嵌入到 $\mathbb{R}^3$ 中的环面, 其参数方程为
\[g(u,v):=\bigl((a+b\cos u)\cos v,(a+b\cos u)\sin v, b\sin u\bigr)\]
令 $U=[0,2\pi]\times [0,2\pi]$, 定义 $f:\ U\rightarrow T^2\subset\mathbb{R}^3$ 为
\[
(\theta,\phi)\mapsto\bigl((a+b\cos\phi)\cos\theta,(a+b\cos\phi)\sin\theta,b\sin\phi\bigr)
\]
下面我们用两种方法来求环面上的黎曼度量.
(法一)
设 $p$ 是环面上一点. 则 $p=\bigl((a+b\cos\phi)\cos\theta,(a+b\cos\phi)\sin\theta,b\sin\phi\bigr)$, 这里 $\theta\in [0,2\pi]$, $\phi\in [0,2\pi]$.
\[
\begin{aligned}
(E_\theta)_p &=\bigl(-(a+b\cos\phi)\sin\theta,(a+b\cos\phi)\cos\theta,0\bigr)\\
(E_\phi)_p &=\bigl(-b\sin\phi\cos\theta,-b\sin\phi\sin\theta,b\cos\phi\bigr)\\
\end{aligned}
\]
因此,
\[
\begin{aligned}
(g_{\theta\theta})_p &=\langle (E_\theta)_p,(E_\theta)_p\rangle=(a+b\cos\phi)^2\\
(g_{\theta\phi})_p &=\langle (E_\theta)_p,(E_\phi)_p\rangle=0\\
(g_{\phi\phi})_p &=\langle (E_\phi)_p,(E_\phi)_p\rangle=b^2
\end{aligned}
\]
因此
\[
(g_{ij})_{2\times 2}=\begin{pmatrix}
(a+b\cos\phi)^2 & 0\\
0 & b^2
\end{pmatrix}
\]
(法二)
对于映射 $f:\ T^2\rightarrow\mathbb{R}^3$
\[
f(\theta,\phi)=(f^1,f^2,f^3)=\bigl((a+b\cos\phi)\cos\theta,(a+b\cos\phi)\sin\theta,b\sin\phi\bigr)
\]
求微分, 得
\[
\begin{aligned}
df^1 &=-(a+b\cos\phi)\sin\theta d\theta-b\sin\phi\cos\theta d\phi\\
df^2 &=(a+b\cos\phi)\cos\theta d\theta-b\sin\phi\sin\theta d\phi\\
df^3 &=b\cos\phi d\phi
\end{aligned}
\]
因此,
\[
f^*\biggl(\sum_{i=1}^{3}dx^i\otimes dx^i\biggr)=\sum_{i=1}^{3}df^i\otimes df^i=(a+b\cos\phi)^2d\theta\otimes d\theta+b^2 d\phi\otimes d\phi
\]
故
\[
(g_{ij}(\theta,\phi))_{2\times 2}=\begin{pmatrix}
(a+b\cos\phi)^2 & 0\\
0 & b^2
\end{pmatrix}
\]
References:
Wilhelm Klingenberg, A Course in Differential Geometry, GTM 51. P.40