CyberSec Research

We characterize $C^*$-simplicity for countable groups by means of the following dichotomy. If a group is $C^*$-simple, then the action on the Poisson boundary is essentially free for a generic measure on the group. If a group is not $C^*$-simple, then the action on the Poisson boundary is not essentially free for a generic measure on the group.

The aim of this paper is threefold. Firstly, we develop the author's previous work on the dynamical relationship between determinantal point processes and CAR algebras. Secondly, we present a novel application of the theory of stochastic processes associated with KMS states for CAR algebras and their quasi-free states. Lastly, we propose a unified theory of algebraic constructions and analysis of stationary processes on point configuration spaces with respect to determinantal point processes. As a byproduct, we establish an algebraic derivation of a determinantal formula for space-time correlations of stochastic processes, and we analyze several limiting behaviors of these processes.

Motivated by the interplay between quadratic algebras, noncommutative geometry, and operator theory, we introduce the notion of quadratic subproduct systems of Hilbert spaces. Specifically, we study the subproduct systems induced by a finite number of complex quadratic polynomials in noncommuting variables, and describe their Toeplitz and Cuntz--Pimsner algebras. Inspired by the theory of graded associative algebras, we define a free product operation in the category of subproduct systems and show that this corresponds to the reduced free product of the Toeplitz algebras. Finally, we obtain results about the K-theory of the Toeplitz and Cuntz--Pimsner algebras of a large class of quadratic subproduct systems.

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.

We prove that a polynomial path of Riemannian metrics on a closed spin manifold induces a continuous field in the spectral propinquity of metric spectral triples.

Given a weak Kac system with duality $(\mathcal{H},V,U)$ arising from regular $\mathrm{C}^{*}$-algebraic locally compact quantum group $(\mathcal{G},\Delta)$, a $\mathrm{C}^{*}$-algebra $A$, and a sufficiently well-behaved coaction $\alpha$, we construct natural lattice isomorphisms from the coaction invariant ideals of $A$ to the dual coaction invariant ideals of full and reduced crossed products associated to $(\mathcal{H},V,U)$. In particular, these lattice isomorphisms are determined by either the maximality or normality of the coaction $\alpha$. This result directly generalizes the main theorem of Gillespie, Kaliszewski, Quigg, and Williams in arXiv:2406.06780, which in turn generalized an older ideal correspondence result of Gootman and Lazar for locally compact amenable groups. Throughout, we also develop basic conventions and motivate through elementary examples how crossed product $\mathrm{C}^{*}$-algebras by quantum groups generalize the classical crossed product theory.

Given a field $K$ and an ample (not necessarily Hausdorff) groupoid $G$, we define the concept of a line bundle over $G$ inspired by the well known concept from the theory of C*-algebras. If $E$ is such a line bundle, we construct the associated twisted Steinberg algebra in terms of sections of $E$, which turns out to extend the original construction introduced independently by Steinberg in 2010, and by Clark, Farthing, Sims and Tomforde in a 2014 paper (originally announced in 2011). We also generalize (strictly, in the non-Hausdorff case) the 2023 construction of (cocycle) twisted Steinberg algebras of Armstrong, Clark, Courtney, Lin, Mccormick and Ramagge. We then extend Steinberg's theory of induction of modules, not only to the twisted case, but to the much more general case of regular inclusions of algebras. Our main result shows that, under appropriate conditions, every irreducible module is induced by an irreducible module over a certain abstractly defined isotropy algebra.

We study the C$^*$ algebra generated by the composition operator $C_a$ acting on the Hardy space $H^2$ of the unit disk, given by $C_af=f\circ\varphi_a$, where $$ \varphi_a(z)=\frac{a-z}{1-\bar{a}z}, $$ for $|a|<1$. Also several operators related to $C_a$ are examined.

A graph of Gelfand-Kirillov dimension three is a connected finite essential graph such that its Leavitt path algebra has Gelfand-Kirillov dimension three. We provide number-theoretic criteria for graphs of Gelfand-Kirillov dimension three to be strong shift equivalent. We then prove that two graphs of Gelfand-Kirillov dimension three are shift equivalent if and only if they are strongly shift equivalent, if and only if their corresponding Leavitt path algebras are graded Morita equivalent, if and only if their graded $K$-theories, $K^{\text{gr}}_0$, are order-preserving $\mathbb{Z}[x, x^{-1}]$-module isomorphic. As a consequence, we obtain that the Leavitt path algebras of graphs of Gelfand-Kirillov dimension three are graded Morita equivalent if and only if their graph $C^*$-algebras are equivariant Morita equivalent, and two graphs $E$ and $F$ of Gelfand-Kirillov dimension three are shift equivalent if and only if the singularity categories $\text{D}_{\text{sg}}(KE/J_E^2)$ and $\text{D}_{\text{sg}}(KF/J_F^2)$ are triangulated equivalent.

In this paper, some Jensen's type inequalities between quaternionic bounded selfadjoint operators on quaternionic Hilbert spaces are proved, using a log-convex function. Also, by applying a specific log-convex function, some particular cases of operator inequalities are obtained.

This paper investigates and classifies a specific class of one-parameter continuous fields of C*-algebras, which can be seen as generalized AI-algebras. Building on the classification of *-homomorphisms between interval algebras by the Cuntz semigroup, along with a selection theorem and a gluing procedure, we employ a 'local-to-global' strategy to achieve our classification result.

A three-functor formalism is the half of a six-functor formalism that supports the projection and base change formulas. In this paper, we provide a three-functor formalism for commutative von Neumann algebras and their modules. Using the Gelfand-Naimark theorem, this gives rise to a three-functor formalism for measure spaces and measurable bundles of Hilbert spaces. We use this to prove Fell absorption for unitary representations of measure groupoids. The three-functor formalism for commutative von Neumann algebras takes values in W*-categories, and we discuss in what sense it is a unitary three-functor formalism.

We introduce and study a unital version of shift equivalence for finite square matrices over the nonnegative integers. In contrast to the classical case, we show that unital shift equivalence does not coincide with one-sided eventual conjugacy. We also prove that unital shift equivalent matrices define one-sided shifts of finite type that are continuously orbit equivalent. Consequently, unitally shift equivalent matrices have isomorphic topological full groups and isomorphic Leavitt path algebras, the latter being related to Hazrat's graded classification conjecture in algebra.

We present a systematic investigation of bimodule quantum Markov semigroups within the framework of quantum Fourier analysis. Building on the structure of quantum symmetries, we introduce the concepts of bimodule equilibrium and bimodule detailed balance conditions, which not only generalize the classical notions of equilibrium and detailed balance but also expose interesting structures of quantum channels. We demonstrate that the evolution of densities governed by the bimodule quantum Markov semigroup is the bimodule gradient flow for the relative entropy with respect to quantum symmetries. Consequently, we obtain bimodule logarithmic Sobelov inequalities and bimodule Talagrand inequality with respect to a hidden density from higher dimensional structure. Furthermore, we establish a bimodule Poincar\'{e} inequality for irreducible inclusions and relative ergodic bimodule quantum semigroups.

Let $\Gamma$ be the fundamental group of a closed orientable surface of genus at least two. Consider the composition of a uniformly random element of $\mathrm{Hom}(\Gamma,S_n)$ with the $(n-1)$-dimensional irreducible representation of $S_n$. We prove the strong convergence in probability as $n\to\infty$ of this sequence of random representations to the regular representation of $\Gamma$. As a consequence, for any closed hyperbolic surface $X$, with probability tending to one as $n\to\infty$, a uniformly random degree-$n$ covering space of $X$ has near optimal relative spectral gap -- ignoring the eigenvalues that arise from the base surface $X$. To do so, we show that the polynomial method of proving strong convergence can be extended beyond rational settings. To meet the requirements of this extension we prove two new kinds of results. First, we show there are effective polynomial approximations of expected values of traces of elements of $\Gamma$ under random homomorphisms to $S_n$. Secondly, we estimate the growth rates of probabilities that a finitely supported random walk on $\Gamma$ is a proper power after a given number of steps.

The amenability of group actions on topological spaces generalizes amenability of groups and has applications in group theory, such as characterizing $C^{*}$--exact groups. We characterized it in terms of Banach algebras. For a topological group $G$ , group amenability can be characterized through the amenability of the convolution Banach algebra $L^1(G)$. Here a Banach algebra $A$ is called amenable if all derivations from $A$ to any dual--type $A$--$A$--Banach bimodules are inner. We extended this result to discrete group actions on compact Hausdorff spaces, using a certain Banach algebra arising from the action and a weakened amenability condition of Banach algebras. We also proved a fixed point characterization of amenable actions, which improves the result by Dong and Wang (2015).

We show that a locally compact group has open unimodular part if and only if the Plancherel weight on its group von Neumann algebra is almost periodic. We call such groups almost unimodular. The almost periodicity of the Plancherel weight allows one to define a Murray-von Neumann dimension for certain Hilbert space modules over the group von Neumann algebra, and we show that for finite covolume subgroups this dimension scales according to the covolume. Using this we obtain a generalization of the Atiyah-Schmid formula in the setting of second countable almost unimodular groups with finite covolume subgroups. Additionally, for the class of almost unimodular groups we present many examples, establish a number of permanence properties, and show that the formal degrees of irreducible and factorial square integrable representations are well behaved.

We introduce generalized pinning fields in conformal field theory that model a large class of critical impurities at large distance, enriching the familiar universality classes. We provide a rigorous definition of such defects as certain unbounded operators on the Hilbert space and prove that when inserted on codimension-one surfaces they factorize the spacetime into two halves. The factorization channels are further constrained by symmetries in the bulk. As a corollary, we solve such critical impurities in the 2d minimal models and establish the factorization phenomena previously observed for localized mass deformations in the 3d ${\rm O}(N)$ model.