Searcharxiv⌕ Search

arXiv subjects

Takeshi Katsura

Publications and source records attributed to Takeshi Katsura.

At least 19 recordsLinked to original sources
arXiv2024

A categorical approach to inverse semigroups, étale groupoids, and their C*-algebras

We introduce a category of inverse semigroup actions and a category of étale groupoids. We show that there are three functors which send inverse semigroups to their spectral actions, inverse semigroup actions to their transformation groupoids, and étale groupoids to their groupoid C*-algebras, respectively. The composition of these functors is naturally equivalent to the already known functor which sends inverse semigroups to their C*-algebras. This result is a categorical version of the theorem by Paterson which states that the inverse semigroup C*-algebras coincide the groupoid C*-algebras of the universal groupoids. We also construct a functor from the category of étale groupoids to the category of inverse semigroup actions sending étale groupoids to their slice actions, and show that this functor is right adjoint to the functor sending inverse semigroup actions to their transformation groupoids.

math.OA↗
arXiv2021

Reproducing kernel Hilbert C*-module and kernel mean embeddings

Kernel methods have been among the most popular techniques in machine learning, where learning tasks are solved using the property of reproducing kernel Hilbert space (RKHS). In this paper, we propose a novel data analysis framework with reproducing kernel Hilbert $C^*$-module (RKHM) and kernel mean embedding (KME) in RKHM. Since RKHM contains richer information than RKHS or vector-valued RKHS (vvRKHS), analysis with RKHM enables us to capture and extract structural properties in such as functional data. We show a branch of theories for RKHM to apply to data analysis, including the representer theorem, and the injectivity and universality of the proposed KME. We also show RKHM generalizes RKHS and vvRKHS. Then, we provide concrete procedures for employing RKHM and the proposed KME to data analysis.

stat.ML↗
arXiv2021

On the magic square C*-algebra of size 4

In this paper, we investigate the structure of the magic square C*-algebra $A(4)$ of size 4. We show that a certain twisted crossed product of $A(4)$ is isomorphic to the homogeneous C*-algebra $M_4(C(\mathbb{R} P^3))$. Using this result, we show that $A(4)$ is isomorphic to the fixed point algebra of $M_4(C(\mathbb{R} P^3))$ by a certain action. From this concrete realization of $A(4)$, we compute the K-groups of $A(4)$ and their generators.

math.OA↗
arXiv2021

Topological graphs and singly generated dynamical systems

In this paper, we introduce the notion of a dual topological graph of a given topological graph, and show that it defines a C*-algebra isomorphic to the C*-algebra of the given one. Repeating to take a dual, and taking a projective limit, we get a singly generated dynamical system with which the associate C*-algebra is isomorphic to the C*-algebra of the given topological graph. This shows that a C*-algebra of an arbitrary topoloical graph has a groupoid model. Similar investigation are done for relative topoloical graphs and partially defined topoloical graphs which are introduced in this paper.

math.OA↗
arXiv2020

Analysis via Orthonormal Systems in Reproducing Kernel Hilbert $C^*$-Modules and Applications

Kernel methods have been among the most popular techniques in machine learning, where learning tasks are solved using the property of reproducing kernel Hilbert space (RKHS). In this paper, we propose a novel data analysis framework with reproducing kernel Hilbert $C^*$-module (RKHM), which is another generalization of RKHS than vector-valued RKHS (vv-RKHS). Analysis with RKHMs enables us to deal with structures among variables more explicitly than vv-RKHS. We show the theoretical validity for the construction of orthonormal systems in Hilbert $C^*$-modules, and derive concrete procedures for orthonormalization in RKHMs with those theoretical properties in numerical computations. Moreover, we apply those to generalize with RKHM kernel principal component analysis and the analysis of dynamical systems with Perron-Frobenius operators. The empirical performance of our methods is also investigated by using synthetic and real-world data.

stat.ML↗
arXiv2014

The K-theoretical range of Cuntz-Krieger algebras

We augment Restorff's classification of purely infinite Cuntz-Krieger algebras by describing the range of his invariant on purely infinite Cuntz-Krieger algebras. We also describe its range on purely infinite graph C*-algebras with finitely many ideals, and provide 'unital' range results for purely infinite Cuntz-Krieger algebras and unital purely infinite graph C*-algebras.

math.OA↗
arXiv2013

Reduction of filtered K-theory and a characterization of Cuntz-Krieger algebras

We show that filtered K-theory is equivalent to a substantially smaller invariant for all real-rank-zero C*-algebras with certain primitive ideal spaces -- including the infinitely many so-called accordion spaces for which filtered K-theory is known to be a complete invariant. As a consequence, we give a characterization of purely infinite Cuntz-Krieger algebras whose primitive ideal space is an accordion space.

math.OA↗
arXiv2013

Identifying AF-algebras that are graph C*-algebras

We consider the problem of identifying exactly which AF-algebras are isomorphic to a graph C*-algebra. We prove that any separable, unital, Type I C*-algebra with finitely many ideals is isomorphic to a graph C*-algebra. This result allows us to prove that a unital AF-algebra is isomorphic to a graph C*-algebra if and only if it is a Type I C*-algebra with finitely many ideals. We also consider nonunital AF-algebras that have a largest ideal with the property that the quotient by this ideal is the only unital quotient of the AF-algebra. We show that such an AF-algebra is isomorphic to a graph C*-algebra if and only if its unital quotient is Type I, which occurs if and only if its unital quotient is isomorphic to M_k for some natural number k. All of these results provide vast supporting evidence for the conjecture that an AF-algebra is isomorphic to a graph C*-algebra if and only if each unital quotient of the AF-algebra is Type I with finitely many ideals, and bear relevance for the intrigiung question of finding K-theoretical criteria for when an extension of two graph C*-algebras is again a graph C*-algebra.

math.OA↗
arXiv2013

Nonseparable UHF algebras II: Classification

For every uncountable cardinal $κ$ there are $2^κ$ nonisomorphic simple AF algebras of density character $κ$ and $2^κ$ nonisomorphic hyperfinite II$_1$ factors of density character $κ$. These estimates are maximal possible. All C*-algebras that we construct have the same Elliott invariant and Cuntz semigroup as the CAR algebra.

math.OA↗
arXiv2013

A simple C*-algebra with finite nuclear dimension which is not Z-stable

We construct a simple C*-algebra with nuclear dimension zero that is not isomorphic to its tensor product with the Jiang-Su algebra Z, and a hyperfinite II_1 factor not isomorphic to its tensor product with the separable hyperfinite II_1 factor R. The proofs use a weakening of the Continuum Hypothesis.

math.OA↗
arXiv2012

On the invariant uniform Roe algebra

Let $Γ$ be a countable discrete group. We show that $Γ$ has the approximation property if and only if $Γ$ is exact and for any operator space $S \subseteq \K(H)$ we have $\Cu(Γ)^Γ \otimes S = (\Cu(Γ) \otimes S)^Γ$, where $\Cu(Γ)$ is the uniform Roe algebra with the right adjoint $Γ$-action. This answers a question of J. Zacharias. We also show that characterisations of several properties of $Γ$ in terms of the reduced group \cast-algebra apply to the invariant uniform Roe algebra $\Cu(Γ)^Γ$.

math.OA↗
arXiv2012

The Ranges of K-theoretic Invariants for Nonsimple Graph Algebras

There are many classes of nonsimple graph C*-algebras that are classified by the six-term exact sequence in K-theory. In this paper we consider the range of this invariant and determine which cyclic six-term exact sequences can be obtained by various classes of graph C*-algebras. To accomplish this, we establish a general method that allows us to form a graph with a given six-term exact sequence of K-groups by splicing together smaller graphs whose C*-algebras realize portions of the six-term exact sequence. As rather immediate consequences, we obtain the first permanence results for extensions of graph C*-algebras. We are hopeful that the results and methods presented here will also prove useful in more general cases, such as situations where the C*-algebras under investigations have more than one ideal and where there are currently no relevant classification theories available.

math.OA↗
arXiv2011

Dynamical systems of type (m,n) and their C*-algebras

Given positive integers n and m, we consider dynamical systems in which n copies of a topological space is homeomorphic to m copies of that same space. The universal such system is shown to arise naturally from the study of a C*-algebra we denote by O_{mn}, which in turn is obtained as a quotient of the well known Leavitt C*-algebra L_{mn}, a process meant to transform the generating set of partial isometries of L{mn} into a tame set. Describing O_{mn} as the crossed-product of the universal (m,n)-dynamical system by a partial action of the free group F_{m+n}, we show that O_{mn} is not exact when n and m are both greater than or equal to 2, but the corresponding reduced crossed-product, denoted O_{mn}^r, is shown to be exact and non-nuclear. Still under the assumption that m,n>=2, we prove that the partial action of F_{m+n} is topologically free and that O_{mn}^r satisfies property (SP) (small projections). We also show that O_{mn}^r admits no finite dimensional representations. The techniques developed to treat this system include several new results pertaining to the theory of Fell bundles over discrete groups.

math.OA↗
arXiv2010

Nonseparable UHF algebras I: Dixmier's problem

There are three natural ways to define UHF (uniformly hyperfinite) C*-algebras, and all three definitions are equivalent for separable algebras. In 1967 Dixmier asked whether the three definitions remain equivalent for not necessarily separable algebras. We give a complete answer to this question. More precisely, we show that in small cardinality two definitions remain equivalent, and give counterexamples in other cases. Our results do not use any additional set-theoretic axioms beyond the usual axioms, namely ZFC.

math.OA↗
arXiv2009

Realizations of AF-algebras as graph algebras, Exel-Laca algebras, and ultragraph algebras

We give various necessary and sufficient conditions for an AF-algebra to be isomorphic to a graph C*-algebra, an Exel-Laca algebra, and an ultragraph C*-algebra. We also explore consequences of these results. In particular, we show that all stable AF-algebras are both graph C*-algebras and Exel-Laca algebras, and that all simple AF-algebras are either graph C*-algebras or Exel-Laca algebras. In addition, we obtain a characterization of AF-algebras that are isomorphic to the C*-algebra of a row-finite graph with no sinks.

math.OA↗