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单词 ProofOfBrouwerFixedPointTheorem
释义

proof of Brouwer fixed point theorem


Proof of the Brouwer fixed point theoremMathworldPlanetmath:

Assume that there does exist a map from f:BnBn with no fixed pointPlanetmathPlanetmath. Then letg(x) be the following map: Start at f(x), draw the ray going through x and then let g(x) bethe first intersectionMathworldPlanetmathPlanetmath of that line with the sphere. This map is continuousMathworldPlanetmathPlanetmath and well defined onlybecause f fixes no point. Also, it is not hard to see that it must be the identityPlanetmathPlanetmath on the boundarysphere. Thus we have a map g:BnSn-1, which is the identity onSn-1=Bn, that is, a retractionMathworldPlanetmathPlanetmathPlanetmath. Now, if i:Sn-1Bn is the inclusionmapMathworldPlanetmath, gi=idSn-1. Applying the reduced homology functorMathworldPlanetmath, we find thatg*i*=idH~n-1(Sn-1), where * indicates the induced map on homologyMathworldPlanetmathPlanetmath.

But, it is a well-known fact that H~n-1(Bn)=0 (since Bn is contractibleMathworldPlanetmath), and thatH~n-1(Sn-1)=. Thus we have an isomorphismMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath of a non-zero group onto itselffactoring through a trivial group, which is clearly impossible. Thus we have a contradictionMathworldPlanetmathPlanetmath,and no such map f exists.

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更新时间:2025/5/5 0:17:27