tj∂φ1+∂x2∂u∂tj∂φ2+…+∂xn∂u∂tj∂φn,(j=1,⋯,m). (2) 矩阵公式定理设函数 u = f ( x 1 , x 2 , ⋯ , x n ) , ∂ u ∂ x 1 , ∂ u ∂ x 2 , ⋯ , ∂ u ∂ x n \displaystyle{ u=f\left(x_{1}, x_{2}, \cdots, x_{n}\right)\ ,\ \frac{\partial u}{\partial x_{1}}, \frac{\partial u}{\partial x_{2}}, \cdots, \frac{\partial u}{\partial x_{n}} } u=f(x1,x2,⋯,xn) , ∂x1∂u,∂x2∂u,⋯,∂xn∂u 在 ( x 1 , x 2 , ⋯ , x n ) \left(x_{1}, x_{2}, \cdots, x_{n}\right) (x1,x2,⋯,xn) 处可微, 函数 x i = φ i ( t 1 , t 2 x_{i}=\varphi_{i}\left(t_{1}, t_{2}\right. xi=φi(t1,t2, ⋯ , t m ) \left.\cdots, t_{m}\right) ⋯,tm) 在点 ( t 1 , t 2 , ⋯ , t m ) \left(t_{1}, t_{2}, \cdots, t_{m}\right) (t1,t2,⋯,tm) 有二阶偏导数, 则复合函数 u = f ( φ 1 ( t 1 , t 2 , ⋯ , t m ) , ⋯ , φ n ( t 1 , t 2 , ⋯ , t m ) ) u=f\left(\varphi_{1}\left(t_{1}, t_{2}, \cdots, t_{m}\right), \cdots, \varphi_{n}\left(t_{1}, t_{2}, \cdots, t_{m}\right)\right) u=f(φ1(t1,t2,⋯,tm),⋯,φn(t1,t2,⋯,tm)) 在点 ( t 1 , t 2 , ⋯ , t m ) \left(t_{1}, t_{2}, \cdots, t_{m}\right) (t1,t2,⋯,tm) 处存在二阶偏导数, 且有公式( i ∈ { 1 , 2 , ⋯ , m } , j = 1 , 2 , ⋯ , m i\in\{1,2, \cdots, m\}\ , \ j=1,2, \cdots, m i∈{1,2,⋯,m} , j=1,2,⋯,m ): 在解决实际问题的时候,我们通常接触的是,一阶、二阶、三阶矩阵的形式,我们下面分别给出: (1) 一阶形式显然,我们可以统一写成如下形式: 接下来根据上述结论我们讨论一种常见的类型,线性复合。 形如 z = f ( u , v ) , u = a x + b y + e , v = c x + d y + f z=f(u,v)\ , \ u=ax+by+e\ , \ v=cx+dy+f z=f(u,v) , u=ax+by+e , v=cx+dy+f 的二元抽象线性复合函数,代入公式可以得到: |
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