请输入您要查询的字词:

 

单词 TheSphereIsIndecomposableAsATopologicalSpace
释义

the sphere is indecomposable as a topological space


Proposition. If for any topological spacesMathworldPlanetmath X and Y the n-dimensional sphere 𝕊n is homeomorphic to X×Y, then either X has exactly one point or Y has exactly one point.

Proof. Recall that the homotopy groupMathworldPlanetmath functor is additive, i.e. πn(X×Y)πn(X)πn(Y). Assume that 𝕊n is homeomorphic to X×Y. Now πn(𝕊n) and thus we have:

πn(𝕊n)πn(X×Y)πn(X)πn(Y).

Since is an indecomposable group, then either πn(X)0 or πn(Y)0.

Assume that πn(Y)0. Consider the map p:X×YY such that p(x,y)=y. Since X×Y is homeomorphic to 𝕊n and πn(Y)0, then p is homotopicMathworldPlanetmathPlanetmath to some constant map. Let y0Y and H:I×X×YY be such that

H(0,x,y)=p(x,y)=y;
H(1,x,y)=y0.

Consider the map F:I×X×YX×Y defined by the formula

F(t,x,y)=(x,H(t,x,y)).

Note that F(0,x,y)=(x,y) and F(1,x,y)=(x,y0) and thus X×{y0} is a deformation retractMathworldPlanetmath of X×Y. But X×Y is a sphere and spheres do not have proper deformation retracts (please see this entry (http://planetmath.org/EveryMapIntoSphereWhichIsNotOntoIsNullhomotopic) for more details). Therefore X×{y0}=X×Y, so Y={y0} has exactly one point.

随便看

 

数学辞典收录了18232条数学词条,基本涵盖了常用数学知识及数学英语单词词组的翻译及用法,是数学学习的有利工具。

 

Copyright © 2000-2023 Newdu.com.com All Rights Reserved
更新时间:2025/5/4 18:39:26