A note on Möbius functions and a breaking process

Links between Möbius functions and inverse semigroups are an interesting topic for study. In this paper, we restrict our interest to the submonoid of right units (as a Möbius monoid) of a combinatorial bisimple inverse monoid. The Möbius functions at the end of this paper are Möbius functions of broken Möbius categories (broken up into two parts) via a submonoid and a right monoid action.

Alexandroff topologies and monoid actions

Given a monoid S acting (on the left) on a set X, all the subsets of X which are invariant with respect to such an action constitute the family of the closed subsets of an Alexandroff topology on X. Conversely, we prove that any Alexandroff topology may be obtained through a monoid action. Based on such a link between monoid actions and Alexandroff topologies, we firstly establish several topological properties for Alexandroff spaces bearing in mind specific examples of monoid actions. Secondly, given an Alexandroff space X with associated topological closure operator σ, we introduce a specific notion of dependence on union of subsets. Then, in relation to such a dependence, we study the family {\mathcal{A}_{\sigma,X}} of the closed subsets Y of X such that, for any {y_{1},y_{2}\in Y}, there exists a third element {y\in Y} whose closure contains both {y_{1}} and {y_{2}}. More in detail, relying on some specific properties of the maximal members of the family {\mathcal{A}_{\sigma,X}}, we provide a decomposition theorem regarding an Alexandroff space as the union (not necessarily disjoint) of a pair of closed subsets characterized by such a dependence. Finally, we refine the study of the aforementioned decomposition through a descending chain of closed subsets of X of which we give some examples taken from specific monoid actions.

SOLVING DIFFERENCE EQUATIONS IN SEQUENCES: UNIVERSALITY AND UNDECIDABILITY

We study solutions of difference equations in the rings of sequences and, more generally, solutions of equations with a monoid action in the ring of sequences indexed by the monoid. This framework includes, for example, difference equations on grids (for example, standard difference schemes) and difference equations in functions on words. On the universality side, we prove a version of strong Nullstellensatz for such difference equations under the assumption that the cardinality of the ground field is greater than the cardinality of the monoid and construct an example showing that this assumption cannot be omitted. On the undecidability side, we show that the following problems are undecidable: