Order theoretic variants of the fundamental theorem of compact semigroups.
We show that if an uncountable regular cardinal τ and τ + 1 embed in a topological group G as closed subspaces then G is not normal. We also prove that an uncountable regular cardinal cannot be embedded in a torsion free Abelian group that is hereditarily normal. These results are corollaries to our main results about ordinals in topological groups. To state the main results, let τ be an uncountable regular cardinal and G a T₁ topological group. We prove, among others, the following statements:...
The following general question is considered. Suppose that is a topological group, and , are subspaces of such that . Under these general assumptions, how are the properties of and related to the properties of ? For example, it is observed that if is closed metrizable and is compact, then is a paracompact -space. Furthermore, if is closed and first countable, is a first countable compactum, and , then is also metrizable. Several other results of this kind are obtained....
In this paper we introduce perfectly supportable semigroups and prove that they are σ-discrete in each Hausdorff shiftinvariant topology. The class of perfectly supportable semigroups includes each semigroup S such that FSym(X) ⊂ S ⊂ FRel(X) where FRel(X) is the semigroup of finitely supported relations on an infinite set X and FSym(X) is the group of finitely supported permutations of X.