Geodesic
The union of two $D$-spaces need not be $D$
Dániel T. Soukup 1   ; Paul J. Szeptycki 2
1 Department of Mathematics University of Toronto Toronto, ON, M5S 1A1 Canada
2 Department of Mathematics and Statistics York University Toronto, ON, M3J 1P3 Canada
Fundamenta Mathematicae, Tome 220 (2013) no. 2, pp. 129-137 Cet article a éte moissonné depuis la source Institute of Mathematics Polish Academy of Sciences

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We construct from $\diamondsuit $ a $T_2$ example of a hereditarily Lindelöf space $X$ that is not a $D$-space but is the union of two subspaces both of which are $D$-spaces. This answers a question of Arhangel'skii.
DOI : 10.4064/fm220-2-3
Keywords: construct diamondsuit example hereditarily lindel space d space union subspaces which d spaces answers question arhangelskii

Dániel T. Soukup  1   ; Paul J. Szeptycki  2

1 Department of Mathematics University of Toronto Toronto, ON, M5S 1A1 Canada
2 Department of Mathematics and Statistics York University Toronto, ON, M3J 1P3 Canada 
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Dániel T. Soukup; Paul J. Szeptycki. The union of two $D$-spaces need not be $D$. Fundamenta Mathematicae, Tome 220 (2013) no. 2, pp. 129-137. doi: 10.4064/fm220-2-3

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