Bases and closures under infinite sums |
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Authors: | Henning Bruhn |
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Institution: | a Universite Pierre et Marie Curie, Paris 6, Combinatoire et Optimisation, Case 189, 4, Place Jussieu, 75252 Paris Cedex 05, France b Technische Universität Graz, Steyrergasse 30, 8010 Graz, Austria |
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Abstract: | Motivated by work of Diestel and Kühn on the cycle spaces of infinite graphs we study the ramifications of allowing infinite sums in a module RM. We show that every generating set in this setup contains a basis if the ground set M is countable, but not necessarily otherwise. Given a family N⊆RM, we determine when the infinite-sum span N of N is closed under infinite sums, i.e. when N=N. We prove that this is the case if R is a field or a finite ring and each element of M lies in the support of only finitely many elements of N. This is, in a sense, best possible. We finally relate closures under infinite sums to topological closures in the product space RM. |
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Keywords: | 15A03 05C63 |
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