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G. Bezhanishvili

Publications and source records attributed to G. Bezhanishvili.

17 recordsLinked to original sources
arXiv2026

Local compactness does not always imply spatiality

It is a well-known result in pointfree topology that every locally compact frame is spatial. Whether this result extends to MT-algebras (McKinsey-Tarski algebras) was an open problem. We resolve it in the negative by constructing a locally compact sober MT-algebra which is not spatial. We also revisit Nöbeling's largely overlooked approach to pointfree topology from the 1950s. We show that his separation axioms are closely related to those in the theory of MT-algebras with the notable exception of Hausdorffness. We prove that Nöbeling's Spatiality Theorem implies the well-known Isbell Spatiality Theorem. We then generalize Nöbeling's Spatiality Theorem by proving that each locally compact $T_{1/2}$-algebra is spatial. The proof utilizes the fact that every nontrivial $T_{1/2}$-algebra contains a closed atom, which we show is equivalent to the axiom of choice.

math.GN↗
arXiv2026

Generalizing Gelfand duality to Nachbin spaces

We introduce the notion of a Nachbin proximity on a bounded archimedean $\ell$-algebra (bal-algebra). We prove that Gelfand duality lifts to yield a dual equivalence between the categories of uniformly complete bal-algebras equipped with a closed Nachbin proximity and of Nachbin spaces (compact ordered spaces). The key ingredients of the proof include appropriate generalizations of the Stone-Weierstrass theorem and Dieudonné's lemma. We also develop an alternate approach by means of bounded archimedean $\ell$-semialgebras (sbal-algebras), from which we derive De Rudder--Hansoul duality.

math.AC↗
arXiv2026

Maximal d-spectra and locally compact Hausdorff spaces

It is an interesting open problem whether every compact Hausdorff space can be realized as the maximal $d$-spectrum of an arithmetic frame. We approach this problem by generalizing the $d$-nucleus to a stably continuous frame. We use Priestley duality to characterize the resulting $\underline d$-nucleus, which allows us to prove that every locally compact Hausdorff space can be realized as the maximal $\underline d$-spectrum of a continuous regular frame. As a corollary, we obtain that every locally Stone space can be realized as the maximal $d$-spectrum of an algebraic regular frame.

math.GN↗
arXiv2025

Maximal $d$-spectra via Priestley duality

We use Priestley duality as a new tool to study maximal $d$-spectra of arithmetic frames, both with and without units. We pay special attention to when the maximal $d$-spectrum is compact or Hausdorff. Various necessary and sufficient conditions are given, including a construction of an arithmetic frame with a unit whose maximal $d$-spectrum is not Hausdorff, thus resolving an open problem in the literature.

math.GN↗
arXiv2025

Dedekind-MacNeille and related completions: subfitness, regularity, and Booleanness

Completions play an important rôle for studying structure by supplying elements that in some sense ``ought to be." Among these, the Dedekind-MacNeille completion is of particular importance. In 1968 Janowitz provided necessary and sufficient conditions for it to be subfit or Boolean. Another natural separation axiom connected to these is regularity. We explore similar characterizations of when closely related completions are subfit, regular, or Boolean. We are mainly interested in the Bruns-Lakser, ideal, and canonical completions, which (unlike the Dedekind-MacNeille completion) satisfy stronger forms of distributivity. The first two are widely used in pointfree topology, while the latter is of crucial importance in the semantics of modal logic.

math.GN↗
arXiv2025

Degrees of join-distributivity via Bruns-Lakser towers

We utilize the Bruns-Lakser completion to introduce Bruns-Lakser towers of a meet-semilattice. This machinery enables us to develop various hierarchies inside the class of bounded distributive lattices, which measure $κ$-degrees of distributivity of bounded distributive lattices and their Dedekind-MacNeille completions. We also use Priestley duality to obtain a dual characterization of the resulting hierarchies. Among other things, this yields a natural generalization of Esakia's representation of Heyting lattices to proHeyting lattices.

math.LO↗
arXiv2024

Semilattice base hierarchy for frames and its topological ramifications

We develop a hierarchy of semilattice bases (S-bases) for frames. For a given (unbounded) meet-semilattice $A$, we analyze the interval in the coframe of sublocales of the frame of downsets of $A$ formed by all frames with the S-base $A$. We give an explicit description of the nuclei associated with these sublocales. We study various degrees of completeness of $A$, which generalize the concepts of extremally disconnected and basically disconnected frames. We also introduce the concepts of D-bases and L-bases, as well as their bounded counterparts, and show how our results specialize and sharpen in these cases. Classic examples that are covered by our approach include zero-dimensional, completely regular, and coherent frames, allowing us to provide a new perspective on these well-studied classes of frames, as well as their spatial counterparts.

math.GN↗
arXiv2024

Algebraic Frames in Priestley duality

We characterize Priestley spaces of algebraic, arithmetic, coherent, and Stone frames. As a corollary, we derive the well-known dual equivalences in pointfree topology involving various categories of algebraic frames.

math.GN↗
arXiv2023

Deriving dualities in pointfree topology from Priestley duality

There are several prominent duality results in pointfree topology. The Hofmann-Lawson duality establishes that the category of continuous frames is dually equivalent to the category of locally compact sober spaces. This restricts to a dual equivalence between the categories of stably continuous frames and stably locally compact spaces, which further restricts to Isbell duality between the categories of compact regular frames and compact Hausdorff spaces. We show how to derive these dualities from Priestley duality for distributive lattices, thus shedding new light on these classic results.

math.GN↗
arXiv2023

De Vries powers and proximity Specker algebras

By de Vries duality [9], the category ${\sf KHaus}$ of compact Hausdorff spaces is dually equivalent to the category ${\sf DeV}$ of de Vries algebras. In [5] an alternate duality for ${\sf KHaus}$ was developed, where de Vries algebras were replaced by proximity Baer-Specker algebras. The functor associating with each compact Hausdorff space a proximity Baer-Specker algebra was described by generalizing the notion of a boolean power of a totally ordered domain to that of a de Vries power. It follows that ${\sf DeV}$ is equivalent to the category ${\sf PBSp}$ of proximity Baer-Specker algebras. The equivalence is obtained by passing through ${\sf KHaus}$, and hence is not choice-free. In this paper we give a direct algebraic proof of this equivalence, which is choice-free. To do so, we give an alternate choice-free description of de Vries powers of a totally ordered domain.

math.RA↗
arXiv2022

Remarks on Hyperspaces for Priestley Spaces

The Vietoris space of a Stone space plays an important role in the coalgebraic approach to modal logic. When generalizing this to positive modal logic, there is a variety of relevant hyperspace constructions based on various topologies on a Priestley space and mechanisms to topologize the hyperspace of closed sets. A number of authors considered hyperspaces of Priestley spaces and their application to the coalgebraic approach to positive modal logic. A mixture of techniques from category theory, pointfree topology, and Priestley duality have been employed. Our aim is to provide a unifying approach to this area of research relying only on a basic familiarity with Priestley duality and related free constructions of distributive lattices.

math.GN↗
arXiv2021

Epimorphisms in varieties of residuated structures

It is proved that epimorphisms are surjective in a range of varieties of residuated structures, including all varieties of Heyting or Brouwerian algebras of finite depth, and all varieties consisting of Goedel algebras, relative Stone algebras, Sugihara monoids or positive Sugihara monoids. This establishes the infinite deductive Beth definability property for a corresponding range of substructural logics. On the other hand, it is shown that epimorphisms need not be surjective in a locally finite variety of Heyting or Brouwerian algebras of width 2. It follows that the infinite Beth property is strictly stronger than the so-called finite Beth property, confirming a conjecture of Blok and Hoogland.

math.LO↗
arXiv2021

A point-free approach to canonical extensions of boolean algebras and bounded archimedean $\ell$-algebras

In \cite{BH20} an elegant choice-free construction of a canonical extension of a boolean algebra $B$ was given as the boolean algebra of regular open subsets of the Alexandroff topology on the poset of proper filters of $B$. We make this construction point-free by replacing the Alexandroff space of proper filters of $B$ with the free frame $\mathcal{L}$ generated by the bounded meet-semilattice of all filters of $B$ (ordered by reverse inclusion) and prove that the booleanization of $\mathcal{L}$ is a canonical extension of $B$. Our main result generalizes this approach to the category $\boldsymbol{\mathit{ba}\ell}$ of bounded archimedean $\ell$-algebras, thus yielding a point-free construction of canonical extensions in $\boldsymbol{\mathit{ba}\ell}$. We conclude by showing that the algebra of normal functions on the Alexandroff space of proper archimedean $\ell$-ideals of $A$ is a canonical extension of $A\in\boldsymbol{\mathit{ba}\ell}$, thus providing a generalization of the result of \cite{BH20} to $\boldsymbol{\mathit{ba}\ell}$.

math.RA↗
arXiv2021

Profiniteness and representability of spectra of Heyting algebras

We prove that there exist profinite Heyting algebras that are not isomorphic to the profinite completion of any Heyting algebra. This resolves an open problem from 2009. More generally, we characterize those varieties of Heyting algebras in which profinite algebras are isomorphic to profinite completions. It turns out that there exists largest such. We give different characterizations of this variety and show that it is finitely axiomatizable and locally finite. From this it follows that it is decidable whether in a finitely axiomatizable variety of Heyting algebras all profinite members are profinite completions. In addition, we introduce and characterize representable varieties of Heyting algebras, thus drawing connection to the classical problem of representing posets as prime spectra.

math.LO↗
arXiv2020

The Vietoris functor and modal operators on rings of continuous functions

We introduce an endofunctor $H$ on the category $bal$ of bounded archimedean $\ell$-algebras and show that there is a dual adjunction between the category $Alg(H)$ of algebras for $H$ and the category $Coalg(V)$ of coalgebras for the Vietoris endofunctor $V$ on the category of compact Hausdorff spaces. We also introduce an endofunctor $Hu$ on the reflective subcategory of $bal$ consisting of uniformly complete objects of $bal$ and show that Gelfand duality lifts to a dual equivalence between $Alg(Hu)$ and $Coalg(V)$. On the one hand, this generalizes a result of \cite{Abr88,KKV04} for the category of coalgebras of the Vietoris endofunctor on the category of Stone spaces. On the other hand, it yields an alternate proof of a recent result of \cite{BCM20a}.

math.RA↗
arXiv2019

Gelfand-Naimark-Stone duality for normal spaces and insertion theorems

Gelfand-Naimark-Stone duality provides an algebraic counterpart of compact Hausdorff spaces in the form of uniformly complete bounded archimedean $\ell$-algebras. In [4] we extended this duality to completely regular spaces. In this article we use this extension to characterize normal, Lindëlof, and locally compact Hausdorff spaces. Our approach gives a different perspective on the classical theorems of Katětov-Tong and Stone-Weierstrass.

math.GN↗