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Spherical map

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Gauss map, normal spherical map

A mapping from a smooth orientable (hyper)surface in a space E ^ {k+} 1 to the (unit) sphere S ^ {k} with centre at the origin of E ^ {k+} 1 . It assigns to a point x \in M ^ {k} the point x ^ \star \in S ^ {k} with position vector \overline{n}\; ( x) — the (unit) normal to M ^ {k} at x . In other words, the spherical map is defined by a multivector constructed from k independent vectors tangent to M ^ {k} :

\overline{n}\; = \frac{\overline{x}\; _ {1} \wedge \dots \wedge \overline{x}\; _ {k} }{| \overline{x}\; _ {1} \wedge \dots \wedge \overline{x}\; _ {k} | }

(here u ^ {1} \dots u ^ {k} are local coordinates of the point x , \overline{x}\; _ {i} = ( \partial \overline{x}\; / \partial u ^ {i} ) , and \overline{x}\; is the position vector of M ^ {k} ). For example, when k = 2 ,

\overline{n}\; = \frac{[ \overline{x}\; _ {u} , \overline{x}\; _ {v} ] }{| [ \overline{x}\; _ {u} , \overline{x}\; _ {v} ] | } ,

where [ \cdot , \cdot ] is the vector product; this simplest case was examined by C.F. Gauss in 1814. The image under the spherical map is called the spherical image of M ^ {k} .

The form

d \overline{n}\; {} ^ {2} = \gamma _ {ij} du ^ {i} du ^ {j}

is the inverse image of the metric form of S ^ {k} , and is called the third fundamental form of the (hyper)surface M ^ {k} . Its corresponding tensor \gamma _ {ij} is related to the tensors g _ {ij} and b _ {ij} of the first and second fundamental forms, respectively, by the relation

\gamma _ {ij} = g ^ {ki} b _ {ik} b _ {jl} ,

while the metric connections corresponding to g _ {ij} and \gamma _ {ij} are adjoint connections.

As well as the spherical map, it is useful in the case of a (hyper)surface that is uniquely projected onto a certain (hyper)plane to consider the so-called normal map \widetilde{n} . For a (hyper)surface defined by the equation

x ^ {k+} 1 = f( x ^ {1} \dots x ^ {k} )

(here x ^ {i} are Cartesian coordinates in E ^ {k+} 1 ), \widetilde{n} is defined thus:

\widetilde{n} = \{ p _ {1} \dots p _ {k} \} ,

where p _ {i} = ( \partial f/ \partial x ^ {i} ) , so \widetilde{n} = n \sqrt {1 + \sum p _ {i} ^ {2} } .

For non-orientable (hyper)surfaces, the so-called non-orientable spherical map is used — a mapping from M ^ {k} into the elliptic space El ^ {k} ( which can be interpreted as the set of straight lines that pass through the centre of E ^ {k+} 1 , i.e. k - dimensional projective space): The line perpendicular to the tangent plane to M ^ {k} at a point x is associated with x \in M ^ {k} .

The spherical map characterizes the curvature of a (hyper)surface in a space. Indeed, the ratio of the area elements of the spherical image dS ^ \star and the surface dS itself at the point x \in M ^ {k} is equal to the total (or Kronecker or outer) curvature K _ {l} — the product of the principal curvatures of M ^ {k} at x :

K _ {l} = \frac{dS ^ \star }{dS} ,\ \ \textrm{ i.e. } K( ds ^ \star ) = K( \frac{ds)}{K _ {l} } .

In precisely the same way, the (integral) curvature of a set F \subset M ^ {k} is equal to the area of its spherical image (i.e. the set F ^ { \star } = \overline{n}\; ( F ) \subset S ^ {k} ):

\tag{1 } \int\limits \int\limits K _ {l} dS = \int\limits \int\limits dS ^ \star .

Generalizations of the spherical map.

1) The tangent representation — the spherical map of a submanifold M ^ {k} to E ^ {N} — is a mapping

M ^ {k} \rightarrow G _ {k,N } ,

where G _ {k,N } is a Grassmann manifold, defined (here) in the following way. Let T _ {x} be the tangent space to M ^ {k} at a point x , which can be considered as a (hyper)plane in E ^ {N} , while T( x) is the k - dimensional subspace that passes through the origin of E ^ {N} parallel to T _ {x} . The mapping x \rightarrow T( x) is also called the spherical map. A generalization of formula (1) holds for k even:

\int\limits _ {T _ {N} ( M ^ {k} ) } \widetilde \Omega = \int\limits _ {M ^ {k} } \Omega ,

here \Omega = \epsilon ^ {i _ {1} \dots i _ {k} } \Omega _ {i _ {1} i _ {2} } \wedge \dots \wedge \Omega _ {i _ {k-} 1 i _ {k} } , where \Omega _ {ij} is the curvature form on M ^ {k} , \widetilde \Omega is the analogous form on G _ {k,N } , and T _ {N} ( M ^ {k} ) is the image of M ^ {k} under the spherical map. The normal map M ^ {k} \rightarrow G _ {N- k N } has a dual definition: The point x \in M ^ {k} is associated with the orthogonal complement to T( x) .

2) A Gauss map of a vector bundle \xi ^ {k} into a vector space F ^ { N } , k \leq N \leq \infty , is an (arbitrary) mapping

g: E( \xi ^ {k} ) \rightarrow F ^ {N}

from the fibre space E( \xi ^ {k} ) that induces a linear monomorphism on each fibre. For the canonical vector bundle \gamma _ {k} ^ {N} ( which is the subbundle of the product ( G _ {N,k} \times \mathbf R ^ {N} , p, G _ {N,k} ) , of which the total space consists of all possible pairs ( V, x) \in G _ {N,k} \times \mathbf R ^ {N} with x \in V ), the mapping ( V, x) \rightarrow x is called the canonical Gauss map. For any fibre bundle \xi ^ {k} , every Gauss map is a composition of a canonical Gauss map and a morphism of fibre bundles; a Gauss map exists if and only if a mapping f: B( \xi ) \rightarrow G _ {N,k} ( where B is the base of the fibre bundle) exists such that \xi and f ^ { \star } ( \gamma _ {k} ^ {N} ) are isomorphic (in particular, for every vector bundle over a paracompact space there is a Gauss map into F ^ { \infty } ). For submanifolds of a Riemannian space, there are several generalizations of spherical maps.

3) An Efimov map relates to surfaces M ^ {2} in a Riemannian space V ^ {3} and is an extension of the above-mentioned concept of adjoint connections. It is defined more formally because of the lack of absolute parallelism in V ^ {3} and the examination of the analogue of the third fundamental form — the square of the covariant differential of the normal — ( D \overline{n}\; ) ^ {2} . The relation between the Gaussian curvatures K( ds ^ \star ) and K( ds) proves to be more complex (a consequence of the inhomogeneity, generally speaking, of the Codazzi equations). This relation remains as before, i.e. K( | Dn | ) = ( K( ds))/K _ {l} ; here K( ds) , K( | Dn | ) are the Gaussian curvatures of the metrics ds and | Dn | ( in the case of V ^ {3} = E ^ {3} , K( ds) = K _ {l} ), and the previous formula K ( | Dn | ) = K( | dn | ) = 1 is obtained, where K _ {l} is the exterior curvature of M ^ {2} in V ^ {3} , for example in the following situation: The normal to M ^ {2} is an eigenvector of the Ricci tensor of the space V ^ {3} ( considered at the points of M ^ {2} ), in other words, M ^ {2} is one of the principal surfaces of this tensor. This is always the case if V ^ {3} is a space of constant curvature.

Finally, the concept of a spherical map is introduced for certain classes of irregular surfaces.

4) The polar mapping is a spherical map from a convex (hyper)surface F ^ { k } into E ^ {k+} 1 that associates to a point x \in F ^ { k } the set \nu ( x) of all unit vectors, drawn from the origin, that are parallel to the normals of the supporting (hyper)planes to F ^ { k } at x . Aleksandrov's theorem: The spherical image \nu ( A) of every Borel set A \subset F ^ { k } is measurable, and the integral curvature K( A) = \mathop{\rm mes} \nu ( A) is a totally-additive function.

References

[1] V.F. Kagan, "Foundations of the theory of surfaces in a tensor setting" , 2 , Moscow-Leningrad (1948) (In Russian)
[2] I.Ya. Bakel'man, A.L. Verner, B.E. Kantor, "Introduction to differential geometry "in the large" " , Moscow (1973) (In Russian)
[3] A.S. Mishchenko, A.T. Fomenko, "A course of differential geometry and topology" , MIR (1988) (Translated from Russian)
[4] A.P. Norden, "Spaces with an affine connection" , Nauka , Moscow-Leningrad (1976) (In Russian)
[5] J.T. Schwartz, "Differential geometry and topology" , Gordon & Breach (1968)
[6] D. Husemoller, "Fibre bundles" , McGraw-Hill (1966)
[7] R.L. Bishop, R.J. Crittenden, "Geometry of manifolds" , Acad. Press (1964)
[8] L.P. Eisenhart, "Riemannian geometry" , Princeton Univ. Press (1949)
[9] H. Busemann, "Convex surfaces" , Interscience (1958)

Comments

References

[a1] M. Spivak, "A comprehensive introduction to differential geometry" , 1979 , Publish or Perish (1975) pp. 1–5
How to Cite This Entry:
Spherical map. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Spherical_map&oldid=48778
This article was adapted from an original article by L.A. Sidorov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article