Wallman compactification and matrices of zeroes and ones |
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Authors: | Alexander Abian Sergio Salbany |
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Institution: | Department of Mathematics, Iowa State University, Ames, Iowa 50011 USA;Mathematical Institute, University of Oxford, Oxford, England |
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Abstract: | Let M be an m by n matrix (where m and n are any finite or infinite cardinals) such that the entries of M are 0's or 1's and M contains the zero row and the rows of M are closed under coordinatewise multiplication. We prove that M can be extended to an m by n′ ? n matrix M′ such that the entries of M′ are 0's or 1's and M′ contains the zero row 0?′ and the extension preserves the zero products. Moreover, the newly introduced columns (if any) are pairwise distinct. Furthermore, if E′ is any set of rows of M′ with the property that for every finite subset of rows r′i of E′ there exists j < n′ such that r′ij = 1, then every subset of rows of E′ has the same property. We rephrase this by saying that if E′ has the finite intersection property then E′ has a nonempty intersection. We also show (this time by Zorn's lemma) that there exists an extension of M with all the abovementioned properties such that the extension preserves products sums, complements and the newly introduced columns (if any) are pairwise distinct in a stricter sense. In effect, our result shows that the classical Wallman compactification theorem can be formulated purely combinatorially requiring no introduction of any topology on n. |
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