English

On special subgroups of fundamental group

Algebraic Topology 2024-01-19 v4

Abstract

Suppose α\alpha is a nonzero cardinal number, I\mathcal I is an ideal on arc connected topological space XX, and PIα(X){\mathfrak P}_{\mathcal I}^\alpha(X) is the subgroup of π1(X)\pi_1(X) (the first fundamental group of XX) generated by homotopy classes of αI\alpha\frac{\mathcal I}{}loops. The main aim of this text is to study PIα(X){\mathfrak P}_{\mathcal I}^\alpha(X)s and compare them. Most interest is in α{ω,c}\alpha\in\{\omega,c\} and I{Pfin(X),{}}\mathcal I\in\{\mathcal P_{fin}(X),\{\varnothing\}\}, where Pfin(X)\mathcal P_{fin}(X) denotes the collection of all finite subsets of XX. We denote P{}α(X){\mathfrak P}_{\{\varnothing\}}^\alpha(X) with Pα(X){\mathfrak P}^\alpha(X). We prove the following statements: \bullet for arc connected topological spaces XX and YY if Pα(X){\mathfrak P}^\alpha(X) is isomorphic to Pα(Y){\mathfrak P}^\alpha(Y) for all infinite cardinal number α\alpha, then π1(X)\pi_1(X) is isomorphic to π1(Y)\pi_1(Y); \bullet there are arc connected topological spaces XX and YY such that π1(X)\pi_1(X) is isomorphic to π1(Y)\pi_1(Y) but Pω(X){\mathfrak P}^\omega(X) is not isomorphic to Pω(Y){\mathfrak P}^\omega(Y); \bullet for arc connected topological space XX we have Pω(X)Pc(X)π1(X){\mathfrak P}^\omega(X)\subseteq{\mathfrak P}^c(X) \subseteq\pi_1(X); \bullet for Hawaiian earring X\mathcal X, the sets Pω(X){\mathfrak P}^\omega({\mathcal X}), Pc(X){\mathfrak P}^c({\mathcal X}), and π1(X)\pi_1({\mathcal X}) are pairwise distinct. So Pα(X){\mathfrak P}^\alpha(X)s and PIα(X){\mathfrak P}_{\mathcal I}^\alpha(X)s will help us to classify the class of all arc connected topological spaces with isomorphic fundamental groups.

Keywords

Cite

@article{arxiv.1704.02802,
  title  = {On special subgroups of fundamental group},
  author = {Fatemah Ayatollah Zadeh Shirazi and Fatemeh Ebrahimifar and Mohammad Ali Mahmoodi},
  journal= {arXiv preprint arXiv:1704.02802},
  year   = {2024}
}

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29 pages