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Search: MSC category 18A20 ( Epimorphisms, monomorphisms, special classes of morphisms, null morphisms )

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1. CJM 2007 (vol 59 pp. 465)

Barr, Michael; Kennison, John F.; Raphael, R.
 Searching for Absolute $\mathcal{CR}$-Epic Spaces In previous papers, Barr and Raphael investigated the situation of a topological space $Y$ and a subspace $X$ such that the induced map $C(Y)\to C(X)$ is an epimorphism in the category $\CR$ of commutative rings (with units). We call such an embedding a $\CR$-epic embedding and we say that $X$ is absolute $\CR$-epic if every embedding of $X$ is $\CR$-epic. We continue this investigation. Our most notable result shows that a Lindel\"of space $X$ is absolute $\CR$-epic if a countable intersection of $\beta X$-neighbourhoods of $X$ is a $\beta X$-neighbourhood of $X$. This condition is stable under countable sums, the formation of closed subspaces, cozero-subspaces, and being the domain or codomain of a perfect map. A strengthening of the Lindel\"of property leads to a new class with the same closure properties that is also closed under finite products. Moreover, all \s-compact spaces and all Lindel\"of $P$-spaces satisfy this stronger condition. We get some results in the non-Lindel\"of case that are sufficient to show that the Dieudonn\'e plank and some closely related spaces are absolute $\CR$-epic. Keywords:absolute $\mathcal{CR}$-epics, countable neighbourhoo9d property, amply LindelÃ¶f, DiuedonnÃ© plankCategories:18A20, 54C45, 54B30

2. CJM 2005 (vol 57 pp. 1121)

Barr, Michael; Raphael, R.; Woods, R. G.
 On $\mathcal{CR}$-epic Embeddings and Absolute $\mathcal{CR}$-epic Spaces We study Tychonoff spaces $X$ with the property that, for all topological embeddings $X\to Y$, the induced map $C(Y) \to C(X)$ is an epimorphism of rings. Such spaces are called \good. The simplest examples of \good spaces are $\sigma$-compact locally compact spaces and \Lin $P$-spaces. We show that \good first countable spaces must be locally compact. However, a bad'' class of \good spaces is exhibited whose pathology settles, in the negative, a number of open questions. Spaces which are not \good abound, and some are presented. Categories:18A20, 54C45, 54B30
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