方向导数
一阶方向导数
方向导数的定义
对函数\(f:v\to \mathbb{R}\),记\(f_{\bm v}:t\to f(\bm x+t\bm v)\),若\(f_{\bm v}\)对\(t\)的微分在\(t=0\)处存在,那么可定义\(f\)在\(\bm x\)处沿向量\(\bm v\)的方向导数为
\[\mathrm{D}_{\bm v}f(\bm x)=\dfrac{\partial f}{\partial \bm v}=\frac{\mathrm{d}f_{\bm v}}{\mathrm{d}t}\bigg|_{t=0}=\lim_{t\to 0}\dfrac{f(\bm x+t\bm v)-f(\bm x)}{t}\\ \]方向导数的性质
- \(\mathrm{D}_{\bm v}(f+g)=\mathrm{D}_{\bm v}f+\mathrm{D}_{\bm v}g\)
- \(\mathrm{D}_{\bm v}(cf)=c\mathrm{D}_{\bm v}f\)
- \(\mathrm{D}_{\bm v}(fg)=g\mathrm{D}_{\bm v}f+f\mathrm{D}_{\bm v}g\)
- \(h:\mathbb{R}\to\mathbb{R},g:v\to \mathbb{R}\),则\(\mathrm{D}_{\bm v}(h\circ g)(\bm x)=h'(g(\bm x))\mathrm{D}_{\bm v}g(\bm x)\)
如果 \(f\) 在 \(\bm x\) 处可微,则沿着任意非零向量 \(\bm v\) 的方向导数都存在,且
\[\mathrm{D}_{\bm v}f(\bm x)=\mathrm{d}f(\bm v)|_{\bm x}=\bm v\cdot\nabla f(\bm x)\\ \]其中 \(\mathrm{d}f|_{\bm x}\) 表示 \(\bm x\) 处的全微分
方向导数与偏导数
对于 \(f:U\subset\mathbb{R^n}\to\mathbb{R}\),由于 \(\dfrac{\partial f}{\partial \bm v}=\bm v\cdot \nabla f\) ,而
\[\nabla f=\begin{pmatrix}\dfrac{\partial f}{\partial x_1}&\dfrac{\partial f}{\partial x_2}&\cdots&\dfrac{\partial f}{\partial x_n}\end{pmatrix}^T \]则
\[\dfrac{\partial f}{\partial \bm v}=v_1\dfrac{\partial f}{\partial x_1}+v_2\dfrac{\partial f}{\partial x_2}+\cdots+v_n\dfrac{\partial f}{\partial x_n}\\ \]二阶方向导数
二阶方向导数的定义
记 \(g=\dfrac{\partial f}{\partial \bm v}\),则 \(\dfrac{\partial g}{\partial \bm v}\)指 \(f\) 沿\(\bm v\)方向的二阶方向导数,记为 \(\mathrm{D}_{\bm{v}}^2 f(\bm{x})\) 或 \(\dfrac{\partial^2 f}{\partial \bm{v}^2}(\bm{x})\)。同时,记\(\dfrac{\partial g}{\partial \bm u}\)为 \(\dfrac{\partial^2 f}{\partial \bm v\partial \bm u}\)
二阶方向导数的性质
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如果 \(f\) 在 \(\bm{x}\) 处可二阶微分,那么 \(\mathrm{D}_{\bm{v}}^2 f(\bm{x})\) 存在
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\(\mathrm{D}_{\bm{v}}^2 f(\bm{x})\) 是一个标量
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如果 \(f\) 在 \(\bm{x}\) 处的二阶偏导数连续,那么 \(\mathrm{D}_{\bm{v}}^2 f(\bm{x})\) 也连续
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如果 \(f\) 在 \(\bm{x}\) 处的二阶偏导数存在,那么 \(\mathrm{D}_{\bm{v}}^2 f(\bm{x}) = \bm{v}^T \bold{H}_f(\bm{x}) \bm{v}\),其中 \(\bold{H}_f(\bm{x})\) 是 \(f\) 在 \(\bm{x}\) 处的 \(\text{Hessian}\) 矩阵