Countable Product Research Articles

Quasi-invariant states

In this paper, we develop the theory of quasi-invariant (respectively, strongly quasi-invariant) states under the action of a group [Formula: see text] of normal ∗-automorphisms of a ∗-algebra (or von Neumann algebra) [Formula: see text]. We prove that these states are naturally associated to left-[Formula: see text]-[Formula: see text]-cocycles. If [Formula: see text] is compact, the structure of strongly [Formula: see text]-quasi-invariant states is determined. For any [Formula: see text]-strongly quasi-invariant state [Formula: see text], we construct a unitary representation associated to the triple [Formula: see text]. We prove, under some conditions, that any quantum Markov chain with commuting, invertible and Hermitian conditional density amplitudes on a countable tensor product of type I factors is strongly quasi-invariant with respect to the natural action of the group [Formula: see text] of local permutations and we give the explicit form of the associated cocycle. This provides a family of nontrivial examples of strongly quasi-invariant states for locally compact groups obtained as inductive limit of an increasing sequence of compact groups.

Strong ergodicity around countable products of countable equivalence relations

Strong ergodicity around countable products of countable equivalence relations

Persistence of spectral projections for stochastic operators on large tensor products

Abstract It is proved that for families of stochastic operators on a countable tensor product, depending smoothly on parameters, any spectral projection persists smoothly, where smoothness is defined using norms based on ideas of Dobrushin. A rigorous perturbation theory for families of stochastic operators with spectral gap is thereby created. It is illustrated by deriving an effective slow two-state dynamics for a three-state probabilistic cellular automaton.

On the product of almost discrete Grothendieck spaces

A topological space X is called almost discrete, if it has precisely one nonisolated point. In this paper, we get that for a countable product X=∏Xi of almost discrete spaces Xi the space Cp(X) of all continuous real-valued functions with the topology of pointwise convergence is a μ-space if, and only if, X is a weak q-space if, and only if, t(X)=ω if, and only if, X is functionally generated by the family of all its countable subspaces.This result makes it possible to solve Archangel'skii's problem on the product of Grothendieck spaces. It is proved that in the model of ZFC, obtained by adding one Cohen real, there are Grothendieck spaces X and Y such that X×Y is not weakly Grothendieck space. In (PFA): the product of any countable family almost discrete Grothendieck spaces is a Grothendieck space.

Geometric Condition For Dependent Choice

We provide a geometric condition which characterises when thePrinciple of Dependent Choice holds in a Fraenkel-Mostowski-Specker permutationmodel. This condition is a slight weakening of requiring the filter of groups tobe closed under countable intersections. We show that this condition holds nontriviallyin a new permutation model we call "the nowhere dense model" and westudy its extensions to uncountable cardinals as well.

A Countable Intersection Like Characterization of Star-Lindelöf Spaces

There have been various studies on star-Lindelöfness but they always explain it in terms of open coverings. So, we have demonstrated in this study a connection between star-Lindelöfness and the family of closed sets that resembles countable intersection property of Lindelöf space. We show that a topological space $X$ is star-Lindelöf if and only if every closed subset's family of $X$ not having the modified non-countable intersection property have non-empty intersection.

An undecidable extension of Morley's theorem on the number of countable models

We show that Morley's theorem on the number of countable models of a countable first-order theory becomes an undecidable statement when extended to second-order logic. More generally, we calculate the number of equivalence classes of equivalence relations obtained by countable intersections of projective sets in several models of set theory. Our methods include random and Cohen forcing, Woodin cardinals and Inner Model Theory.

Countable products and countable direct sums of compact metrizable spaces in the absence of the Axiom of Choice

The main aim of the article is to show, in the absence of the Axiom of Choice, relationships between the following, independent of ZF, statements: “Every countable product of compact metrizable spaces is separable (respectively, compact)” and “Every countable product of compact metrizable spaces is metrizable”. Statements related to the above-mentioned ones are also studied. Permutation models (among them two new ones) are shown in which a countable direct sum (also a countable product) of metrizable spaces need not be metrizable, countable unions of countable sets are countable and there is a countable family of non-empty sets of size at most 2ℵ0 which does not have a choice function. A new permutation model is constructed in which every uncountable compact metrizable space is of size at least 2ℵ0 but a denumerable family of denumerable sets need not have a multiple choice function.

End spaces and tree-decompositions

We present a systematic investigation into how tree-decompositions of finite adhesion capture topological properties of the space formed by a graph together with its ends. As main results, we characterise when the ends of a graph can be distinguished, and characterise which subsets of ends can be displayed by a tree-decomposition of finite adhesion.In particular, we show that a subset Ψ of the ends of a graph G can be displayed by a tree-decomposition of finite adhesion if and only if Ψ is Gδ (a countable intersection of open sets) in |G|, the topological space formed by a graph together with its ends. Since the undominated ends of a graph are easily seen to be Gδ, this provides a structural explanation for Carmesin's result that the set of undominated ends can always be displayed.

An explicit self-dual construction of complete cotorsion pairs in the relative context

Let $R\to A$ be a homomorphism of associative rings, and let $(\mathcal F,\mathcal C)$ be a hereditary complete cotorsion pair in $R\mathsf{-Mod}$. Let $(\mathcal F_A,\mathcal C_A)$ be the cotorsion pair in $A\mathsf{-Mod}$ in which $\mathcal F_A$ is the class of all left $A$-modules whose underlying $R$-modules belong to $\mathcal F$. Assuming that the $\mathcal F$-resolution dimension of every left $R$-module is finite and the class $\mathcal F$ is preserved by the coinduction functor $\operatorname{Hom}_R(A,-)$, we show that $\mathcal C_A$ is the class of all direct summands of left $A$-modules finitely (co)filtered by $A$-modules coinduced from $R$-modules from $\mathcal C$. Assuming that the class $\mathcal F$ is closed under countable products and preserved by the functor $\operatorname{Hom}_R(A,-)$, we prove that $\mathcal C_A$ is the class of all direct summands of left $A$-modules cofiltered by $A$-modules coinduced from $R$-modules from $\mathcal C$, with the decreasing filtration indexed by the natural numbers. A combined result, based on the assumption that countable products of modules from $\mathcal F$ have finite $\mathcal F$-resolution dimension bounded by $k$, involves cofiltrations indexed by the ordinal $\omega+k$. The dual results also hold, provable by the same technique going back to the author's monograph on semi-infinite homological algebra arXiv:0708.3398. In addition, we discuss the $n$-cotilting and $n$-tilting cotorsion pairs, for which we obtain better results using a suitable version of a classical Bongartz-Ringel lemma. As an illustration of the main results of the paper, we consider certain cotorsion pairs related to the contraderived and coderived categories of curved DG-modules.

Inverse limit slender groups

Classically, an abelian group $G$ is said to be <em>slender</em> if every homomorphism from the countable product $\mathbb Z^{\mathbb N}\mathbb N$ to $G$ factors through the projection to some finite product $\mathbb Z^n$. Various authors have proposed ge

A UNIVERSAL CHARACTERIZATION OF STANDARD BOREL SPACES

AbstractWe prove that the category $\mathsf {SBor}$ of standard Borel spaces is the (bi-)initial object in the 2-category of countably complete Boolean (countably) extensive categories. This means that $\mathsf {SBor}$ is the universal category admitting some familiar algebraic operations of countable arity (e.g., countable products and unions) obeying some simple compatibility conditions (e.g., products distribute over disjoint unions). More generally, for any infinite regular cardinal $\kappa $ , the dual of the category $\kappa \mathsf {Bool}_{\kappa }$ of $\kappa $ -presented $\kappa $ -complete Boolean algebras is (bi-)initial in the 2-category of $\kappa $ -complete Boolean ( $\kappa $ -)extensive categories.

Chaotic behaviour of countable products of homeomorphism groups

ABSTRACT Relationships between a chaotic behaviour and closely related properties of topological transitivity, sensitivity to initial conditions, density of closed orbits of homeomorphism groups and their countable products are investigated. We provide a large number of new examples of chaotic groups of homeomorphisms of countable products of various metrizable topological spaces, including infinite-dimensional topological manifolds, whose factors can be as noncompact surfaces, so triangulable closed manifolds of an arbitrary dimension.

Topologically mixing extensions of endomorphisms on Polish groups

In this paper we study the dynamics of continuous endomorphisms on Polish groups. We offer necessary and sufficient conditions for a continuous endomorphism on a Polish group to be weakly mixing. We prove that any continuous endomorphism of an abelian Polish group can be extended in a natural way to a topologically mixing endomorphism on the countable infinite product of said group.

A Model in Which the Separation Principle Holds for a Given Effective Projective Sigma-Class

In this paper, we prove the following: If n≥3, there is a generic extension of L—the constructible universe—in which it is true that the Separation principle holds for both effective (lightface) classes Σn1 and Πn1 of sets of integers. The result was announced long ago by Leo Harrington with a sketch of the proof for n=3; its full proof has never been presented. Our methods are based on a countable product of almost-disjoint forcing notions independent in the sense of Jensen–Solovay.

Countable tensor products of Hermite spaces and spaces of Gaussian kernels

In recent years finite tensor products of reproducing kernel Hilbert spaces (RKHSs) of Gaussian kernels on the one hand and of Hermite spaces on the other hand have been considered in tractability analysis of multivariate problems. In the present paper we study countably infinite tensor products for both types of spaces. We show that the incomplete tensor product in the sense of von Neumann may be identified with an RKHS whose domain is a proper subset of the sequence space RN. Moreover, we show that each tensor product of spaces of Gaussian kernels having square-summable shape parameters is isometrically isomorphic to a tensor product of Hermite spaces; the corresponding isomorphism is given explicitly, respects point evaluations, and is also an L2-isometry. This result directly transfers to the case of finite tensor products. Furthermore, we provide regularity results for Hermite spaces of functions of a single variable.

Compactivorous sets in Banach spaces

A set E E in a Banach space X X is compactivorous if for every compact set K K in X X there is a nonempty, (relatively) open subset of K K which can be translated into E E . In a separable Banach space, this is a sufficient condition which guarantees the Haar nonnegligibility of Borel subsets. We give some characterisations of this property in both separable and nonseparable Banach spaces and prove an extension of the main theorem to countable products of locally compact Polish groups.

Sets with large intersection properties in metric spaces

In this work we reproduce the characterization of Gs-sets from the euclidean setting [11] to more general metric spaces. These sets have Hausdorff dimension at least s and are closed by countable intersections, which is particularly useful to estimate the dimension of the so called sets of α-approximable points.

Multiset Modules

Multiset modules and their properties are introduced in this paper. Some interesting properties are obtained, such as the countable intersection of multiset modules is multiset module, but the union need not be. Also, the sub-multiset module is defined and illustrated with suitable examples. Homomorphism and isomorphism in the contest of multisets are defined, and some valuable theorems are proved. Then the quotient module is proposed, and the relation that M/ker f is isomorphic to Im f for a multiset homomorphism f. Multiset modules drawn from a ℤ module are of particular interest and proved that if L ∈ ML[ℤM], then L is an mset group under addition, and conversely, every mset abelian group drawn from ℤ is an element of ML[ℤM].

Γ-convergence of Onsager–Machlup functionals: II. Infinite product measures on Banach spaces

We derive Onsager–Machlup functionals for countable product measures on weighted ℓ p subspaces of the sequence space . Each measure in the product is a shifted and scaled copy of a reference probability measure on that admits a sufficiently regular Lebesgue density. We study the equicoercivity and Γ-convergence of sequences of Onsager–Machlup functionals associated to convergent sequences of measures within this class. We use these results to establish analogous results for probability measures on separable Banach or Hilbert spaces, including Gaussian, Cauchy, and Besov measures with summability parameter 1 ⩽ p ⩽ 2. Together with part I of this paper, this provides a basis for analysis of the convergence of maximum a posteriori estimators in Bayesian inverse problems and most likely paths in transition path theory.