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Given a Riemannian metric g, the scalar curvature Scal is defined as the trace of the Ricci curvature tensor with respect to the metric: [1] = . The scalar curvature cannot be computed directly from the Ricci curvature since the latter is a (0,2)-tensor field; the metric must be used to raise an index to obtain a (1,1)-tensor field in order to take the trace.
The Weyl tensor is invariant with respect to a conformal change of metric: if two metrics are related as ~ = for some positive scalar function , then ~ = . In dimensions 2 and 3 the Weyl tensor vanishes, but in 4 or more dimensions the Weyl tensor can be non-zero.
A positive curvature corresponds to the inverse square radius of curvature; an example is a sphere or hypersphere. An example of negatively curved space is hyperbolic geometry (see also: non-positive curvature). A space or space-time with zero curvature is called flat.
In Riemannian geometry, a branch of mathematics, the prescribed scalar curvature problem is as follows: given a closed, smooth manifold M and a smooth, real-valued function ƒ on M, construct a Riemannian metric on M whose scalar curvature equals ƒ. Due primarily to the work of J. Kazdan and F. Warner in the 1970s, this problem is well understood.
Here is a short list of global results concerning manifolds with positive Ricci curvature; see also classical theorems of Riemannian geometry. Briefly, positive Ricci curvature of a Riemannian manifold has strong topological consequences, while (for dimension at least 3), negative Ricci curvature has no topological
Here is the Ricci curvature tensor and represents the Lie derivative. If there exists a function f : M → R {\displaystyle f:M\rightarrow \mathbb {R} } such that V = ∇ f {\displaystyle V=\nabla f} we call ( M , g ) {\displaystyle (M,g)} a gradient Ricci soliton and the soliton equation becomes
The only regular (of class C 2) closed surfaces in R 3 with constant positive Gaussian curvature are spheres. [2] If a sphere is deformed, it does not remain a sphere, proving that a sphere is rigid. A standard proof uses Hilbert's lemma that non-umbilical points of extreme principal curvature have non-positive Gaussian curvature. [3]
In mathematical physics, n-dimensional de Sitter space (often denoted dS n) is a maximally symmetric Lorentzian manifold with constant positive scalar curvature.It is the Lorentzian [further explanation needed] analogue of an n-sphere (with its canonical Riemannian metric).