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It assigns a tensor to each point of a Riemannian manifold (i.e., it is a tensor field). It is a local invariant of Riemannian metrics that measures the failure of the second covariant derivatives to commute. A Riemannian manifold has zero curvature if and only if it is flat, i.e. locally isometric to the Euclidean space. [1]
The Kulkarni–Nomizu product is an important tool for constructing new tensors from existing tensors on a Riemannian manifold. Let A {\displaystyle A} and B {\displaystyle B} be symmetric covariant 2-tensors.
A metric tensor g is positive-definite if g(v, v) > 0 for every nonzero vector v. A manifold equipped with a positive-definite metric tensor is known as a Riemannian manifold. Such a metric tensor can be thought of as specifying infinitesimal distance on the manifold.
A smooth manifold endowed with a Riemannian metric is a Riemannian manifold, denoted (,). [3] A Riemannian metric is a special case of a metric tensor. A Riemannian metric is not to be confused with the distance function of a metric space, which is also called a metric.
A Riemannian metric is a metric with a positive definite signature (v, 0). A Lorentzian metric is a metric with signature ( p , 1) , or (1, p ) . There is another notion of signature of a nondegenerate metric tensor given by a single number s defined as ( v − p ) , where v and p are as above, which is equivalent to the above definition when ...
In Riemannian geometry and pseudo-Riemannian geometry, the trace-free Ricci tensor (also called traceless Ricci tensor) of a Riemannian or pseudo-Riemannian -manifold (,) is the tensor defined by Z = Ric − 1 n R g , {\displaystyle Z=\operatorname {Ric} -{\frac {1}{n}}Rg,}
Although individually, the Weyl tensor and Ricci tensor do not in general determine the full curvature tensor, the Riemann curvature tensor can be decomposed into a Weyl part and a Ricci part. This decomposition is known as the Ricci decomposition, and plays an important role in the conformal geometry of Riemannian manifolds.
Riemannian geometry is the branch of differential geometry that studies Riemannian manifolds, defined as smooth manifolds with a Riemannian metric (an inner product on the tangent space at each point that varies smoothly from point to point). This gives, in particular, local notions of angle, length of curves, surface area and volume.