Strongly s-dense injective hull and Banaschewski’s theorems for acts

For a class 𝓜 of monomorphisms of a category, mathematicians usually use different types of essentiality. Essentiality is an important notion closely related to injectivity. Banaschewski defines and gives sufficient conditions on a category 𝓐 and a subclass 𝓜 of its monomorphisms under which 𝓜-injectivity well-behaves with respect to the notions such as 𝓜-absolute retract and 𝓜-essentialness. In this paper, 𝓐 is taken to be the category of acts over a semigroup S and 𝓜sd to be the class of strongly s-dense monomorphisms. We study essentiality with respect to strongly s-dense monomorphisms of acts. Depending on a class 𝓜 of morphisms of a category 𝓐, In some literatures, three different types of essentialness are considered. Each has its own benefits in regards with the behavior of 𝓜-injectivity. We will show that these three different definitions of essentiality with respect to the class of strongly s-dense monomorphisms are equivalent. Also, the existence and the explicit description of a strongly s-dense injective hull for any given act which is equivalent to the maximal such essential extension and minimal strongly s-dense injective extension with respect to strongly s-dense monomorphism is investigated. At last we conclude that strongly s-dense injectivity well behaves in the category Act-S.

Absolute retractness of automata on directed complete posets

The notion of retractness, which is about having left inverses (reflection) for monomorphisms, is crucial in most branches of mathematics. One very important notion related to it is injectivity, which is about extending morphisms to larger domains and plays a fundamental role in many areas of mathematics as well as in computer science, under the name of complete or partial objects. Absolute retractness is tightly related to injectivity and is in fact equivalent to it in many categories. In this paper, combining the two important notions of actions of semigroups and directed complete posets, which are both crucial abstraction and useful in mathematics as well as in computer science, we consider the category Dcpo-[Formula: see text] of actions of a directed complete semigroup on directed complete posets, and study absolute retractness with respect to both monomorphisms and embeddings in this category. Among other things, we show that absolute retract ([Formula: see text]-)dcpo’s are complete but the converse is not necessarily true. Investigating the converse, we find that if we add the property of being a countable chain to completeness, over some kinds of dcpo-monoids such as dcpo-groups and commutative monoids, we get absolute retractness. Furthermore, we show that there are absolute retract [Formula: see text]-dcpo’s, which are not chains.

Supercompactness of spaces of capacities and its consequences

It is proved that the space of normalized real-valued capacities on an openly generated compactum is an absolute retract. Existence of a normal binary subbase for closed subsets in the space of normalized lattice-valued capacities on an openly generated compactum is also shown, which implies that this space is a Dugundji space.

Paths in hyperspaces

<p>We prove that the hyperspace of closed bounded sets with the Hausdor_ topology, over an almost convex metric space, is an absolute retract. Dense subspaces of normed linear spaces are examples of, not necessarily connected, almost convex metric spaces. We give some necessary conditions for the path-wise connectedness of the Hausdorff metric topology on closed bounded sets. Finally, we describe properties of a separable metric space, under which its hyperspace with the Wijsman topology is path-wise connected.</p>