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Ilan Hirshberg

Publications and source records attributed to Ilan Hirshberg.

At least 19 recordsLinked to original sources
arXiv2026

Obstructions to homomorphisms between homogeneous C*-algebras

We develop a method to find new obstructions to the existence of homomorphisms between homogeneous C*-algebras with prescribed behavior on K-theory. As an application, we give a complete answer to a problem posed by Blackadar in 1993 concerning the existence of unital homomorphisms between algebras of matrix-valued functions on even spheres which are injective on K_0: we show that if n, d, k, r are natural numbers and k is not in the range n, n+1, ..., n+d-1, then there is no unital homomorphism from C (S^{2n}, M_r) to C (S^{2k}, M_{dr}) which is injective on K_0.

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arXiv2026

An isomorphism theorem for infinite reduced free products

Let C_1, C_2, ... be a sequence of separable unital C*-algebras, equipped with faithful tracial states and satisfying a mild condition. Let A be a unital direct limit of one dimensional NCCW complexes, also equipped with a faithful tracial state. Suppose there is a unital trace preserving embedding of A in the Jiang-Su algebra which is an isomorphism on K-theory. (For example, A could be C([0,1]) with Lebesgue measure, or the Jiang-Su algebra itself.) Let D be the infinite reduced free product of the algebras C_n. Then the reduced free product A*D is isomorphic to D. If D is exact and the factors satisfy a blockwise real rank zero condition, then in place of A we can use C(X) for any contractible compact metric space X and any faithful tracial state on C(X). An example consequence is that the reduced free product of infinitely many copies of C([0,1]), with Lebesgue measure, is isomorphic to the reduced free product of infinitely many copies of the Jiang-Su algebra.

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arXiv2024

Powers averaging for actions on $C(X)$-algebras

Given a unital $C(X)$-algebra $A$ discrete group $Γ$ and an action $α: Γ\to \text{aut}(A)$ which leaves $C(X)$ invariant and such that $C(X)\rtimes_{α,r} Γ$ is simple, and a $2$-cocycle $ω$, we obtain a bijective correspondence between maximal $Γ$-invariant ideals of $A$ and maximal ideals in $A\rtimes_{α,ω,r} Γ$. In particular, $A\rtimes_{α,ω,r} Γ$ is simple if and only if $A$ has no $Γ$-invariant ideals.

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arXiv2024

Long thin covers and nuclear dimension

We establish finite nuclear dimension for crossed product C*-algebras arising from various classes of possibly non-free topological actions, including arbitrary actions of finitely generated virtually nilpotent groups on finite dimensional spaces, certain amenable actions of hyperbolic groups, and certain allosteric actions of wreath products. We obtain these results by introducing a new notion of dimension for topological dynamical systems, called the long thin covering dimension, which involves a suitable version of Rokhlin-type towers with controlled overlaps for possibly non-free actions.

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arXiv2023

A dichotomy for central sequence algebras

We prove that the central sequence algebra of a separable C*-algebra is either subhomogeneous or non-exact, confirming a conjecture of Enders and Shulman. We also prove analogous dichotomy for other massive C*-algebras.

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arXiv2023

Intermediate crossed product $C^*$-algebras

Let $B$ be a separable $C^*$-algebra, let $Γ$ be a discrete countable group, let $α: Γ\to \text{Aut}(B)$ be an action, and let $A$ be an invariant subalgebra. We find certain freeness conditions which guarantee that any intermediate $C^*$-algebra $A \rtimes_{α,r} Γ\subseteq C \subseteq B \rtimes_{α,r} Γ$ is a crossed product of an intermediate invariant subalgebra $A \subseteq C_0 \subseteq B$ by $Γ$. Those are used to generalize related results by Suzuki.

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arXiv2023

Values of Rokhlin dimension for actions of compact groups

We show that any finite group admits actions on simple AF algebras with unique trace which have arbitrarily large finite values of Rokhlin dimension with commuting towers. We show similar results for actions of compact Lie groups, with AH algebras with no dimension growth in place of AF algebras. We also relate Rokhlin dimension to the G-index for actions of compact Lie groups on commutative C*-algebras.

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arXiv2021

Strongly outer actions of amenable groups on $\mathcal{Z}$-stable nuclear $C^*$-algebras

Let $A$ be a separable, unital, simple, $\mathcal{Z}$-stable, nuclear $C^*$-algebra, and let $α\colon G\to \mathrm{Aut}(A)$ be an action of a discrete, countable, amenable group. Suppose that the orbits of the action of $G$ on $T(A)$ are finite and that their cardinality is bounded. We show that $α$ is strongly outer if and only if $α\otimes\mathrm{id}_{\mathcal{Z}}$ has the weak tracial Rokhlin property. If $G$ is moreover residually finite, these conditions are also equivalent to $α\otimes\mathrm{id}_{\mathcal{Z}}$ having finite Rokhlin dimension (in fact, at most 2). If $\partial_eT(A)$ is furthermore compact, has finite covering dimension, and the orbit space $\partial_eT(A)/G$ is Hausdorff, we generalize results by Matui and Sato to show that $α$ is cocycle conjugate to $α\otimes\mathrm{id}_{\mathcal{Z}}$, even if $α$ is not strongly outer. In particular, in this case the equivalences above hold for $α$ in place of $α\otimes\mathrm{id}_{\mathcal{Z}}$. In the course of the proof, we develop equivariant versions of complemented partitions of unity and uniform property $Γ$ as technical tools of independent interest.

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arXiv2021

The nuclear dimension of $C^*$-algebras associated to topological flows and orientable line foliations

We show that for any locally compact Hausdorff space $Y$ with finite covering dimension and for any continuous flow $\mathbb{R} \curvearrowright Y$, the resulting crossed product $C^*$-algebra $C_0(Y) \rtimes \mathbb{R}$ has finite nuclear dimension. This generalizes previous results for free flows, where this was proved using Rokhlin dimension techniques. As an application, we obtain bounds for the nuclear dimension of $C^*$-algebras associated to one-dimensional orientable foliations. This result is analogous to the one we obtained earlier for non-free actions of $\mathbb{Z}$. Some novel techniques in our proof include the use of a conditional expectation constructed from the inclusion of a clopen subgroupoid, as well as the introduction of what we call fiberwise groupoid coverings that help us build a link between foliation $C^*$-algebras and crossed products.

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arXiv2020

Rokhlin dimension: duality, tracial properties, and crossed products

We study compact group actions with finite Rokhlin dimension, particularly in relation to crossed products. For example, we characterize the duals of such actions, generalizing previous partial results for the Rokhlin property. As an application, we determine the ideal structure of their crossed products. Under the assumption of so-called commuting towers, we show that taking crossed products by such actions preserves a number of relevant classes of C*-algebras, including: $D$-absorbing C*-algebras, where $D$ is a strongly self-absorbing C*-algebra, stable C*-algebras, C*-algebras with finite nuclear dimension (or decomposition rank), C*-algebras with finite stable rank (or real rank), and C*-algebras whose K-theory is either trivial, rational, or $n$-divisible for $n\in\mathbb{N}$. The combination of nuclearity and the UCT is also shown to be preserved by these actions. Some of these results are new even in the well-studied case of the Rokhlin property. Additionally, and under some technical assumptions, we show that finite Rokhlin dimension with commuting towers implies the (weak) tracial Rokhlin property. At the core of our arguments is a certain local approximation of the crossed product by a continuous $C(X)$-algebra with fibers that are stably isomorphic to the underlying algebra. The space $X$ is computed in some cases of interest, and we use its description to construct a $\mathbb{Z}_2$-action on a unital AF-algebra and on a unital Kirchberg algebra satisfying the UCT, whose Rokhlin dimensions with and without commuting towers are finite but do not agree.

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arXiv2020

Radius of comparison and mean cohomological independence dimension

We introduce a notion of mean cohomological independence dimension for actions of discrete amenable groups on compact metrizable spaces, as a variant of mean dimension, and use it to obtain lower bounds for the radius of comparison of the associated crossed product C*-algebras. Our general theory gives the following for the minimal subshifts constructed by Dou in 2017. Let G be a countable amenable group, let Z be a polyhedron, and let T be Dou's subshift of Z^G (which also depends on a density parameter). Then the radius of comparison of the crossed product is greater than r (1/2) mdim (T) - 2, in which r depends on the density parameter and is close to 1 when the density parameter is close to 1. If Z is even dimensional and has nonvanishing rational cohomology in degree dim (Z), then the radius of comparison of the crossed product is greater than (1/2) mdim (T) - 1, regardless of what the density parameter is.

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arXiv2020

Rokhlin-type properties, approximate innerness and Z-stability

We establish four results concerning connections between actions on separable C*-algebras with Rokhlin-type properties and absorption of the Jiang-Su algebra Z. For actions of residually finite groups or of the reals which have finite Rokhlin dimension with commuting towers, we show that if the action of any nontrivial group element is approximately inner then the C*-algebra acted upon is Z-stable. Without the assumption on approximate innerness, we show that the crossed product has good divisibility properties under mild assumptions. We also establish an analogous result for the generalized tracial Rokhlin property and tracial versions of approximate innerness and Z-absorption for actions of finite groups and of the integers. For actions of a single automorphism which have the Rokhlin property, we show that a condition which is strictly weaker than requiring that some power of the automorphism is approximately inner is sufficient to obtain that the crossed product absorbs Z even when the original algebra is not.

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arXiv2020

Strongly outer actions of amenable groups on $\mathcal{Z}$-stable $C^*$-algebras

Let $A$ be a separable, unital, simple, $\mathcal{Z}$-stable, nuclear $C^*$-algebra, and let $α\colon G\to \mathrm{Aut}(A)$ be an action of a countable amenable group $G$. If the trace space $T(A)$ is a Bauer simplex and the action of $G$ on $\partial_eT(A)$ has finite orbits and Hausdorff orbit space, we show that $α$ is strongly outer if and only if $α\otimes\mathrm{id}_{\mathcal{Z}}$ has the weak tracial Rokhlin property. If $G$ is moreover residually finite, then these conditions are also equivalent to $α\otimes\mathrm{id}_{\mathcal{Z}}$ having finite Rokhlin dimension (in fact, at most 2). When the covering dimension of $\partial_eT(A)$ is finite, we prove that $α$ is cocycle conjugate to $α\otimes\mathrm{id}_{\mathcal{Z}}$. In particular, the equivalences above hold for $α$ in place of $α\otimes\mathrm{id}_{\mathcal{Z}}$.

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arXiv2020

A rigid hyperfinite type $\mathrm{II}_1$ factor

We show that it is relatively consistent with ZFC that there exists a hyperfinite type $\mathrm{II}_1$-factor of density character $\aleph_1$ which is not isomorphic to its opposite, does not have any outer automorphisms, and has trivial fundamental group.

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