TITLE: Riemannian/Ricci curvature for round n-sphere
QUESTION [11 upvotes]: What is the best way to see that the Ricci scalar curvature of $(S^n(r),g_{round})$ is a constant $n(n-1)/r^2$ ? I essentially only see this value stated in the literature, but no computation associated with it, so I assume it is a straightforward calculation. But...
I keep doing the calculation, and it gets messy... Not sure of the best basis to view the round metric in, and whether to go straight through definitions with Riemannian/Ricci curvature or just use sectional curvature. So in particular, what's the best way to see that the sectional/Riemannian curvature is $1/r^2$ ?

REPLY [5 votes]: There is a compelling way to see this using the Gauss equation. Let us consider the sphere $S^n \subset \mathbb R^{n+1}$. Choose a point $p \in S^n$ and an orthonormal basis $\{e_i\}$ of $T_pS^n$ in which the second fundamental form is diagonalized, thus 
$$D_{e_i}\nu = \lambda_ie_i,$$ where $\nu$ is the normal vector ($\nu$ is the position vector in this case) and $D_{e_i}$ is the usual directional derivative in $\mathbb R^n$. Then the Gauss equation reads $$\mathrm{sec_{\mathbb R^{n+1}}}(e_i,e_j) = \mathrm{sec_{S^n}}(e_i,e_j) - \lambda_i\lambda_j.$$ On the other hand it is easy to calculate that $$2\mathrm{Hess}r = \frac{2}{r} g_r,$$ where $g_r = r^2ds^2_{n}$ and $g = dr^2 + g_r$ is the Euclidean metric on $\mathbb R^{n+1}$ written in spherical coordinates. Here $ds^2_{n}$ is the round metric on $S^{n}$. Thus $\lambda_i = \frac{1}{r}$ and since $\mathrm{sec_{\mathbb R^n}(e_i,e_j)} = 0$ it follows that $$\mathrm{sec_{S^n}}(e_i,e_j)= \frac{1}{r^2}.$$ 
Remark: This method can be found in Chapter 3 of Peter Petersen's great book "Riemannian Geometry."