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Hilbert's theorem (differential geometry)

In differential geometry, Hilbert's theorem (1901) states that there exists no complete regular surface of constant negative gaussian curvature immersed in . This theorem answers the question for the negative case of which surfaces in can be obtained by isometrically immersing complete manifolds with constant curvature.

HistoryEdit

  • Hilbert's theorem was first treated by David Hilbert in, "Über Flächen von konstanter Krümmung" (Trans. Amer. Math. Soc. 2 (1901), 87-99).
  • A different proof was given shortly after by E. Holmgren, "Sur les surfaces à courbure constante négative," (1902).
  • A far leading generalization was obtained by Nikolai Efimov in 1975.[1]

ProofEdit

The proof of Hilbert's theorem is elaborate and requires several lemmas. The idea is to show the nonexistence of an isometric immersion

 

of a plane   to the real space  . This proof is basically the same as in Hilbert's paper, although based in the books of Do Carmo and Spivak.

Observations: In order to have a more manageable treatment, but without loss of generality, the curvature may be considered equal to minus one,  . There is no loss of generality, since it is being dealt with constant curvatures, and similarities of   multiply   by a constant. The exponential map   is a local diffeomorphism (in fact a covering map, by Cartan-Hadamard theorem), therefore, it induces an inner product in the tangent space of   at  :  . Furthermore,   denotes the geometric surface   with this inner product. If   is an isometric immersion, the same holds for

 .

The first lemma is independent from the other ones, and will be used at the end as the counter statement to reject the results from the other lemmas.

Lemma 1: The area of   is infinite.
Proof's Sketch:
The idea of the proof is to create a global isometry between   and  . Then, since   has an infinite area,   will have it too.
The fact that the hyperbolic plane   has an infinite area comes by computing the surface integral with the corresponding coefficients of the First fundamental form. To obtain these ones, the hyperbolic plane can be defined as the plane with the following inner product around a point   with coordinates  

 

Since the hyperbolic plane is unbounded, the limits of the integral are infinite, and the area can be calculated through

 

Next it is needed to create a map, which will show that the global information from the hyperbolic plane can be transfer to the surface  , i.e. a global isometry.   will be the map, whose domain is the hyperbolic plane and image the 2-dimensional manifold  , which carries the inner product from the surface   with negative curvature.   will be defined via the exponential map, its inverse, and a linear isometry between their tangent spaces,

 .

That is

 ,

where  . That is to say, the starting point   goes to the tangent plane from   through the inverse of the exponential map. Then travels from one tangent plane to the other through the isometry  , and then down to the surface   with another exponential map.

The following step involves the use of polar coordinates,   and  , around   and   respectively. The requirement will be that the axis are mapped to each other, that is   goes to  . Then   preserves the first fundamental form.
In a geodesic polar system, the Gaussian curvature   can be expressed as

 .

In addition K is constant and fulfills the following differential equation

 

Since   and   have the same constant Gaussian curvature, then they are locally isometric (Minding's Theorem). That means that   is a local isometry between   and  . Furthermore, from the Hadamard's theorem it follows that   is also a covering map.
Since   is simply connected,   is a homeomorphism, and hence, a (global) isometry. Therefore,   and   are globally isometric, and because   has an infinite area, then   has an infinite area, as well.  

Lemma 2: For each   exists a parametrization  , such that the coordinate curves of   are asymptotic curves of   and form a Tchebyshef net.

Lemma 3: Let   be a coordinate neighborhood of   such that the coordinate curves are asymptotic curves in  . Then the area A of any quadrilateral formed by the coordinate curves is smaller than  .

The next goal is to show that   is a parametrization of  .

Lemma 4: For a fixed  , the curve  , is an asymptotic curve with   as arc length.

The following 2 lemmas together with lemma 8 will demonstrate the existence of a parametrization  

Lemma 5:   is a local diffeomorphism.

Lemma 6:   is surjective.

Lemma 7: On   there are two differentiable linearly independent vector fields which are tangent to the asymptotic curves of  .

Lemma 8:   is injective.

Proof of Hilbert's Theorem:
First, it will be assumed that an isometric immersion from a complete surface   with negative curvature exists:  

As stated in the observations, the tangent plane   is endowed with the metric induced by the exponential map   . Moreover,   is an isometric immersion and Lemmas 5,6, and 8 show the existence of a parametrization   of the whole  , such that the coordinate curves of   are the asymptotic curves of  . This result was provided by Lemma 4. Therefore,   can be covered by a union of "coordinate" quadrilaterals   with  . By Lemma 3, the area of each quadrilateral is smaller than  . On the other hand, by Lemma 1, the area of   is infinite, therefore has no bounds. This is a contradiction and the proof is concluded.  

See alsoEdit

  • Nash embedding theorem, states that every Riemannian manifold can be isometrically embedded into some Euclidean space.

ReferencesEdit

  1. ^ Ефимов, Н. В. Непогружаемость полуплоскости Лобачевского. Вестн. МГУ. Сер. мат., мех. — 1975. — No 2. — С. 83—86.
  • Manfredo do Carmo, Differential Geometry of Curves and Surfaces, Prentice Hall, 1976.
  • Spivak, Michael, A Comprehensive Introduction to Differential Geometry, Publish or Perish, 1999.