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Riemannian circle : ウィキペディア英語版 | Riemannian circle
In metric space theory and Riemannian geometry, the Riemannian circle (named after Bernhard Riemann) is 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 2, as opposed to the ''extrinsic'' 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. == Properties == The diameter of the Riemannian circle is π, in contrast with the usual value of 2 for the Euclidean diameter of the unit circle. 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).
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