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The envelope of a subcategory in topology and group theory. (English) Zbl 1145.18300

Summary: A collection of results are presented which are loosely centered around the notion of reflective subcategory. For example, it is shown that reflective subcategories are orthogonality classes, that the morphisms orthogonal to a reflective subcategory are precisely the morphisms inverted under the reflector, and that each subcategory has a largest “envelope” in the ambient category in which it is reflective. Moreover, known results concerning the envelopes of the category of sober spaces, spectral spaces, and jacspectral spaces, respectively, are summarized and reproved. Finally, attention is focused on the envelopes of one-object subcategories, and examples are considered in the category of groups.

MSC:

18B30 Categories of topological spaces and continuous mappings (MSC2010)
20J15 Category of groups
54B30 Categorical methods in general topology
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