An Explicit Isometric Reduction of the Unit Sphere into an Arbitrarily Small Ball
Foundations of Computational Mathematics(2017)
摘要
Spheres are known to be rigid geometric objects: they cannot be deformed isometrically , i.e., while preserving the length of curves, in a twice differentiable way. An unexpected result by Nash (Ann Math 60:383–396, 1954 ) and Kuiper (Indag Math 17:545–555, 1955 ) shows that this is no longer the case if one requires the deformations to be only continuously differentiable. A remarkable consequence of their result makes possible the isometric reduction of a unit sphere inside an arbitrarily small ball. In particular, if one views the Earth as a round sphere, the theory allows to reduce its diameter to that of a terrestrial globe while preserving geodesic distances. Here, we describe the first explicit construction and visualization of such a reduced sphere. The construction amounts to solve a nonlinear PDE with boundary conditions. The resulting surface consists of two unit spherical caps joined by a C^1 fractal equatorial belt. An intriguing question then arises about the transition between the smooth and the C^1 fractal geometries. We show that this transition is similar to the one observed when connecting a Koch curve to a line segment.
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关键词
Isometric embedding,Convex integration,Sphere reduction,Boundary conditions,Primary 35-04,Secondary 53C21,53C23,53C42,57R40
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