Ultrafilter semigroups generated by direct sums |
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Authors: | Onesmus Shuungula Yevhen Zelenyuk Yuliya Zelenyuk |
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Institution: | 1.School of Mathematics,University of the Witwatersrand,Wits,South Africa |
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Abstract: | Let λ be an infinite cardinal and for every ordinal α<λ, let A α be a set with a distinguished element 0 α ∈A α . The direct sum of sets A α , α<λ, is the subset \(X=\bigoplus_{\alpha<\lambda}A_{\alpha}\) of the Cartesian product ∏α<λ A α consisting of all x with finite supp?(x)={α<λ:x(α)≠0 α }. Endow X with a topology by taking as a neighborhood base at x∈X the subsets of the form {y∈X:y(α)=x(α) for all α<γ} where γ<λ. Let Ult?(X) denote the set of all nonprincipal ultrafilters on X converging to 0∈X. There is a natural partial semigroup operation on X which induces a semigroup operation on Ult?(X). We show that if direct sums X and Y are homeomorphic, then the semigroups Ult?(X) and Ult?(Y) are isomorphic. |
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