# Relative geometry

The geometry of a configuration composed of two surfaces $ S _ {0} : \mathbf n = \mathbf n ( u ^ {1} , u ^ {2} ) $
and $ S: \mathbf r = \mathbf r ( u ^ {1} , u ^ {2} ) $
that are in Peterson correspondence. The analogy between this correspondence and the spherical map makes it possible to introduce the concepts of a relative area, Gaussian and mean curvature, etc., and in particular of a relatively-minimal surface (see [1]).

An examination of the derivation of the equations for the reference frame $ \partial \mathbf r / \partial u ^ {1} $, $ \partial \mathbf r / \partial u ^ {2} $, $ \mathbf n $ leads to the concept of the interior relative geometry of a surface $ S $( see [2]). This is the geometry of an affine connection (or more precisely, an equi-affine connection) without torsion. The concept of a second-order geometry similar to the geometry of the spherical map has been introduced (see [3]).

Within relative geometry it is possible to include in an overall scheme not only the geometry of Euclidean surfaces and pseudo-Euclidean spaces, but also the geometry of affine differential geometry. The vector $ \mathbf n $ of the affine normal is characterized by the fact that the asymptotic net of the surface $ S $ is a Chebyshev net (see [3]).

A further generalization of relative geometry is the theory of normalized surfaces (see [4]). With each point of a surface $ S $ in a projective space two straight lines are connected: the first-order normal which passes through a point $ A $ of the surface but having no other common points with the tangent plane $ \alpha $, and the second-order normal belonging to $ \alpha $ but not passing through $ A $. Two interior geometries conjugated through an asymptotic net are defined on $ S $. The construction of relative geometries allows many generalizations (see [4]).

#### References

[1] | E. Müller, Monatsh. Math. und Physik , 31 (1921) pp. 3–19 |

[2] | A.P. Norden, "Sur l'inclusion des théories métriques et affines des surfaces dans la géométrie des systèmes spécifiques" C.R. Acad. Sci. Paris , 192 (1931) pp. 135–137 |

[3] | A.P. Norden, "On the intrinsic geometry of second kind hypersurfaces in affine space" Izv. Vyzov. Mat. , 4 (1958) pp. 172–183 (In Russian) |

[4] | A.P. Norden, "Spaces with an affine connection" , Nauka , Moscow-Leningrad (1976) (In Russian) |

**How to Cite This Entry:**

Relative geometry.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Relative_geometry&oldid=48496