Density character of subgroups of topological groups
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- by Arkady G. Leiderman, Sidney A. Morris and Mikhail G. Tkachenko PDF
- Trans. Amer. Math. Soc. 369 (2017), 5645-5664 Request permission
Abstract:
We give a complete characterization of subgroups of separable topological groups. Then we show that the following conditions are equivalent for an $\omega$-narrow topological group $G$: (i) $G$ is homeomorphic to a subspace of a separable regular space; (ii) $G$ is topologically isomorphic to a subgroup of a separable topological group; (iii) $G$ is topologically isomorphic to a closed subgroup of a separable path-connected, locally path-connected topological group.
A pro-Lie group is a projective limit of finite-dimensional Lie groups. We prove here that an almost connected pro-Lie group is separable if and only if its weight is not greater than the cardinality $\mathfrak {c}$ of the continuum. It is deduced from this that an almost connected pro-Lie group is separable if and only if it is homeomorphic to a subspace of a separable Hausdorff space. It is also proved that a locally compact (even feathered) topological group $G$ which is a subgroup of a separable Hausdorff topological group is separable, but the conclusion is false if it is assumed only that $G$ is homeomorphic to a subspace of a separable Tychonoff space.
We show that every precompact (abelian) topological group of weight less than or equal to $\mathfrak {c}$ is topologically isomorphic to a closed subgroup of a separable pseudocompact (abelian) group of weight $\mathfrak {c}$. This result implies that there is a wealth of closed non-separable subgroups of separable pseudocompact groups. An example is also presented under the Continuum Hypothesis of a separable countably compact abelian group which contains a non-separable closed subgroup.
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Additional Information
- Arkady G. Leiderman
- Affiliation: Department of Mathematics, Ben-Gurion University of the Negev, P.O.B. 653, Beer Sheva, Israel
- MR Author ID: 214471
- ORCID: 0000-0002-2257-1635
- Email: arkady@math.bgu.ac.il
- Sidney A. Morris
- Affiliation: Faculty of Science, Federation University Australia, P.O.B. 663, Ballarat, Victoria, 3353, Australia — and — Department of Mathematics and Statistics, La Trobe University, Melbourne, Victoria, 3086, Australia
- MR Author ID: 127180
- Email: morris.sidney@gmail.com
- Mikhail G. Tkachenko
- Affiliation: Departamento de Matemáticas, Universidad Autónoma Metropolitana, Avenida San Rafael Atlixco 186, Col. Vicentina, Del. Iztapalapa, C.P. 09340, México, D.F., Mexico
- MR Author ID: 190563
- Email: mich@xanum.uam.mx
- Received by editor(s): December 29, 2014
- Received by editor(s) in revised form: August 10, 2015, and September 10, 2015
- Published electronically: December 22, 2016
- © Copyright 2016 American Mathematical Society
- Journal: Trans. Amer. Math. Soc. 369 (2017), 5645-5664
- MSC (2010): Primary 54D65; Secondary 22D05
- DOI: https://doi.org/10.1090/tran/6832
- MathSciNet review: 3646774