TITLE: Relationship beween Ricci curvature and sectional curvature
QUESTION [11 upvotes]: Let $(M,g)$ be a Riemannian manifold and assume that for all orthonormal $v,z$ the sectional curvatures is bounded from below i.e. $K(v,z) \geq C$, where $C > 0$. Is it in this case possible for the Ricci curvature to vanish? Or is this condition, on the sectional curvature, very strong? Sorry if the question is too trivial :).
Gunam

REPLY [10 votes]: You basically just have to look at the definitions: Given a unit-length tangent vector $x\in T_pM$, we obtain the Ricci curvature of $x$ at $p$ by extending $x=z_n$ to an orthonormal basis $z_1, \ldots, z_n$. And then
$$\mathrm{Ric}_p(x) = \frac{1}{n-1}\sum_{i=1}^{n-1} \langle R(x,z_i)x,z_i\rangle$$
where $R$ denotes the Riemannian curvature tensor.
On the other hand, the sectional curvature $K(x, z_i)$ for $i<n$ is given by (remember the $z_i$ are orthonormal) 
$$K(x, z_i) = \frac{\langle R(x,z_i)x,z_i\rangle}{\Vert x\Vert\, \Vert z_i\Vert-|\langle x, z_i\rangle|^2}=\langle R(x,z_i)x,z_i\rangle$$
Since $K(x,z_i) \ge C$ by your assumption, we obtain
$$\mathrm{Ric}_p(x) = \frac{1}{n-1}\sum_{i=1}^{n-1} \langle R(x,z_i)x,z_i\rangle \ge C > 0$$
So you get a lower bound on the Ricci curvature.
Attention: All sign conventions are as can be found in DoCarmo's "Riemannian Geometry".