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Semi-elliptic space

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A projective -space in which the metric is defined by a given absolute, which is the aggregate of an imaginary quadratic cone Q_0 with an (n-m_0-1)-flat vertex T_0, an (n-m_0-2)-imaginary cone Q_1 with an (n-m_1-1)-flat vertex T_1 in the (n-m_0-1)-plane T_0, etc., up to an (n-m_{r-1}-2)-imaginary cone Q_{r-1} with an (n-m_{r-1}-2)-flat vertex T_{r-1} and a non-degenerate imaginary (n-m_{r-1}-2)-quadratic Q_r in the (n-m_{r-1}-1)-plane T_{r-1}, 0\leq m_0<m_1<\dots<m_{r-1}<n. The indices of the cones Q_k, k=0,\dots,r-1, are: l_a=m_0-m_{a-1}, 0<a<r; l_r=n-m_{r-1}. A semi-elliptic space is denoted by S_n^{m_0\dots m_{r-1}}.

In case the cone Q_0 is a pair of merging planes coinciding with the plane T_0 (for m_0=0), the space with the improper plane T_0 is called the semi-Euclidean space R_n^{m_1\dots m_{r-1}}.

The distance between two points X and Y is defined according to the position of the straight line XY with respect to the planes T_0,\dots,T_{r-1}. If, in particular, the line XY does not intersect the plane T_0, then the distance between the points X and Y is defined in terms of the scalar product, analogously to the distance in a quasi-elliptic space. If, however, the line XY intersects the plane T_0 but does not intersect the plane T_1, or intersects the plane T_{a-1} but does not intersect the plane T_a, the distance between the points is defined using the scalar square of the difference of the corresponding vectors of the points X and Y.

According to the position with respect to the planes of the absolute in a semi-elliptic space, one distinguishes four types of straight lines.

The angles between planes in a semi-elliptic space are defined analogously to angles between planes in a quasi-elliptic space, that is, by using distances in the dual space.

A projective metric in a semi-elliptic space is a metric of a very general type. A particular case of the metric in a semi-elliptic space is, for example, the metric of a quasi-elliptic space. In particular, the 2-plane S_2^0 coincides with the Euclidean and S_2^1 with the co-Euclidean plane; the 3-space S_3^1 with the quasi-elliptic and S_3^0 with the Euclidean 3-space; the 3-space S_3^{01} is Galilean, S_3^{012} is a flag space, etc. The 3-space S_3^{12} corresponds by the duality principle to the Galilean 3-space \Gamma_3 and is called the co-Galilean space. (The absolute of a co-Galilean space consists of a pair of imaginary planes (the cone Q_0) and a point T_1 on the straight line T_0 of intersection of these planes.)

The motions of a semi-elliptic space are the collineations of it taking the absolute into itself. In the case m_a=n-m_{r-a-1}-1, l_a=l_{r-a}, the semi-elliptic space is dual to itself, and has co-motions defined in it analogously to co-motions in a quasi-elliptic space.

The motions, and the motions and co-motions form Lie groups. The motions (as well as the co-motions) are described by orthogonal operators.

A semi-elliptic space is a semi-Riemannian space.

References

[1] B.A. Rozenfel'd, "Non-Euclidean spaces" , Moscow (1969) (In Russian)
[a1] B.A. [B.A. Rozenfel'd] Rosenfel'd, "A history of non-euclidean geometry" , Springer (1988) (Translated from Russian)
How to Cite This Entry:
Semi-elliptic space. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Semi-elliptic_space&oldid=53601
This article was adapted from an original article by L.A. Sidorov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article