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Girth (geometry) : ウィキペディア英語版 | Girth (geometry) In three-dimensional geometry, the girth of a geometric object, in a certain direction, is the perimeter of its parallel projection in that direction.〔.〕〔.〕 For instance, the girth of a unit cube in a direction parallel to one of the three coordinate axes is four: it projects to a unit square, which has four as its perimeter. ==Surfaces of constant girth== The girth of a sphere in any direction equals the circumference of its equator, or of any of its great circles. More generally, if ''S'' is a surface of constant width ''w'', then every projection of ''S'' is a curve of constant width, with the same width ''w''. All curves of constant width have the same perimeter, the same value ''w'' as the circumference of a circle with that width (this is Barbier's theorem). Therefore, every surface of constant width is also a surface of constant girth: its girth in all directions is the same number ''w''. Hermann Minkowski proved, conversely, that every convex surface of constant girth is also a surface of constant width.〔〔
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