# Beltrami interpretation

A realization of a part of the Lobachevskii plane on a pseudo-sphere — a surface of constant negative curvature. In the Beltrami interpretation, the geodesic lines (cf. Geodesic line) and their segments on the pseudo-sphere play the role of straight lines and their segments on the Lobachevskii plane. The isometric mappings of the pseudo-sphere onto itself represent the movements in the Lobachevskii plane, with preservation of horocircles. The lengths, angles and areas on the pseudo-sphere correspond to the lengths, angles and areas on the Lobachevskii plane. Under these conditions, to each theorem of Lobachevskii planimetry which refers to a part of the Lobachevskii plane there corresponds an analogous theorem of the intrinsic geometry of the pseudo-sphere. The Beltrami interpretation realizes a part of the Lobachevskii plane, but the entire Lobachevskii plane cannot be realized in three-dimensional Euclidean space as a regular surface (Hilbert's theorem).

The Beltrami interpretation was proposed by E. Beltrami in 1868 [1]. This publication represented the first realization of Lobachevskii's "imaginary geometry" in three-dimensional Euclidean space.

For other interpretations of the Lobachevskii geometry see Klein interpretation; Poincaré model.

#### References

[1] | E. Beltrami, "Saggio di interpretazione della geometria non-euclidea" , Opere Mat. , 1 (1868) pp. 374–405 |

[2] | V.F. Kagan, "Foundations of geometry" , 1 , Moscow-Leningrad (1949) (In Russian) |

#### Comments

#### References

[a1] | F. Klein, "Vorlesungen über Nicht-Euklidische Geometrie" , Springer (1968) |

**How to Cite This Entry:**

Beltrami interpretation. E.V. Shikin (originator),

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Beltrami_interpretation&oldid=13708