arXiv subjects

David Pask

Publications and source records attributed to David Pask.

At least 55 records · Page 3Linked to original sources

arXiv2008

Reducibility of Covers of AFT shifts

In this paper we show that the reducibility structure of several covers of sofic shifts is a flow invariant. In addition, we prove that for an irreducible subshift of almost finite type the left Krieger cover and the past set cover are reducible. We provide an example which shows that there are non almost finite type shifts which have reducible left Krieger covers. As an application we show that the Matsumoto algebra of an irreducible, strictly sofic shift of almost finite type is not simple.

math.DS↗

arXiv2008

C*-algebras associated to coverings of k-graphs

A covering of k-graphs (in the sense of Pask-Quigg-Raeburn) induces an embedding of universal C*-algebras. We show how to build a (k+1)-graph whose universal algebra encodes this embedding. More generally we show how to realise a direct limit of k-graph algebras under embeddings induced from coverings as the universal algebra of a (k+1)-graph. Our main focus is on computing the K-theory of the (k+1)-graph algebra from that of the component k-graph algebras. Examples of our construction include a realisation of the Kirchberg algebra \mathcal{P}_n whose K-theory is opposite to that of \mathcal{O}_n, and a class of AT-algebras that can naturally be regarded as higher-rank Bunce-Deddens algebras.

math.OA↗

arXiv2008

Coverings of k-graphs

k-graphs are higher-rank analogues of directed graphs which were first developed to provide combinatorial models for operator algebras of Cuntz-Krieger type. Here we develop the theory of covering spaces for k-graphs, obtaining a satisfactory version of the usual topological classification in terms of subgroups of a fundamental group. We then use this classification to describe the C*-algebras of covering k-graphs as crossed products by coactions of homogeneous spaces, generalizing recent results on the C*-algebras of graphs.

math.OA↗

arXiv2008

Coverings of skew-products and crossed products by coactions

Consider a projective limit G of finite groups G_n. Fix a compatible family δ^n of coactions of the G_n on a C*-algebra A. From this data we obtain a coaction δof G on A. We show that the coaction crossed product of A by δis isomorphic to a direct limit of the coaction crossed products of A by the δ^n. If A = C*(Λ) for some k-graph Λ, and if the coactions δ^n correspond to skew-products of Λ, then we can say more. We prove that the coaction crossed-product of C*(Λ) by δmay be realised as a full corner of the C*-algebra of a (k+1)-graph. We then explore connections with Yeend's topological higher-rank graphs and their C*-algebras.

math.OA↗

arXiv2008

A family of 2-graphs arising from two-dimensional subshifts

Higher-rank graphs (or $k$-graphs) were introduced by Kumjian and Pask to provide combinatorial models for the higher-rank Cuntz-Krieger $C^*$-algebras of Robertson and Steger. Here we consider a family of finite 2-graphs whose path spaces are dynamical systems of algebraic origin, as studied by Schmidt and others. We analyse the $C^*$-algebras of these 2-graphs, find criteria under which they are simple and purely infinite, and compute their $K$-theory. We find examples whose $C^*$-algebras satisfy the hypotheses of the classification theorem of Kirchberg and Phillips, but are not isomorphic to the $C^*$-algebras of ordinary directed graphs.

math.OA↗

arXiv2007

Crossed products of k-graph C*-algebras by Z^l

An action of Z^l by automorphisms of a k-graph induces an action of Z^l by automorphisms of the corresponding k-graph C*-algebra. We show how to construct a (k+l)-graph whose C*-algebra coincides with the crossed product of the original k-graph algebra by Z^l. We then investigate the structure of the crossed-product C*-algebra.

math.OA↗

arXiv2007

Strong Shift Equivalence of $C^*$-correspondences

We define a notion of strong shift equivalence for $C^*$-correspondences and show that strong shift equivalent $C^*$-correspondences have strongly Morita equivalent Cuntz-Pimsner algebras. Our analysis extends the fact that strong shift equivalent square matrices with non-negative integer entries give stably isomorphic Cuntz-Krieger algebras.

math.OA↗

arXiv2007

Noncommutative manifolds from graph and $k$-graph $C^*$-algebras

In previous papers, we constructed smooth (1,\infty)-summable semfinite spectral triples for graph algebras with a faithful trace, and (k,\infty)-summable semifinite spectral triples for k-graph algebras. In this paper we identify classes of graphs and k-graphs which satisfy a version of Connes' conditions for noncommutative manifolds.

math.OA↗

arXiv2007

The Noncommutative Geometry of k-graph C*-Algebras

This paper is comprised of two related parts. First we discuss which k-graph algebras have faithful gauge invariant traces, where the gauge action of $\T^k$ is the canonical one. We give a sufficient condition for the existence of such a trace, identify the C*-algebras of k-graphs satisfying this condition up to Morita equivalence, and compute their K-theory. For k-graphs with faithful gauge invariant trace, we construct a smooth $(k,\infty)$-summable semifinite spectral triple. We use the semifinite local index theorem to compute the pairing with K-theory. This numerical pairing can be obtained by applying the trace to a KK-pairing with values in the K-theory of the fixed point algebra of the $\T^k$ action. As with graph algebras, the index pairing is an invariant for a finer structure than the isomorphism class of the algebra.

math.OA↗

arXiv2005

Rank-two graphs whose C^*-algebras are direct limits of circle algebras

We describe a class of rank-2 graphs whose C^*-algebras are AT algebras. For a subclass which we call rank-2 Bratteli diagrams, we compute the K-theory of the C*-algebra. We identify rank-2 Bratteli diagrams whose C*-algebras are simple and have real-rank zero, and characterise the K-invariants achieved by such algebras. We give examples of rank-2 Bratteli diagrams whose C*-algebras contain as full corners the irrational rotation algebras and the Bunce-Deddens algebras.

math.OA↗

arXiv2005

The Noncommutative Geometry of Graph $C^*$-Algebras I: The Index Theorem

We investigate conditions on a graph $C^*$-algebra for the existence of a faithful semifinite trace. Using such a trace and the natural gauge action of the circle on the graph algebra, we construct a smooth $(1,\infty)$-summable semfinite spectral triple. The local index theorem allows us to compute the pairing with $K$-theory. This produces invariants in the $K$-theory of the fixed point algebra, and these are invariants for a finer structure than the isomorphism class of $C^*(E)$.

math.FA↗

arXiv2005

C*-algebras of labelled graphs

We describe a class of $C^*$-algebras which simultaneously generalise the ultragraph algebras of Tomforde and the shift space $C^*$-algebras of Matsumoto. In doing so we shed some new light on the different $C^*$-algebras that may be associated to a shift space. Finally, we show how to associate a simple $C^*$-algebra to an irreducible sofic shift.

math.OA↗

arXiv2004

A dual graph construction for higher-rank graphs, and $K$-theory for finite 2-graphs

Given a $k$-graph $Λ$ and an element $p$ of $\NN^k$, we define the dual $k$-graph, $pΛ$. We show that when $Λ$ is row-finite and has no sources, the $C^*$-algebras $C^*(Λ)$ and $C^*(pΛ)$ coincide. We use this isomorphism to apply Robertson and Steger's results to calculate the $K$-theory of $C^*(Λ)$ when $Λ$ is finite and strongly connected and satisfies the aperiodicity condition.

math.OA↗

arXiv2004

Fundamental groupoids of k-graphs

k-graphs are higher-rank analogues of directed graphs which were first developed to provide combinatorial models for operator algebras of Cuntz-Krieger type. Here we develop a theory of the fundamental groupoid of a k-graph, and relate it to the fundamental groupoid of an associated graph called the 1-skeleton. We also explore the failure, in general, of k-graphs to faithfully embed into their fundamental groupoids.

math.CO↗

arXiv2003

Flow equivalence of graph algebras

This paper explores the effect of various graphical constructions upon the associated graph $C^*$-algebras. The graphical constructions in question arise naturally in the study of flow equivalence for topological Markov chains. We prove that out-splittings give rise to isomorphic graph algebras, and in-splittings give rise to strongly Morita equivalent $C^*$-algebras. We generalise the notion of a delay as defined by Drinen to form in-delays and out-delays. We prove that these constructions give rise to Morita equivalent graph $C^*$-algebras. We provide examples which suggest that our results are the most general possible in the setting of the $C^*$-algebras of arbitrary directed graphs.

math.OA↗

arXiv2002

Actions of ${\mathbb Z}^k$ associated to higher rnak graphs

An action of ${\mathbb Z}^k$ is associated to a higher rank graph $Λ$ satisfying a mild assumption. This generalises the construction of a topological Markov shift arising from a nonnegative integer matrix. We show that the stable Ruelle algebra of $Λ$ is strongly Morita equivalent to $C^*(Λ)$. Hence, if $Λ$ satisfies the aperiodicity condition, the stable Ruelle algebra is simple, stable and purely infinite.

math.OA↗

arXiv2002

Some intrinsic properties of simple graph $C^*$-algebras

To a directed graph $E$ is associated a $C^*$-algebra $C^* (E)$ called a graph $C^*$-algebra. There is a canonical action $γ$ of ${\bf T}$ on $C^* (E)$, called the gauge action. In this paper we present necessary and sufficient conditions for the fixed point algebra $C^* (E)^γ$ to be simple. Our results also yield some structure theorems for simple graph algebras.

math.OA↗

arXiv2002

Coverings of Directed Graphs and Crossed Products of C*-algebras by Coactions of Homogeneous Spaces

The graph C*-algebra of a directed graph E is the universal C*-algebra generated by a family of partial isometries satisfying relations which reflect the path structure of E. In the first part of this paper we consider coverings of directed graphs: morphisms p:F->E which are local isomorphisms. We show that the graph algebra C*(F) can be recovered from C*(E) as a crossed product by a coaction of a homogeneous space associated to the fundamental group π_1 (E). These crossed products provide a good model for a general theory of crossed products by homogeneous spaces. The second part of the paper is devoted to building a framework for studying crossed products by homogeneous spaces using Rieffel's theory of proper actions.

math.OA↗