# curvature form of the unit sphere

Trying to compute explicitly the curvature form of the unit sphere and got the following result: The parametrization of the unit 2-sphere $S^2$ is well known:

$n=(\sin\theta\cos\phi, \sin\theta\sin\phi, \cos\theta)$, let $$e_1=\frac{\partial}{\partial\theta}=(\cos\theta\cos\phi, \cos\theta\sin\phi, -\sin\theta)$$ $$e_2=\frac{1}{\sin\theta}\frac{\partial}{\partial\phi}=(-\sin\phi, \cos\phi,0)$$ be an orthonormal frame defined on an open set U of $S^2$, let $\theta^1=d\theta, \theta^2=\sin\theta d\phi$ be the dual basis. It's easy to compute that the Levi-Civita connection D on $S^2$ gives the following (where P is the orthogonal projection to the tangent space of $S^2$): $$D_{e_1}{e_1}=D_{e_1}{e_2}=0$$ $$D_{e_2}{e_1}=P(\frac{1}{\sin\theta}\frac{\partial}{\partial\phi}(\cos\theta\cos\phi, \cos\theta\sin\phi, -\sin\theta))=\cot\theta e_2$$ $$D_{e_2}{e_2}=P(\frac{1}{\sin\theta}\frac{\partial}{\partial\phi}(-\sin\phi, \cos\phi,0))=-\cot\theta e_1$$
So the connection form with respect to the frame $\{e_1, e_2\}$ is: $$\omega=\begin{bmatrix}0 & \cot\theta \theta^2 \\-\cot\theta\theta^2 & 0 \end{bmatrix}=\begin{bmatrix}0 & \cos\theta d\phi \\-\cos\theta d\phi & 0 \end{bmatrix}.$$
Hence the curvature form is: $$\Omega=d\omega-\omega\wedge\omega=\begin{bmatrix}0 & -\sin\theta d\theta\wedge d\phi \\\sin\theta d\theta\wedge d\phi & 0 \end{bmatrix}$$
To verify let's compute the Gaussian curvature: $$k=R(e_1, e_2, e_1, e_2)=\Omega_1^2(e_1,e_2)=-\sin\theta d\theta\wedge d\phi(\frac{\partial}{\partial\theta}, \frac{1}{\sin\theta}\frac{\partial}{\partial\phi})=-1$$ But where is this negative sign coming from?

Oh just realized that I missed a negative sign at the last step: $$K=-R(e_1, e_2, e_1, e_2)=1$$

• There are always two conventions of curvature, differs by a negative sign. – user99914 Feb 21 '15 at 8:00