O Teorema de Borsuk-Ulam: uma versão fraca associada a grupos topológicos
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Data
2017-09-25
Autores
Marini, Mirela Cristina
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Universidade Estadual Paulista (Unesp)
Resumo
O Teorema de Borsuk-Ulam clássico afirma que: “Se f : Sn → IRn é uma aplicação contínua, entãoexisteumponto x em Sn talque f(x) = f(−x), ouequivalentemente f(x) = f(A(x)), onde Sn indica a esfera unitária n-dimensional e A : Sn → Sn é a aplicação antipodal”. Se pensamos na superfície terrestre como uma esfera, o caso n = 2 pode ser ilustrado dizendo-se que em cada instante, existe sempre um par de pontos antipodais na superfície da Terra com mesma temperatura e pressão barométrica (supondo que a temperatura e a pressão variam continuamente na superfície). Este trabalho é baseado no artigo “Some generalizations of the Borsuk-Ulam Theorem” de Vendrúsculo, Desideri e Pergher (2011), [8], e tem como principal objetivo apresentar um estudo de uma versão fraca do Teorema de Borsuk-Ulam associada a grupos topológicos. Diz-se que {(X,T);G}, onde X é um espaço topológico equipado por uma involução livre T e G é um grupo topológico, “satisfaz uma versão fraca do Teorema de Borsuk-Ulam”, abreviadamente, “satisfaz WBUT”, se, para cada aplicação contínua f : X → G, temos que o conjunto {x ∈ X; f(x) · f(T(x))−1 ∈ 2G} é diferente do vazio, onde f(T(x))−1 é o simétrico de f(T(x)) em G e 2G = {g ∈ G; g = g−1}. Neste trabalho, relacionamos essa condição fraca com a condição geral de “satisfazer o Teorema de Borsuk-Ulam” (ou “satisfazer BUT”) dada também pelos autores; apresentamos alguns exemplos; considerando G = T2 (toro), detalhamos a demonstração de um resultado que estabelece um critério algébrico para que {(X,T);T2} satisfaça a condição WBUT e de um resultado que dá uma equivalência entre a versão fraca WBUT para triplas {(S,T);T2} e a condição BUT para {(S,T);IR2}, sendo S uma superfície fechada. Por fim, apresentamos um invariante topológico obtido da versão WBUT. Tal invariante, por nós definido, é similar ao obtido da condição BUT e apresentado pelos autores citados.
The classical Borsuk-Ulam Theorem states that: “If f : Sn → IRn is any continuous map, then there exists a point x in Sn such that f(x) = f(−x), or equivalently f(x) = f(A(x)), where Sn denotes the n-dimensional unit sphere and A : Sn → Sn is the antipodal map”. If we think of the Earth’s surface as a sphere, the case n = 2 can be illustrated by saying that at every instant there is always a pair of antipodal points on the Earth’s surface with the same temperature and barometric pressure (assuming that the temperature and pressure vary continuously in the surface). This work is based on the article “Some generalizations of Borsuk-Ulam Theorem” by Ven drúsculo, Desideri and Pergher (2011), [8], and has the main purpose of presenting a study of a weak version of the Borsuk-Ulam Theorem associated with topolog ical groups. It is said that {(X,T);G}, where X is a topological space equipped with a free involution T and G is a topological group “satisfies a Weak version of the Borsuk-Ulam Theorem”, abbreviatedly, “satisfies WBUT” if, given any continuous map f : X → Y , the set {x ∈ X; f(x) · f(T(x))−1 ∈ 2G} is non empty, where f(T(x))−1 is the symmetric of f(T(x)) in G and 2G = {g ∈ G; g = g−1}. In this work, we relate this weak condition with the more general condition of “satisfying the Borsuk-Ulam Theorem” (or “satisfying BUT”) also given by the authors; we present some examples; considering G = T2 (torus), we detail the proof of a result that establishes an algebraic criterion for {(X,T);T2} satisfy the condition WBUT, and of a result that gives an equivalence between the weak version WBUT for triples {(S,T);T2} and the condition BUT for {(S,T);IR2}, where S is a closed surface and T is a free involution on S. Finally, we present a topological invariant obtained from the WBUT version. Such invariant, defined by us, is similar to that obtained from the BUT condition and presented by the cited authors.
The classical Borsuk-Ulam Theorem states that: “If f : Sn → IRn is any continuous map, then there exists a point x in Sn such that f(x) = f(−x), or equivalently f(x) = f(A(x)), where Sn denotes the n-dimensional unit sphere and A : Sn → Sn is the antipodal map”. If we think of the Earth’s surface as a sphere, the case n = 2 can be illustrated by saying that at every instant there is always a pair of antipodal points on the Earth’s surface with the same temperature and barometric pressure (assuming that the temperature and pressure vary continuously in the surface). This work is based on the article “Some generalizations of Borsuk-Ulam Theorem” by Ven drúsculo, Desideri and Pergher (2011), [8], and has the main purpose of presenting a study of a weak version of the Borsuk-Ulam Theorem associated with topolog ical groups. It is said that {(X,T);G}, where X is a topological space equipped with a free involution T and G is a topological group “satisfies a Weak version of the Borsuk-Ulam Theorem”, abbreviatedly, “satisfies WBUT” if, given any continuous map f : X → Y , the set {x ∈ X; f(x) · f(T(x))−1 ∈ 2G} is non empty, where f(T(x))−1 is the symmetric of f(T(x)) in G and 2G = {g ∈ G; g = g−1}. In this work, we relate this weak condition with the more general condition of “satisfying the Borsuk-Ulam Theorem” (or “satisfying BUT”) also given by the authors; we present some examples; considering G = T2 (torus), we detail the proof of a result that establishes an algebraic criterion for {(X,T);T2} satisfy the condition WBUT, and of a result that gives an equivalence between the weak version WBUT for triples {(S,T);T2} and the condition BUT for {(S,T);IR2}, where S is a closed surface and T is a free involution on S. Finally, we present a topological invariant obtained from the WBUT version. Such invariant, defined by us, is similar to that obtained from the BUT condition and presented by the cited authors.
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Palavras-chave
Teorema de Borsuk-Ulam, Versão Fraca do Teorema de Borsuk Ulam, Involução Livre, Grupos Topológicos, Superfície, Borsuk-Ulam Theorem, Weak Version of Borsuk-Ulam Theorem, Free Involution, Topological Groups, Surfaces