## A.V. Arhangel'skii

*Perfect mappings in topological groups, cross-complementary subsets and quotients *

Comment.Math.Univ.Carolinae 44,4 (2003) 701-709. **Abstract:**The following general question is considered. Suppose that $G$ is a topological group, and $F$, $M$ are subspaces of $G$ such that $G=MF$. Under these general assumptions, how are the properties of $F$ and $M$ related to the properties of $G$? For example, it is observed that if $M$ is closed metrizable and $F$ is compact, then $G$ is a paracompact $p$-space. Furthermore, if $M$ is closed and first countable, $F$ is a first countable compactum, and $FM=G$, then $G$ is also metrizable. Several other results of this kind are obtained. An extensive use is made of the following old theorem of N. Bourbaki [5]: if $F$ is a compact subset of a topological group $G$, then the natural mapping of the product space $G\times F$ onto $G$, given by the product operation in $G$, is perfect (that is, closed continuous and the fibers are compact). This fact provides a basis for applications of the theory of perfect mappings to topological groups. Bourbaki's result is also generalized to the case of Lindel\"of subspaces of topological groups; with this purpose the notion of a $G_\delta $-closed mapping is introduced. This leads to new results on topological groups which are $P$-spaces.

**Keywords:** topological group, quotient group, locally compact subgroup, quotient mapping, perfect mapping, paracompact $p$-space, metrizable group, countable tightness

**AMS Subject Classification:** 22A05, 54H11, 54D35, 54D60

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