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A Riemannian space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252701.png" /> for which the sectional curvature <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252702.png" /> is constant in all two-dimensional directions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252703.png" />; if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252704.png" />, then the space is said to be of constant curvature <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252705.png" />. By Schur's theorem, a Riemannian space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252706.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252707.png" />, is a space of constant curvature if for any point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252708.png" /> the sectional curvatures <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c0252709.png" /> in the directions of every two-dimensional subspace <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527010.png" /> of the tangent space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527011.png" /> are the same. The curvature tensor of a space of constant curvature is expressed in terms of the curvature <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527012.png" /> and the metric tensor <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527013.png" /> by the formula
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A Riemannian space $M$ for which the sectional curvature $K(\sigma)$ is constant in all two-dimensional directions $\sigma$; if $K(\sigma)=k$, then the space is said to be of constant curvature $k$. By Schur's theorem, a Riemannian space $M^n$, $n>2$, is a space of constant curvature if for any point $p\in M$ the sectional curvatures $K(\sigma)$ in the directions of every two-dimensional subspace $\sigma$ of the tangent space $T_pM$ are the same. The curvature tensor of a space of constant curvature is expressed in terms of the curvature $k$ and the metric tensor $g_{ij}$ by the formula
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527014.png" /></td> </tr></table>
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$$R_{jlk}^i=k(\delta_k^ig_{jl}-\delta_l^ig_{jk}).$$
  
 
A space of a constant curvature is a locally symmetric space.
 
A space of a constant curvature is a locally symmetric space.
  
Up to an isometry there exists a unique complete simply-connected <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527015.png" />-dimensional Riemannian space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527016.png" /> of constant curvature <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527017.png" />. For <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527018.png" /> it is [[Euclidean space|Euclidean space]], for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527019.png" /> it is the sphere of radius <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527020.png" />, for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527021.png" /> it is [[Lobachevskii space|Lobachevskii space]].
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Up to an isometry there exists a unique complete simply-connected $n$-dimensional Riemannian space $S^n(k)$ of constant curvature $k$. For $k=0$ it is [[Euclidean space|Euclidean space]], for $k>0$ it is the sphere of radius $1/\sqrt k$, for $k<0$ it is [[Lobachevskii space|Lobachevskii space]].
  
The spaces <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527022.png" /> are maximal homogeneous spaces, i.e. their group of motions has maximum possible dimension <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527023.png" />. All maximal homogeneous Riemannian spaces different from <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527024.png" /> are exhausted by projective (elliptic) spaces obtained from spheres by identification of antipodal points.
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The spaces $S^n(k)$ are maximal homogeneous spaces, i.e. their group of motions has maximum possible dimension $n(n+1)/2$. All maximal homogeneous Riemannian spaces different from $S^n(k)$ are exhausted by projective (elliptic) spaces obtained from spheres by identification of antipodal points.
  
Complete but multiply-connected spaces of constant curvature are called space forms. They are obtained by factorizing a simply-connected space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527025.png" /> by a freely-acting discrete group of motions of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527026.png" />. All space forms of positive curvature are known. The problem of classifying space forms of zero curvature and negative curvature has not yet (1983) been completely solved.
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Complete but multiply-connected spaces of constant curvature are called space forms. They are obtained by factorizing a simply-connected space $S^n(k)$ by a freely-acting discrete group of motions of $S^n(k)$. All space forms of positive curvature are known. The problem of classifying space forms of zero curvature and negative curvature has not yet (1983) been completely solved.
  
 
Spaces of constant curvature are distinguished from the other Riemannian spaces by one of the following characteristic properties: 1) spaces of constant curvature satisfy the axiom of planes, i.e. through every point and in the direction of every plane element at this point there passes a totally-geodesic submanifold; and 2) a space of constant curvature is a locally projectively-flat space, i.e. it admits locally projective mappings into Euclidean space.
 
Spaces of constant curvature are distinguished from the other Riemannian spaces by one of the following characteristic properties: 1) spaces of constant curvature satisfy the axiom of planes, i.e. through every point and in the direction of every plane element at this point there passes a totally-geodesic submanifold; and 2) a space of constant curvature is a locally projectively-flat space, i.e. it admits locally projective mappings into Euclidean space.
  
The notion of a space of constant curvature does not have the property of  "well-posedness"  ( "correctness" ): a space with slowly varying sectional curvatures may be very different from a space of constant curvature. However, certain common properties of spaces of constant curvature, for example the topological structure, are preserved (the Hadamard–Cartan theorem, the sphere theorem, etc., see [[Curvature|Curvature]], [[#References|[2]]]). In the class of pseudo-Riemannian spaces of constant curvature the situation is completely different: Any pseudo-Riemannian space of dimension exceeding 2 and with sectional curvature of fixed sign is a space of constant curvature.
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The notion of a space of constant curvature does not have the property of  "well-posedness"  ("correctness"): a space with slowly varying sectional curvatures may be very different from a space of constant curvature. However, certain common properties of spaces of constant curvature, for example the topological structure, are preserved (the Hadamard–Cartan theorem, the sphere theorem, etc., see [[Curvature|Curvature]], [[#References|[2]]]). In the class of pseudo-Riemannian spaces of constant curvature the situation is completely different: Any pseudo-Riemannian space of dimension exceeding 2 and with sectional curvature of fixed sign is a space of constant curvature.
  
 
Spaces of constant curvature are also locally conformally flat, i.e. they admit locally conformal mappings into Euclidean space.
 
Spaces of constant curvature are also locally conformally flat, i.e. they admit locally conformal mappings into Euclidean space.
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====Comments====
 
====Comments====
A Riemannian manifold of constant curvature is said to be elliptic, hyperbolic or flat according as the sectional curvature is positive, negative or zero. References [[#References|[a1]]] and [[#References|[1]]] contain a proof of Schur's theorem and give explicit constant curvature metrics. The classification of the compact spaces of constant positive curvature was done by J.A. Wolf. A Riemannian space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527027.png" /> is a locally symmetric space if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/c/c025/c025270/c02527028.png" />, cf. also [[Symmetric space|Symmetric space]].
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A Riemannian manifold of constant curvature is said to be elliptic, hyperbolic or flat according as the sectional curvature is positive, negative or zero. References [[#References|[a1]]] and [[#References|[1]]] contain a proof of Schur's theorem and give explicit constant curvature metrics. The classification of the compact spaces of constant positive curvature was done by J.A. Wolf. A Riemannian space $M$ is a locally symmetric space if $\nabla R=0$, cf. also [[Symmetric space|Symmetric space]].
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  S. Kobayashi,  K. Nomizu,  "Foundations of differential geometry" , '''1''' , Interscience  (1963)  pp. Chapt. V, VI</TD></TR></table>
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  S. Kobayashi,  K. Nomizu,  "Foundations of differential geometry" , '''1''' , Interscience  (1963)  pp. Chapt. V, VI</TD></TR></table>

Revision as of 10:29, 30 August 2014

A Riemannian space $M$ for which the sectional curvature $K(\sigma)$ is constant in all two-dimensional directions $\sigma$; if $K(\sigma)=k$, then the space is said to be of constant curvature $k$. By Schur's theorem, a Riemannian space $M^n$, $n>2$, is a space of constant curvature if for any point $p\in M$ the sectional curvatures $K(\sigma)$ in the directions of every two-dimensional subspace $\sigma$ of the tangent space $T_pM$ are the same. The curvature tensor of a space of constant curvature is expressed in terms of the curvature $k$ and the metric tensor $g_{ij}$ by the formula

$$R_{jlk}^i=k(\delta_k^ig_{jl}-\delta_l^ig_{jk}).$$

A space of a constant curvature is a locally symmetric space.

Up to an isometry there exists a unique complete simply-connected $n$-dimensional Riemannian space $S^n(k)$ of constant curvature $k$. For $k=0$ it is Euclidean space, for $k>0$ it is the sphere of radius $1/\sqrt k$, for $k<0$ it is Lobachevskii space.

The spaces $S^n(k)$ are maximal homogeneous spaces, i.e. their group of motions has maximum possible dimension $n(n+1)/2$. All maximal homogeneous Riemannian spaces different from $S^n(k)$ are exhausted by projective (elliptic) spaces obtained from spheres by identification of antipodal points.

Complete but multiply-connected spaces of constant curvature are called space forms. They are obtained by factorizing a simply-connected space $S^n(k)$ by a freely-acting discrete group of motions of $S^n(k)$. All space forms of positive curvature are known. The problem of classifying space forms of zero curvature and negative curvature has not yet (1983) been completely solved.

Spaces of constant curvature are distinguished from the other Riemannian spaces by one of the following characteristic properties: 1) spaces of constant curvature satisfy the axiom of planes, i.e. through every point and in the direction of every plane element at this point there passes a totally-geodesic submanifold; and 2) a space of constant curvature is a locally projectively-flat space, i.e. it admits locally projective mappings into Euclidean space.

The notion of a space of constant curvature does not have the property of "well-posedness" ("correctness"): a space with slowly varying sectional curvatures may be very different from a space of constant curvature. However, certain common properties of spaces of constant curvature, for example the topological structure, are preserved (the Hadamard–Cartan theorem, the sphere theorem, etc., see Curvature, [2]). In the class of pseudo-Riemannian spaces of constant curvature the situation is completely different: Any pseudo-Riemannian space of dimension exceeding 2 and with sectional curvature of fixed sign is a space of constant curvature.

Spaces of constant curvature are also locally conformally flat, i.e. they admit locally conformal mappings into Euclidean space.

References

[1] J.A. Wolf, "Spaces of constant curvature" , Publish or Perish (1977)
[2] Yu.D. Burago, V.A. Zalgaller, "Convex sets in Riemannian spaces of non-negative curvature" Russian Math. Surveys , 32 (1977) pp. 1–57 Uspekhi Mat. Nauk , 32 : 3 (1977) pp. 3–55


Comments

A Riemannian manifold of constant curvature is said to be elliptic, hyperbolic or flat according as the sectional curvature is positive, negative or zero. References [a1] and [1] contain a proof of Schur's theorem and give explicit constant curvature metrics. The classification of the compact spaces of constant positive curvature was done by J.A. Wolf. A Riemannian space $M$ is a locally symmetric space if $\nabla R=0$, cf. also Symmetric space.

References

[a1] S. Kobayashi, K. Nomizu, "Foundations of differential geometry" , 1 , Interscience (1963) pp. Chapt. V, VI
How to Cite This Entry:
Constant curvature, space of. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Constant_curvature,_space_of&oldid=14702
This article was adapted from an original article by D.D. Sokolov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article