arXiv subjects

M. Anoussis

Publications and source records attributed to M. Anoussis.

15 recordsLinked to original sources

arXiv2023

Homomorphisms of $L^1$ algebras and Fourier algebras

We investigate conditions for the extendibility of continuous algebra homomorphisms $ϕ$ from the Fourier algebra $A(F)$ of a locally compact group $F$ to the Fourier-Stieltjes algebra $B(G)$ of a locally compact group $G$ to maps between the corresponding $L^\infty$ algebras which are weak* continuous. When $ϕ$ is completely bounded and $F$ is amenable, it is induced by a piecewise affine map $α: Y\to F$ where $Y\subseteq G$. We show that extendibility of $ϕ$ is equivalent to $α$ being an open map. We also study the dual problem for contractive homomorphisms $ϕ: L^1(F)\to M(G)$. We show that $ϕ$ induces a w* continuous homomorphism between the von Neumann algebras of the groups if and only if the naturally associated map $θ$ (Greenleaf [1965], Stokke [2011]) is a proper map.

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arXiv2022

Topological Radicals of Semicrossed Products

We characterize the hypocompact radical of a semicrossed product in terms of properties of the dynamical system. We show that an element A of a semicrossed product is in the hypocompact radical if and only if the Fourier coefficients of A vanish on the closure of the recurrent points and the 0-Fourier coefficient vanishes also on the largest perfect subset of X.

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arXiv2022

Idempotents of large norm and homomorphisms of Fourier algebras

We provide necessary and sufficient conditions for the existence of idempotents of arbitrarily large norms in the Fourier algebra A(G) and the Fourier-Stieltjes algebra B(G) of a locally compact group G. We prove that the existence of idempotents of arbitrarily large norm in B(G) implies the existence of homomorphisms of arbitrarily large norm from A(H) into B(G) for every locally compact group H. A partial converse is also obtained: the existence of homomorphisms of arbitrarily large norm from A(H) into B(G) for some amenable locally compact group H implies the existence of idempotents of arbitrarily large norm in B(G).

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arXiv2021

Synthetic properties of locally compact groups: preservation and transference

Using techniques from TRO equivalence of masa bimodules we prove various transference results: We show that when $α$ is a group homomorphism which pushes forward the Haar measure of $G$ to a measure absolutely continuous with respect to the Haar measure on $H$, then $(α\timesα)^{-1}$ preserves sets of compact operator synthesis, and conversely when $α$ is onto. We also prove similar preservation results for operator Ditkin sets and operator M-sets, obtaining preservation results for M-sets as corollaries. Some of these results extend or complement existing results of Ludwig, Shulman, Todorov and Turowska.

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arXiv2021

Homomorphisms of Fourier algebras and transference results

We prove that if $ρ: A(H) \to B(G)$ is a homomorphism between the Fourier algebra of a locally compact group $H$ and the Fourier-Stieltjes algebra of a locally compact group $G$ induced by a mixed piecewise affine map $α: G \to H$, then $ρ$ extends to a w*-w* continuous map between the corresponding $L^\infty$ algebras if and only if $α$ is an open map. Using techniques from TRO equivalence of masa bimodules we prove various transference results: We show that when $α$ is a group homomorphism which pushes forward the Haar measure of $G$ to a measure absolutely continuous with respect to the Haar measure of $H$, then $(α\timesα)^{-1}$ preserves sets of compact operator synthesis, and conversely when $α$ is onto. We also prove similar preservation properties for operator Ditkin sets and operator M-sets, obtaining preservation properties for M-sets as corollaries. Some of these results extend or complement existing results of Ludwig, Shulman, Todorov and Turowska.

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arXiv2021

Compact multiplication operators on semicrossed products

We characterize the compact multiplication operators on a semi-crossed product in terms of the corresponding dynamical system. We also characterize the compact elements of this algebra and determine the ideal they generate.

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arXiv2019

Norms of vector functionals

We examine the question of when, and how, the norm of a vector functional on an operator algebra can be controlled by the invariant subspace lattice of the algebra. We introduce a related operator algebraic property, and show that it is satisfied by all von Neumann algebras and by all CSL algebras. We exhibit examples of operator algebras that do not satisfy the property or any scaled version of it.

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arXiv2016

Topological Radicals of Nest Algebras

Let N be a nest on a Hilbert space H and AlgN the corresponding nest algebra. We determine the hypocompact radical of AlgN . Other topological radicals are also characterized.

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arXiv2016

Compact multiplication operators on nest algebras

Let N be a nest on a Hilbert space H and AlgN the corresponding nest algebra. We obtain a characterization of the compact and weakly compact multiplication operators defined on nest algebras. This characterization leads to a description of the closed ideal generated by the compact elements of AlgN . We also show that there is no non-zero weakly compact multiplication operator on AlgN / (AlgN $\cap$ K(H)).

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arXiv2014

Ideals of $A(G)$ and bimodules over maximal abelian selfadjoint algebras

This paper is concerned with weak* closed masa-bimodules generated by A(G)-invariant subspaces of VN(G). An annihilator formula is established, which is used to characterise the weak* closed subspaces of B(L^2(G)) which are invariant under both Schur multipliers and a canonical action of M(G) on B(L^2(G)) via completely bounded maps. We study the special cases of extremal ideals with a given null set and, for a large class of groups, we establish a link between relative spectral synthesis and relative operator synthesis.

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arXiv2014

Ideals of the Fourier algebra, supports and harmonic operators

We examine the common null spaces of families of Herz-Schur multipliers and apply our results to study jointly harmonic operators and their relation with jointly harmonic functionals. We show how an annihilation formula obtained in J. Funct. Anal. 266 (2014), 6473-6500 can be used to give a short proof as well as a generalisation of a result of Neufang and Runde concerning harmonic operators with respect to a normalised positive definite function. We compare the two notions of support of an operator that have been studied in the literature and show how one can be expressed in terms of the other.

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arXiv2010

Operator algebras from the discrete Heisenberg semigroup

We study reflexivity and structure properties of operator algebras generated by representations of the discrete Heisenberg semi-group. We show that the left regular representation of this semi-group gives rise to a semi-simple reflexive algebra. We exhibit an example of a representation which gives rise to a non-reflexive algebra. En route, we establish reflexivity results for subspaces of $H^{\infty}(\bb{T})\otimes\cl B(\cl H)$.

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arXiv2008

S-numbers of elementary operators on C*-algebras

We study the s-numbers of elementary operators acting on C*-algebras. The main results are the following: If $τ$ is any tensor norm and $a,b\in B(H)$ are such that the sequences $s(a),s(b)$ of their singular numbers belong to a stable Calkin space $J$ then the sequence of approximation numbers of $a\otimes_τ b$ belongs to $J$. If $A$ is a C*-algebra, $J$ is a stable Calkin space, $s$ is an s-number function, and $a_i, b_i \in A,$ $i=1,...,m$ are such that $s(π(a_i)), s(π(b_i)) \in J$, $i=1,...,m$ for some faithful representation $π$ of $A$ then $s(\sum_{i=1}^{m} M_{a_i,b_i})\in J$. The converse implication holds if and only if the ideal of compact elements of $A$ has finite spectrum. We also prove a quantitative version of a result of Ylinen.

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arXiv2007

Angles in C*-algebras

In this work we characterise the C*-algebras A generated by projections with the property that every pair of projections in A has positive angle, as certain extensions of abelian algebras by algebras of compact operators. We show that this property is equivalent to a lattice theoretic property of projections and also to the property that the set of finite-dimensional *-subalgebras of A is directed.

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arXiv2006

On Mixing and Ergodicity in Locally Compact Motion Groups

Let $G$ be a semi-direct product $G=A\times_ϕK$ with $A$ Abelian and $K$ compact. We characterize spread-out probability measures on $G$ that are mixing by convolutions by means of their Fourier transforms. A key tool is a spectral radius formula for the Fourier transform of a regular Borel measure on $G$ that we develop, and which is analogous to the well-known Beurling--Gelfand spectral radius formula. For spread-out probability measures on $G$, we also characterize ergodicity by means of the Fourier transform of the measure. Finally, we show that spread-out probability measures on such groups are mixing if and only if they are weakly mixing.

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