arXiv subjects

B. K. Kwaśniewski

Publications and source records attributed to B. K. Kwaśniewski.

12 recordsLinked to original sources

arXiv2026

C*-irreducible regular inclusions, Galois correspondence and aperiodicity

We characterise C*-irreducible regular C*-inclusions using a number of different conditions considered by different authors. In particular, we show that all C*-irreducible regular inclusions $A\subseteq B$ are modelled by outer Fell bundles $(B_{g})_{g\in G}$ over discrete groups with a simple unit fibre $A=B_1$. In this case, we prove a bijection between intermediate C*-algebras $A\subseteq C \subseteq B$ and subgroups $H$ of $G$. This extends the Galois correspondence for reduced crossed products by discrete group actions established by Cameron-Smith. We relate it to the Galois correspondences of Izumi and Mukohara for fixed-point algebras of actions of compact abelian groups, and the mixed inclusion of a fixed-point algebra in a reduced crossed product considered by Echterhoff-Rørdam. In addition, using a recent result of Geffen-Ursu, we show that the inclusion of a fixed-point subalgebra $A\subseteq B$ of an action of $\mathbb{T}$ or $\mathbb{Z}/p$ for a square-free number $p>0$ is aperiodic if and only if $A$ detects ideals in $B$. We apply this to give examples of C*-irreducible inclusions coming from Cuntz-Pimsner algebras, including crossed products by endomorphisms or transfer operators. In particular, we characterise when a core subalgebra of a graph C*-algebra is C*-irreducible. Lastly, we show that a general regular topologically graded C*-inclusion $A\subseteq B$ is aperiodic and has a unique pseudo-expectation provided $A$ detects ideals in all intermediate C*-algebras of $B$. This partially answers a question by Pitts-Zarikian.

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arXiv2026

Banach algebra crossed products by inverse semigroup actions

We give a self-contained and simplified presentation of the theory of covariant representations for inverse semigroup actions on Banach algebras, which was recently introduced in the authors and A. Mckee in the twisted case. The main result of this note is a general universal description of the associated Banach algebra crossed product, that allows disintegration of all representations of the crossed product. Such a disintegration was studied so far only for actions on spaces.

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arXiv2025

Twisted operator algebras of self-similar groupoid actions on arbitrary graphs

We study self-similar groupoid actions on arbitrary directed graphs together with $\mathbb{T}$-valued twists that exhaust the second cohomology group of the associated Zappa-Szép product category. We define and analyse the associated universal, reduced, and essential $C^*$-algebras, along with their Toeplitz versions and core subalgebras. In fact, we develop our theory in the more general setting of $L^P$-operator algebras, where $P\subseteq [1,\infty]$ is any non-empty set of parameters. This includes $C^*$-algebras, $L^p$-operator algebras and symmetrised $L^{p,*}$-operator algebras for $p\in [1,\infty]$, as special cases. We use three complementary approaches: twisted inverse semigroups, twisted ample groupoids, and $C^*$-correspondences. We provide, in terms of the self-similar action, general characterisations of topological freeness, minimality, Hausdorffness, finite non-Hausdorffness, effectiveness, and local contractiveness for the associated ample groupoids. We generalise the classical Cuntz--Krieger Uniqueness and Coburn--Toeplitz Uniqueness Theorems for graph $C^*$-algebras to twisted $L^P$-operator algebras of self-similar groupoid actions. We characterise when the natural inclusions are Cartan, give checkable criteria for simplicity and pure infiniteness of the essential algebras, and discuss when the universal and reduced algebras coincide. We also provide conditions that ensure the singular ideals vanish. Using the groupoid model we show that for any $P\subseteq [1,\infty]$, the $L^P$-operator algebra of a contracting self-similar action is simple if and only if the corresponding Steinberg algebra is simple. Using the Toeplitz-Pimsner model, we prove that for the universal groupoid of any self-similar groupoid action on a row-finite graph, the singular $C^*$-algebraic ideal always vanishes.

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arXiv2025

Topologically free actions and ideals in twisted Banach algebra crossed products

We generalize the influential $C^*$-algebraic result of Kawamura-Tomiyama and Archbold-Spielberg for crossed products of discrete transformation groups to the realm of Banach algebras and twisted actions. We prove that topological freeness is equivalent to the intersection property for all reduced twisted Banach algebra crossed products coming from subgroups, and in the untwisted case to a generalised intersection property for a full $L^p$-operator algebra crossed product for any $p\in [1,\infty]$. This gives efficient simplicity criteria for various Banach algebra crossed products. We also use it to identify the prime ideal space of some crossed products as the quasi-orbit space of the action. For amenable actions we prove that the full and reduced twisted $L^p$-operator algebras coincide.

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arXiv2023

Ideal structure and pure infiniteness of inverse semigroup crossed products

Let $A\subseteq B$ be a $C^*$-inclusion. We give efficient conditions under which $A$ separates ideals in $B$, and $B$ is purely infinite if every positive element in $A$ is properly infinite in $B$. We specialise to the case when $B$ is a crossed product for an inverse semigroup action by Hilbert bimodules or a section $C^*$-algebra of a Fell bundle over an étale, possibly non-Hausdorff, groupoid. Then our theory works provided $B$ is the recently introduced essential crossed product and the action is essentially exact and residually aperiodic or residually topologically free. These last notions are developed in the article.

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arXiv2020

Stone duality and quasi-orbit spaces for generalised C*-inclusions

Let $A$ and $B$ be $C^*$-algebras with $A\subseteq M(B)$. Exploiting the duality between sober spaces and spatial locales, and the adjunction between restriction and induction for ideals in $A$ and $B$, we identify conditions that allow to define a quasi-orbit space and a quasi-orbit map for $A\subseteq M(B)$. These objects generalise classical notions for group actions. We characterise when the quasi-orbit space is an open quotient of the primitive ideal space of $A$ and when the quasi-orbit map is open and surjective. We apply these results to cross section $C^*$-algebras of Fell bundles over locally compact groups, regular $C^*$-inclusions, tensor products, relative Cuntz--Pimsner algebras, and crossed products for actions of locally compact Hausdorff groupoids and quantum groups.

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arXiv2016

Pure infiniteness and ideal structure of $C^*$-algebras associated to Fell bundles

We investigate structural properties of the reduced cross-sectional algebra $C^*_r(\mathcal{B})$ of a Fell bundle $\mathcal{B}$ over a discrete group $G$. Conditions allowing one to determine the ideal structure of $C^*_r(\mathcal{B})$ are studied. Notions of aperiodicity, paradoxicality and $\mathcal{B}$-infiniteness for the Fell bundle $\mathcal{B}$ are introduced and investigated by themselves and in relation to the partial dynamical system dual to $\mathcal{B}$. Several criteria of pure infiniteness of $C^*_r(\mathcal{B})$ are given. It is shown that they generalize and unify corresponding results obtained in the context of crossed products, by the following duos: Laca, Spielberg; Jolissaint, Robertson; Sierakowski, Rørdam; Giordano, Sierakowski and Ortega, Pardo. For exact, separable Fell bundles satisfying the residual intersection property primitive ideal space of $C^*_r(\mathcal{B})$ is determined. The results of the paper are shown to be optimal when applied to graph $C^*$-algebras. Applications to a class of Exel-Larsen crossed products are presented.

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arXiv2015

Exel's crossed product and crossed products by completely positive maps

We introduce crossed products of a $C^*$-algebra $A$ by a completely positive map $\varrho:A\to A$ relative to an ideal in $A$. They generalize various crossed products by endomorphisms when $\varrho$ is multiplicative. When $A$ is commutative they include $C^*$-algebras associated to Markov operators by Ionescu, Muhly, Vega, and to topological relations by Brenken, but in general they are not modeled by topological quivers popularized by Muhly and Tomforde. We show that Exel's crossed product $A\rtimes_{α,L} \mathbb{N}$, generalized to the case where $A$ is not necessarily unital, is the crossed product of $A$ by the transfer operator $L$ relative to the ideal generated by $α(A)$. We give natural conditions under which $α(A)$ is uniquely determined by $L$, and hence $A\rtimes_{α,L} \mathbb{N}$ depends only on $L$. Moreover, the $C^*$-algebra $\mathcal{O}(A,α,L)$ associated to $(A,α,L)$ by Exel and Royer always coincides with our unrelative crossed product by $L$. As another non-trivial application of our construction we extend a result of Brownlowe, Raeburn and Vittadello, by showing that the $C^*$-algebra of an arbitrary infinite graph $E$ can be realized as a crossed product of the diagonal algebra $\mathcal{D}_E$ by a `Perron-Frobenious' operator $L$. The important difference is that in general there is no endomorphism $α$ of $\mathcal{D}_E$ making $(\mathcal{D}_E,α,L)$ an Exel system.

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arXiv2014

Crossed products for interactions and graph algebras

We consider Exel's interaction $(V,H)$ over a unital $C^*$-algebra $A$, such that $V(A)$ and $H(A)$ are hereditary subalgebras of $A$. For the associated crossed product, we obtain a uniqueness theorem, ideal lattice description, simplicity criterion and a version of Pimsner-Voiculescu exact sequence. These results cover the case of crossed products by endomorphisms with hereditary ranges and complementary kernels. As model examples of interactions not coming from endomorphisms we introduce and study in detail interactions arising from finite graphs. The interaction $(V,H)$ associated to a graph $E$ acts on the core $F_E$ of the graph algebra $C^*(E)$. By describing a partial homeomorphism of $\widehat{F}_E$ dual to $(V,H)$ we find Cuntz-Krieger uniqueness theorem, criteria for gauge-invariance of all ideals and simplicity of $C^*(E)$ as results concerning reversible noncommutative dynamics. We also provide a new approach to calculation of $K$-theory of $C^*(E)$ using only an induced partial automorphism of $K_0(F_E)$ and the six-term exact sequence.

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arXiv2013

Extensions of C*-dynamical systems to systems with complete transfer operators

Starting from an arbitrary endomorphism $α$ of a unital C*-algebra $A$ we construct a bigger C*-algebra $B$ and extend $α$ onto $B$ in such a way that the extended endomorphism $α$ has a unital kernel and a hereditary range, i.e. there exists a unique non-degenerate transfer operator for $(B,α)$, called the complete transfer operator. The pair $(B,α)$ is universal with respect to a suitable notion of a covariant representation and depends on a choice of an ideal in $A$. The construction enables a natural definition of the crossed product for arbitrary $α$.

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arXiv2011

C*-algebras generalizing both relative Cuntz-Pimsner and Doplicher-Roberts algebras

We introduce and analyse the structure of C*-algebras arising from ideals in right tensor C*-precategories, which naturally generalize both relative Cuntz-Pimsner and Doplicher-Roberts algebras. We establish an explicit intrinsic construction of the considered algebras and a number of key results such as structure theorem, gauge-invariant uniqueness theorem or description of the gauge-invariant ideal structure. In particular, these statements give a new insight into the corresponding results for relative Cuntz-Pimsner algebras and are applied to Doplicher-Roberts associated to C*-correspondences.

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arXiv2009

On transfer operators for C*-dynamical systems

The theme of the paper is the question of existence and basic structure of transfer operators for endomorphisms of a unital C*-algebra. We establish a complete description of non-degenerate transfer operators, characterize complete transfer operators and clarify their role in crossed product constructions. Also, we give necessary and sufficient conditions for existence of transfer operators for commutative systems, and discuss their form for endomorphisms of B(H) which is relevant to Kadison-Singer problem.

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