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Update 2024-06-08-DifferentialGeometry-NOTES-07.md
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peng00bo00 committed Jun 14, 2024
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9 changes: 5 additions & 4 deletions _posts/2024-06-08-DifferentialGeometry-NOTES-07.md
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$$
\varphi = \frac{1}{r!} \varphi_{i_1 \cdots i_r} \mathrm{d} u^{i_1} \wedge \cdots \wedge \mathrm{d} u^{i_r}
$$
是定义在区域$$D$$上的一个$$r$$次外微分式。用如下的方式定义$$r+1$$次外微分式:
是定义在区域$$D$$上的一个$$r$$次外微分式。用如下的方式定义$$r+1$$次外微分式:
$$
\begin{aligned}
\mathrm{d} \varphi &= \frac{1}{r!} \mathrm{d} \varphi_{i_1 \cdots i_r} \mathrm{d} u^{i_1} \wedge \cdots \wedge \mathrm{d} u^{i_r} \\
&= \frac{1}{r!} \frac{\partial \varphi_{i_1 \cdots i_r}}{\partial u^j} \mathrm{d} u^j \wedge \mathrm{d} u^{i_1} \wedge \cdots \wedge \mathrm{d} u^{i_r}
\end{aligned}
$$
$$
称为$$\varphi$$**外微分**。如果$$\varphi: D \rightarrow \mathbb{R}$$是定义在$$D$$上的连续可微函数(即零次外微分式),则它的外微分$$\mathrm{d} \varphi$$就是它的普通微分。
{:.success}

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在另一个曲纹坐标系$$(\tilde{u}^1, ..., \tilde{u}^n)$$下的表示是
$$
\varphi = \frac{1}{r!} \tilde{\varphi}_{i_1 \cdots i_r} (\tilde{u}^1, ..., \tilde{u}^n) \mathrm{d} \tilde{u}^{i_1} \wedge \cdots \wedge \mathrm{d} \tilde{u}^{i_r}
$$
其中假定$$\varphi_{i_1 \cdots i_r}$$$$\tilde{\varphi}_{i_1 \cdots i_r}$$对下指标都是反对称的,则有
$$
其中假定$$\varphi_{i_1 \cdots i_r}$$$$\tilde{\varphi}_{i_1 \cdots i_r}$$对下指标都是反对称的,则有
$$
\mathrm{d} \varphi_{i_1 \cdots i_r} \wedge \mathrm{d} u^{i_1} \wedge \cdots \wedge \mathrm{d} u^{i_r} = \mathrm{d} \tilde{\varphi}_{i_1 \cdots i_r} \wedge \mathrm{d} \tilde{u}^{i_1} \wedge \cdots \wedge \mathrm{d} \tilde{u}^{i_r}
$$
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