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A proof of the recursion formula relating the volume of the n-ball and an (n − 2)-ball can be given using the proportionality formula above and integration in cylindrical coordinates. Fix a plane through the center of the ball. Let r denote the distance between a point in the plane and the center of the sphere, and let θ denote the azimuth.
Any closed topological n-ball is homeomorphic to the closed n-cube [0, 1] n. An n-ball is homeomorphic to an m-ball if and only if n = m. The homeomorphisms between an open n-ball B and R n can be classified in two classes, that can be identified with the two possible topological orientations of B. A topological n-ball need not be smooth; if it ...
In mathematics and more specifically in topology, a homeomorphism (from Greek roots meaning "similar shape", named by Henri Poincaré), [2] [3] also called topological isomorphism, or bicontinuous function, is a bijective and continuous function between topological spaces that has a continuous inverse function.
Some authors use the term Alexander trick for the statement that every homeomorphism of can be extended to a homeomorphism of the entire ball .. However, this is much easier to prove than the result discussed above: it is called radial extension (or coning) and is also true piecewise-linearly, but not smoothly.
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A homeomorphism is a special case of a homotopy equivalence, in which g ∘ f is equal to the identity map id X (not only homotopic to it), and f ∘ g is equal to id Y. [ 6 ] : 0:53:00 Therefore, if X and Y are homeomorphic then they are homotopy-equivalent, but the opposite is not true.
The case n = 2 can also be proven by contradiction based on a theorem about non-vanishing vector fields. For n > 2, however, proving the impossibility of the retraction is more difficult. One way is to make use of homology groups: the homology H n−1 (D n) is trivial, while H n−1 (S n−1) is infinite cyclic. This shows that the retraction ...
The connected sum of two n-manifolds is defined by removing an open ball from each manifold and taking the quotient of the disjoint union of the resulting manifolds with boundary, with the quotient taken with regards to a homeomorphism between the boundary spheres of the removed balls. This results in another n-manifold. [7]