Riemannian circle

Riemannian circle

In metric space theory and Riemannian geometry, the term Riemannian circle refers to a great circle equipped with its great-circle distance. In more detail, the term refers to the circle equipped with its "intrinsic" Riemannian metric of a compact 1-dimensional manifold of total length 2π, as opposed to the "extinsic" metric obtained by restriction of the Euclidean metric to the unit circle in the plane. Thus, the distance between a pair of points is defined to be the length of the shorter of the two arcs into which the circle is partitioned by the two points.

Diameter

The diameter of the Riemannian circle is π, in contrast with the usual value, 2, of the Euclidean diameter of the unit circle.

Gromov's filling conjecture

A long-standing open problem, posed by Mikhail Gromov, concerns the calculation of the filling area of the Riemannian circle.

Isometric imbedding

The inclusion of the Riemannian circle as the equator (or any great circle) of the 2-sphere of constant Gaussian curvature +1, is an isometric imbedding in the sense of metric spaces (there is no isometric imbedding of the Riemannian circle in Hilbert space in this sense).

References

*Gromov, M.: "Filling Riemannian manifolds", "Journal of Differential Geometry" 18 (1983), 1–147.


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