arXiv subjects

Gábor Szabó

Publications and source records attributed to Gábor Szabó.

17 recordsLinked to original sources

arXiv2026

Corona algebras and strongly self-absorbing $\mathrm{C}^{\ast}$-dynamics

This article concerns the structure of $\mathrm{C}^{\ast}$-algebraic group actions induced on corona algebras from a given $σ$-unital $\mathrm{C}^{\ast}$-dynamical system over a locally compact group $G$. We prove that such actions satisfy the so-called dynamical folding property, which generalizes a fundamental property observed for corona algebras in works of Manuilov--Thomsen and Phillips--Weaver. We then focus on corona actions induced from $G$-$\mathrm{C}^{\ast}$-dynamics that are assumed to absorb a given strongly self-absorbing and unitarily regular $G$-action $γ$. It is proved that these corona actions are $γ$-saturated, which is a stronger property than being separably $γ$-stable. Conversely, if one assumes that the underlying $\mathrm{C}^{\ast}$-dynamics absorbs the trivial action on the compact operators, then $γ$-saturation of the corona action is equivalent to the original action being $γ$-absorbing. These results are a dynamical version of recent work by Farah and the third-named author.

math.OA↗

arXiv2026

Classification of anomalous actions of finite groups with the Rokhlin property

Given a finite group G, we develop a generalization of the fundamental classification of Rokhlin G-actions on C*-algebras to the setting of anomalous G-actions with the Rokhlin property. For Rokhlin G-kernels on C*-algebras covered by the classification program, this implies that the induced G-action on some known invariants determines the conjugacy class. Extending a result of Izumi, we show that G-kernels with the Rokhlin property on Kirchberg algebras are classified by their anomaly and the induced module structure on the K-theory groups. We explore K-theoretic obstructions for the existence of Rokhlin G-kernels on general separable C*-algebras and use these to characterise which G-module structures arise as K-groups of Kirchberg algebras admitting a Rokhlin G-kernel with a given lifting obstruction.

math.OA↗

arXiv2026

The uniqueness theorem for Kasparov theory

Answering a question of Carrión et al in their recent landmark paper on C*-algebra classification, we prove a general uniqueness theorem for $KK$-theory. Given arbitrary separable C*-algebras $A$ and $B$ and a Cuntz pair consisting of two absorbing representations $φ,ψ: A\to\mathcal{M}(B\otimes\mathcal{K})$, the induced element of $KK(A,B)$ vanishes if and only if $φ$ and $ψ$ are strongly asymptotically unitarily equivalent. This improves upon the Lin-Dadarlat-Eilers stable uniqueness theorem. The conclusion is deduced by first showing the $K_1$-injectivity of an auxiliary C*-algebra associated to the C*-pair $(A,B)$, which is sometimes called the Paschke dual algebra in the literature. Most of the article is concerned with the treatment of an umbrella theorem, which yields such a uniqueness theorem for other variants of $KK$-theory. This encompasses nuclear $KK$-theory, ideal-related $KK$-theory, equivariant $KK$-theory, or any combinations thereof.

math.OA↗

arXiv2025

On a Rokhlin property for abelian group actions on C$^*$-algebras

In this article, we study the so-called abelian Rokhlin property for actions of locally compact, abelian groups on C$^*$-algebras. We propose a unifying framework for obtaining various duality results related to this property. The abelian Rokhlin property coincides with the known Rokhlin property for actions by the reals (i.e., flows), but is not identical to the known Rokhlin property in general. The main duality result we obtain is a generalisation of a duality for flows proved by Kishimoto in the case of Kirchberg algebras. We consider also a slight weakening of the abelian Rokhlin property, which allows us to show that all traces on the crossed product C$^*$-algebra are canonically induced from invariant traces on the the coefficient C$^*$-algebra.

math.OA↗

arXiv2025

Coronas and strongly self-absorbing C*-algebras

Let $\mathcal D$ be a strongly self-absorbing $\mathrm{C}^*$-algebra. Given any separable $\mathrm{C}^*$-algebra $A$, our two main results assert the following. If $A$ is $\mathcal D$-stable, then the corona algebra of $A$ is $\mathcal D$-saturated, i.e., $\mathcal D$ embeds unitally into the relative commutant of every separable $\mathrm{C}^*$-subalgebra. Conversely, assuming that the stable corona of $A$ is separably $\mathcal D$-stable, we prove that $A$ is $\mathcal D$-stable. This generalizes recent work by the first-named author on the structure of the Calkin algebra. As an immediate corollary, it follows that the multiplier algebra of a separable $\mathcal D$-stable $\mathrm{C}^*$-algebra is separably $\mathcal D$-stable. Appropriate versions of the aforementioned results are also obtained when $A$ is not necessarily separable. The article ends with some non-trivial applications.

math.OA↗

arXiv2025

The stable uniqueness theorem for equivariant Kasparov theory

This paper examines and strengthens the Cuntz-Thomsen picture of equivariant Kasparov theory for arbitrary second-countable locally compact groups, in which elements are given by certain pairs of cocycle representations between C*-dynamical systems. The main result is a stable uniqueness theorem that generalizes a fundamental characterization of ordinary $KK$-theory by Lin and Dadarlat-Eilers. Along the way, we prove an equivariant Cuntz-Thomsen picture analog of the fact that the equivalence relation of homotopy agrees with the (a priori stronger) equivalence relation of stable operator homotopy. The results proved in this paper will be employed as the technical centerpiece in forthcoming work of the authors to classify certain amenable group actions on Kirchberg algebras by equivariant Kasparov theory.

math.OA↗

arXiv2024

Classification of equivariantly $\mathcal{O}_2$-stable amenable actions on nuclear $\mathrm{C}^\ast$-algebras

Given a second-countable, locally compact group $G$, we consider amenable $G$-actions on separable, stable, nuclear $\mathrm{C}^\ast$-algebras that are isometrically shift-absorbing and tensorially absorb the trivial action on the Cuntz algebra $\mathcal{O}_2$. We show that such actions are classified up to cocycle conjugacy by the induced $G$-action on the primitive ideal space. In the special case when $G$ is exact, we prove a unital version of our classification theorem. For compact groups, we obtain a classification up to conjugacy.

math.OA↗

arXiv2024

Equivariant property Gamma and the tracial local-to-global principle for C*-dynamics

We consider the notion of equivariant uniform property Gamma for actions of countable discrete groups on C*-algebras that admit traces. In case the group is amenable and the C*-algebra has a compact tracial state space, we prove that this property implies a kind of tracial local-to-global principle for the C*-dynamical system, generalizing a recent such principle for C*-algebras exhibited in work of Castillejos et al. For actions on simple nuclear $\mathcal{Z}$-stable C*-algebras, we use this to prove that equivariant uniform property Gamma is equivalent to equivariant $\mathcal{Z}$-stability, generalizing a result of Gardella-Hirshberg-Vaccaro.

math.OA↗

arXiv2024

Dynamical McDuff-type properties for group actions on von Neumann algebras

We consider the notion of strong self-absorption for continuous actions of locally compact groups on the hyperfinite II$_1$-factor and characterize when such an action is tensorially absorbed by another given action on any separably acting von Neumann algebra. This extends the well-known McDuff property for von Neumann algebras and is analogous to the core theorems around strongly self-absorbing C$^*$-dynamics. Given a countable discrete group $G$ and an amenable action $G\curvearrowright M$ on any separably acting semi-finite von Neumann algebra, we establish a type of measurable local-to-global principle: If a given strongly self-absorbing $G$-action is suitably absorbed at the level of each fibre in the direct integral decomposition of $M$, then it is tensorially absorbed by the action on $M$. As a direct application of Ocneanu's theorem, we deduce that if $M$ has the McDuff property, then every amenable $G$-action on $M$ has the equivariant McDuff property, regardless whether $M$ is assumed to be injective or not. By employing Tomita-Takesaki theory, we can extend the latter result to the general case where $M$ is not assumed to be semi-finite.

math.OA↗

arXiv2023

Equivariant property (SI) revisited, II

We investigate Matui-Sato's notion of property (SI) for C*-dynamics, this time with a focus on actions of possibly non-amenable groups. The main result is a generalization of earlier work: For any countable group $Γ$ and any non-elementary separable simple nuclear C*-algebra $A$ with strict comparison, every amenable $Γ$-action on $A$ has equivariant property (SI). This is deduced from a more general statement involving relative property (SI) for certain inclusions into ultraproducts. The article concludes with a few consequences of this result.

math.OA↗

arXiv2023

The dynamical Kirchberg-Phillips theorem

Let $G$ be a second-countable, locally compact group. In this article we study amenable $G$-actions on Kirchberg algebras that admit an approximately central embedding of a canonical quasi-free action on the Cuntz algebra $\mathcal{O}_\infty$. If $G$ is discrete, this coincides with the class of amenable and outer $G$-actions on Kirchberg algebras. We show that the resulting $G$-C*-dynamical systems are classified by equivariant Kasparov theory up to cocycle conjugacy. This is the first classification theory of its kind applicable to actions of arbitrary locally compact groups. Among various applications, our main result solves a conjecture of Izumi for actions of discrete amenable torsion-free groups, and recovers the main results of recent work by Izumi-Matui for actions of poly-$\mathbb{Z}$ groups.

math.OA↗

arXiv2022

On embeddings of extensions of almost finite actions into cubical shifts

For a countable amenable group $G$ and a fixed dimension $m\geq 1$, we investigate when it is possible to embed a $G$-space $X$ into the $m$-dimensional cubical shift $([0,1]^m)^G$. We focus our attention on systems that arise as an extension of an almost finite $G$-action on a totally disconnected space $Y$, in the sense of Matui and Kerr. We show that if such a $G$-space $X$ has mean dimension less than $m/2$, then $X$ embeds into the $(m+1)$-dimensional cubical shift. If the distinguished factor $G$-space $Y$ is assumed to be a subshift of finite type, then this can be improved to an embedding into the $m$-dimensional cubical shift. This result ought to be viewed as the generalization of a theorem by Gutman-Tsukamoto for $G=\mathbb Z$ to actions of all amenable groups, and represents the first result supporting the Lindenstrauss-Tsukamoto conjecture for actions of groups other than $G=\mathbb{Z}^k$.

math.DS↗

arXiv2018

Approaching the UCT problem via crossed products of the Razak-Jacelon algebra

We show that the UCT problem for separable, nuclear $\mathrm C^*$-algebras relies only on whether the UCT holds for crossed products of certain finite cyclic group actions on the Razak-Jacelon algebra. This observation is analogous to and in fact recovers a characterization of the UCT problem in terms of finite group actions on the Cuntz algebra $\mathcal O_2$ established in previous work by the authors. Although based on a similar approach, the new conceptual ingredients in the finite context are the recent advances in the classification of stably projectionless $\mathrm C^*$-algebras, as well as a known characterization of the UCT problem in terms of certain tracially AF $\mathrm C^*$-algebras due to Dadarlat.

math.OA↗

arXiv2018

On diagonal quasi-free automorphisms of simple Cuntz-Krieger algebras

We show that an outer action of a finite abelian group on a simple Cuntz-Krieger algebra is strongly approximately inner in the sense of Izumi if the action is given by diagonal quasi-free automorphisms and the associated matrix is aperiodic. This is achieved by an approximate cohomology vanishing-type argument for the canonical shift restricted to the relative commutant of the set of domain projections of the canonical generating isometries in the fixed point algebra.

math.OA↗

arXiv2017

Enhancing diamond fluorescence via optimized nanorod dimer configurations

Optical response of silicon (SiV) and nitrogen (NV) vacancy diamond color centers coupled to silver and gold nanorod dimers was numerically inspected. Optimization of the coupled emitter - nanorod dimer configurations was performed to attain the highest possible fluorescence enhancement by enhancing the excitation and emission of color centers simultaneously through plasmonic antenna resonances. To minimize losses conditional optimization was realized by setting a criterion regarding the minimum quantum efficiency of the coupled system (cQE). Restricted symmetric and allowed asymmetric antenna designs were also inspected to prove the potential advantages of asymmetric configurations tuneability. Among all inspected systems the highest 2.59*10^8 fluorescence enhancement with 46.08% cQE was achieved in case of NV color center coupled to asymmetric silver nanoantenna dimer. This is 3.17-times larger than the enhancement in corresponding symmetric configuration, which has larger 68.52% cQE. In case of SiV color center the highest 1.04*10^8 fluorescence enhancement with 37.83% cQE was achieved via asymmetric silver nanoantenna dimer. This is 1.06-times larger than the enhancement in the corresponding symmetric configuration, which has larger 57.46% cQE. The highest fluorescence enhancement achieved by gold nanorods is 4.75*10^4 with 21.8% cQE, which was shown in case of SiV color center coupled to asymmetric dimer. The attained enhancement is 8.48- (92.42-) times larger than the fluorescence enhancement achievable via symmetric (asymmetric) gold nanorod dimer coupled to SiV (NV) color center.

physics.optics↗

arXiv2016

The spatial Rokhlin property for actions of compact quantum groups

We introduce the spatial Rokhlin property for actions of coexact compact quantum groups on $\mathrm{C}^*$-algebras, generalizing the Rokhlin property for both actions of classical compact groups and finite quantum groups. Two key ingredients in our approach are the concept of sequentially split $*$-homomorphisms, and the use of braided tensor products instead of ordinary tensor products. We show that various structure results carry over from the classical theory to this more general setting. In particular, we show that a number of $\mathrm{C}^*$-algebraic properties relevant to the classification program pass from the underlying $\mathrm{C}^*$-algebra of a Rokhlin action to both the crossed product and the fixed point algebra. Towards establishing a classification theory, we show that Rokhlin actions exhibit a rigidity property with respect to approximate unitary equivalence. Regarding duality theory, we introduce the notion of spatial approximate representability for actions of discrete quantum groups. The spatial Rokhlin property for actions of a coexact compact quantum group is shown to be dual to spatial approximate representability for actions of its dual discrete quantum group, and vice versa.

math.OA↗

arXiv2016

Sequentially split $$-homomorphisms between $\mathrm{C}^$-algebras

We define and examine sequentially split $*$-homomorphisms between $\mathrm{C}^*$-algebras and $\mathrm{C}^*$-dynamical systems. For a $*$-homomorphism, the property of being sequentially split can be regarded as an approximate weakening of being a split-injective inclusion of $\mathrm{C}^*$-algebras. We show for a sequentially split $*$-homomorphism that a multitude of $\mathrm{C}^*$-algebraic approximation properties pass from the target algebra to the domain algebra, including virtually all important approximation properties currently used in the classification theory of $\mathrm{C}^*$-algebras. We also discuss various settings in which sequentially split $*$-homomorphisms arise naturally from context. One particular class of examples arises from compact group actions with the Rokhlin property. This allows us to recover and extend the presently known permanence properties of Rokhlin actions with a unified conceptual approach and a simple proof. Moreover, this perspective allows us to obtain new results about such actions, such as a generalization of Izumi's original $K$-theory formula for the fixed point algebra, or duality between the Rokhlin property and approximate representability.

math.OA↗