Riemannian geometry
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Riemannian geometry is the branch of differential geometry that studies Riemannian manifolds, smooth manifolds with a Riemannian metric, i.e. with 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. From those, some other global quantities can be derived by integrating local contributions.
Riemannian geometry originated with the vision of Bernhard Riemann expressed in his inaugural lecture "Ueber die Hypothesen, welche der Geometrie zu Grunde liegen" ("On the Hypotheses on which Geometry is Based"). It is a very broad and abstract generalization of the differential geometry of surfaces in R^{3}. Development of Riemannian geometry resulted in synthesis of diverse results concerning the geometry of surfaces and the behavior of geodesics on them, with techniques that can be applied to the study of differentiable manifolds of higher dimensions. It enabled the formulation of Einstein's general theory of relativity, made profound impact on group theory and representation theory, as well as analysis, and spurred the development of algebraic and differential topology.
Introduction
Riemannian geometry was first put forward in generality by Bernhard Riemann in the 19th century. It deals with a broad range of geometries whose metric properties vary from point to point, including the standard types of NonEuclidean geometry.
Any smooth manifold admits a Riemannian metric, which often helps to solve problems of differential topology. It also serves as an entry level for the more complicated structure of pseudoRiemannian manifolds, which (in four dimensions) are the main objects of the theory of general relativity. Other generalizations of Riemannian geometry include Finsler geometry.
There exists a close analogy of differential geometry with the mathematical structure of defects in regular crystals. Dislocations and Disclinations produce torsions and curvature.^{[1]}^{[2]}
The following articles provide some useful introductory material:
 Metric tensor
 Riemannian manifold
 LeviCivita connection
 Curvature
 Curvature tensor
 List of differential geometry topics
 Glossary of Riemannian and metric geometry
Classical theorems
What follows is an incomplete list of the most classical theorems in Riemannian geometry. The choice is made depending on its importance and elegance of formulation. Most of the results can be found in the classic monograph by Jeff Cheeger and D. Ebin (see below).
The formulations given are far from being very exact or the most general. This list is oriented to those who already know the basic definitions and want to know what these definitions are about.
General theorems
 Gauss–Bonnet theorem The integral of the Gauss curvature on a compact 2dimensional Riemannian manifold is equal to 2πχ(M) where χ(M) denotes the Euler characteristic of M. This theorem has a generalization to any compact evendimensional Riemannian manifold, see generalized GaussBonnet theorem.
 Nash embedding theorems also called fundamental theorems of Riemannian geometry. They state that every Riemannian manifold can be isometrically embedded in a Euclidean space R^{n}.
Geometry in large
In all of the following theorems we assume some local behavior of the space (usually formulated using curvature assumption) to derive some information about the global structure of the space, including either some information on the topological type of the manifold or on the behavior of points at "sufficiently large" distances.
Pinched sectional curvature
 Sphere theorem. If M is a simply connected compact ndimensional Riemannian manifold with sectional curvature strictly pinched between 1/4 and 1 then M is diffeomorphic to a sphere.
 Cheeger's finiteness theorem. Given constants C, D and V, there are only finitely many (up to diffeomorphism) compact ndimensional Riemannian manifolds with sectional curvature K ≤ C, diameter ≤ D and volume ≥ V.
 Gromov's almost flat manifolds. There is an ε_{n} >0 such that if an ndimensional Riemannian manifold has a metric with sectional curvature K ≤ ε_{n} and diameter ≤ 1 then its finite cover is diffeomorphic to a nil manifold.
Sectional curvature bounded below
 CheegerGromoll's Soul theorem. If M is a noncompact complete nonnegatively curved ndimensional Riemannian manifold, then M contains a compact, totally geodesic submanifold S such that M is diffeomorphic to the normal bundle of S (S is called the soul of M.) In particular, if M has strictly positive curvature everywhere, then it is diffeomorphic to R^{n}. G. Perelman in 1994 gave an astonishingly elegant/short proof of the Soul Conjecture: M is diffeomorphic to R^{n} if it has positive curvature at only one point.
 Gromov's Betti number theorem. There is a constant C = C(n) such that if M is a compact connected ndimensional Riemannian manifold with positive sectional curvature then the sum of its Betti numbers is at most C.
 Grove–Petersen's finiteness theorem. Given constants C, D and V, there are only finitely many homotopy types of compact ndimensional Riemannian manifolds with sectional curvature K ≥ C, diameter ≤ D and volume ≥ V.
Sectional curvature bounded above
 The Cartan–Hadamard theorem states that a complete simply connected Riemannian manifold M with nonpositive sectional curvature is diffeomorphic to the Euclidean space R^{n} with n = dim M via the exponential map at any point. It implies that any two points of a simply connected complete Riemannian manifold with nonpositive sectional curvature are joined by a unique geodesic.
 The geodesic flow of any compact Riemannian manifold with negative sectional curvature is ergodic.
 If M is a complete Riemannian manifold with sectional curvature bounded above by a strictly negative constant k then it is a CAT(k) space. Consequently, its fundamental group Γ = π_{1}(M) is Gromov hyperbolic. This has many implications for the structure of the fundamental group:


 it is finitely presented;
 the word problem for Γ has a positive solution;
 the group Γ has finite virtual cohomological dimension;
 it contains only finitely many conjugacy classes of elements of finite order;
 the abelian subgroups of Γ are virtually cyclic, so that it does not contain a subgroup isomorphic to Z×Z.

Ricci curvature bounded below
 Myers theorem. If a compact Riemannian manifold has positive Ricci curvature then its fundamental group is finite.
 Bochner's formula. If a compact Riemannian nmanifold has nonnegative Ricci curvature, then its first Betti number is at most n, with equality if and only if the Riemannian manifold is a flat torus.
 Splitting theorem. If a complete ndimensional Riemannian manifold has nonnegative Ricci curvature and a straight line (i.e. a geodesic that minimizes distance on each interval) then it is isometric to a direct product of the real line and a complete (n1)dimensional Riemannian manifold that has nonnegative Ricci curvature.
 Bishop–Gromov inequality. The volume of a metric ball of radius r in a complete ndimensional Riemannian manifold with positive Ricci curvature has volume at most that of the volume of a ball of the same radius r in Euclidean space.
 Gromov's compactness theorem. The set of all Riemannian manifolds with positive Ricci curvature and diameter at most D is precompact in the GromovHausdorff metric.
Negative Ricci curvature
 The isometry group of a compact Riemannian manifold with negative Ricci curvature is discrete.
 Any smooth manifold of dimension n ≥ 3 admits a Riemannian metric with negative Ricci curvature.^{[3]} (This is not true for surfaces.)
Positive scalar curvature
 The ndimensional torus does not admit a metric with positive scalar curvature.
 If the injectivity radius of a compact ndimensional Riemannian manifold is ≥ π then the average scalar curvature is at most n(n1).
See also
 Shape of the universe
 Basic introduction to the mathematics of curved spacetime
 Normal coordinates
 Systolic geometry
 Riemann–Cartan geometry in Einstein–Cartan theory (motivation)
 Riemann's minimal surface
Notes
 ^ Kleinert, Hagen (1989). "Gauge Fields in Condensed Matter Vol II": 743–1440.
 ^ Kleinert, Hagen (2008). "Multivalued Fields in Condensed Matter, Electromagnetism, and Gravitation" (PDF): 1–496.
 ^ Joachim Lohkamp has shown (Annals of Mathematics, 1994) that any manifold of dimension greater than two admits a metric of negative Ricci curvature.
References
 Books
 Berger, Marcel (2000), Riemannian Geometry During the Second Half of the Twentieth Century, University Lecture Series, 17, Rhode Island: American Mathematical Society, ISBN 0821820524. (Provides a historical review and survey, including hundreds of references.)
 Cheeger, Jeff; Ebin, David G. (2008), Comparison theorems in Riemannian geometry, Providence, RI: AMS Chelsea Publishing; Revised reprint of the 1975 original.
 Gallot, Sylvestre; Hulin, Dominique; Lafontaine, Jacques (2004), Riemannian geometry, Universitext (3rd ed.), Berlin: SpringerVerlag.
 Jost, Jürgen (2002), Riemannian Geometry and Geometric Analysis, Berlin: SpringerVerlag, ISBN 3540426272.
 Petersen, Peter (2006), Riemannian Geometry, Berlin: SpringerVerlag, ISBN 0387982124
 Papers
 Brendle, Simon; Schoen, Richard M. (2007), Classification of manifolds with weakly 1/4pinched curvatures, arXiv:0705.3963
External links
 Riemannian geometry by V. A. Toponogov at the Encyclopedia of Mathematics
 Weisstein, Eric W. "Riemannian Geometry". MathWorld.