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Borsuk–Ulam theorem : ウィキペディア英語版
Borsuk–Ulam theorem
In mathematics, the Borsuk–Ulam theorem (BUT), states that every continuous function from an ''n''-sphere into Euclidean ''n''-space maps some pair of antipodal points to the same point. Here, two points on a sphere are called antipodal if they are in exactly opposite directions from the sphere's center.
Formally: if f: S^n \to R^n is continuous then there exists an x\in S^n such that: f(-x)=f(x).
The case n=1 can be illustrated by saying that there always exist a pair of opposite points on the earth's equator with the same temperature. The same is true for any circle. This assumes the temperature varies continuously.
The case n=2 is often illustrated by saying that at any moment, there is always a pair of antipodal points on the Earth's surface with equal temperatures and equal barometric pressures.
BUT has several equivalent statements in terms of odd functions. Recall that S^n is the ''n''-sphere and B^n is the ''n''-ball:
* If g: S^n \to R^n is a continuous odd function, then there exists an x\in S^n such that: g(x)=0.
* If g: B^n \to R^n is a continuous function which is odd on S^ (the boundary of B^n), then there exists an x\in B^n such that: g(x)=0.
==History==

According to , the first historical mention of the statement of BUT appears in . The first proof was given by , where the formulation of the problem was attributed to Stanislaw Ulam. Since then, many alternative proofs have been found by various authors, as collected by .

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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