Traces of functions of generalized Sobolev classes

Considering the Sobolev type function classes on a metric space equipped with a Borel measure we address the question of compactness of embeddings of the space of traces into Lebesgue spaces on the sets of less “dimension.” Also, we obtain compactness conditions for embeddings of the traces of the classical Sobolev spaces W _p ¹ on the “zero” cusp with a Hölder singularity at the vertex.     

Approximation of analytic by Borel sets and definable countable chain conditions

LetI be a σ-ideal on a Polish space such that each set fromI is contained in a Borel set fromI. We say thatI fails to fulfil theΣ ₁ ¹ countable chain condition if there is aΣ ₁ ¹ equivalence relation with uncountably many equivalence classes none of which is inI. Assuming definable determinacy, we show that if the family of Borel sets fromI is definable in the codes of Borel sets, then eachΣ ₁ ¹ set is equal to a Borel set modulo a set fromI iffI fulfils theΣ ₁ ¹ countable chain condition. Further we characterize the σ-idealsI generated by closed sets that satisfy the countable chain condition or, equivalently in this case, the approximation property forΣ ₁ ¹ sets mentioned above. It turns out that they are exactly of the formMGR(F)={A : ∀F ∈ F A ∩F is meager inF} for a countable family F of closed sets. In particular, we verify partially a conjecture of Kunen by showing that the σ-ideal of meager sets is the unique σ-ideal onR, or any Polish group, generated by closed sets which is invariant under translations and satisfies the countable chain condition. Research partially supported by NSF grant DMS-9317509.     

Algebraically and verbally closed subgroups and retracts of finitely generated nilpotent groups

We study algebraically and verbally closed subgroups and retracts of finitely generated nilpotent groups. A special attention is paid to free nilpotent groups and the groups UT _n (Z) of unitriangular (n×n)-matrices over the ring Z of integers for arbitrary n. We observe that the sets of retracts of finitely generated nilpotent groups coincides with the sets of their algebraically closed subgroups. We give an example showing that a verbally closed subgroup in a finitely generated nilpotent group may fail to be a retract (in the case under consideration, equivalently, fail to be an algebraically closed subgroup). Another example shows that the intersection of retracts (algebraically closed subgroups) in a free nilpotent group may fail to be a retract (an algebraically closed subgroup) in this group. We establish necessary conditions fulfilled on retracts of arbitrary finitely generated nilpotent groups. We obtain sufficient conditions for the property of being a retract in a finitely generated nilpotent group. An algorithm is presented determining the property of being a retract for a subgroup in free nilpotent group of finite rank (a solution of a problem of Myasnikov). We also obtain a general result on existentially closed subgroups in finitely generated torsion-free nilpotent with cyclic center, which in particular implies that for each n the group UT _n (Z) has no proper existentially closed subgroups.     

On the index sets of Σ-subsets of the real numbers

We compute the levels of complexity in analytical and arithmetical hierarchies for the sets of the Σ-formulas defining in the hereditarily finite superstructure over the ordered field of the reals the classes of open, closed, clopen, nowhere dense, dense subsets of ? ⁿ , first category subsets in ? ⁿ as well as the sets of pairs of Σ-formulas corresponding to the relations of set equality and inclusion which are defined by them. It is also shown that the complexity of the set of the Σ-formulas defining connected sets is at least Π ₁ ¹ .     

The isomorphism problem for classes of computable fields

Theories of classification distinguish classes with some good structure theorem from those for which none is possible. Some classes (dense linear orders, for instance) are non-classifiable in general, but are classifiable when we consider only countable members. This paper explores such a notion for classes of computable structures by working out several examples. One motivation is to see whether some classes whose set of countable members is very complex become classifiable when we consider only computable members. We follow recent work by Goncharov and Knight in using the degree of the isomorphism problem for a class to distinguish classifiable classes from non-classifiable. For real closed fields we show that the isomorphism problem is ¹₁ complete (the maximum possible), and for others we show that it is of relatively low complexity. We show that the isomorphism problem for algebraically closed fields, Archimedean real closed fields, or vector spaces is ⁰₃ complete.     

Canonical and existential groups in universal classes of Abelian groups

Universal classes of Abelian groups are classified in terms of sets of finitely generated groups closed with respect to the discrimination operator. The notions of a principal universal class and a canonical group for such a class are introduced. For any universal class K, the class K^ec of existentially closed groups generated by the universal theory of K is described. It is proved that K^ec is axiomatizable and, therefore, the universal theory of K has a model companion.     

The Hausdorff-Ershov Hierarchy in Euclidean Spaces

The topological arithmetical hierarchy is the effective version of the Borel hierarchy. Its class Δ^ta₂ is just large enough to include several types of pointsets in Euclidean spaces ℝ^k which are fundamental in computable analysis. As a crossbreed of Hausdorff's difference hierarchy in the Borel class Δ^B₂ and Ershov's hierarchy in the class Δ⁰₂ of the arithmetical hierarchy, the Hausdorff-Ershov hierarchy introduced in this paper gives a powerful classification within Δ^ta₂. This is based on suitable characterizations of the sets from Δ^ta₂ which are obtained in a close analogy to those of the Δ^B₂ sets as well as those of the Δ⁰₂ sets. A helpful tool in dealing with resolvable sets is contributed by the technique of depth analysis. On this basis, the hierarchy properties, in particular the strict inclusions between classes of different levels, can be shown by direct constructions of witness sets. The Hausdorff-Ershov hierarchy runs properly over all constructive ordinals, in contrast to Ershov's hierarchy whose denotation-independent version collapses at level ω². Also, some new characterizations of concepts of decidability for pointsets in Euclidean spaces are presented.     

On the Convergence of ω1 Sequences of Real Functions

We study sets of points at which ω₁ sequences of real functions from a given class F converge. As F we consider continuous functions, first class of Baire, Borel measurable functions, functions with Baire property and Lebesgue measurable functions. Connections of those problem with additional set-theoretic axioms are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Difference sets with few character values

The known families of difference sets can be subdivided into three classes: difference sets with Singer parameters, cyclotomic difference sets, and difference sets with gcd \((v,n)>1\) . It is remarkable that all the known difference sets with gcd \((v,n)>1\) have the so-called character divisibility property. Jungnickel and Schmidt (Difference sets: an update. London Math. Soc. Lecture Note Ser., vol. 245, pp. 89–112, Cambridge University Press, Cambridge 1997) posed the problem of constructing difference sets with gcd \((v,n)>1\) that do not satisfy this property. In an attempt to attack this problem, we use difference sets with three nontrivial character values as candidates, and get some necessary conditions.     

Convergence properties of local solutions of sequences of mathematical programming problems in general spaces

This paper gives several sets of sufficient conditions that alocal solutionx ^k exists of the problem \(\min _{R^k } f^k (x)\) ,k=1, 2,..., such that {x ^k } has cluster points that arelocal solutions of a problem of the form min _R f(x). The results are based on a well-known concept of topological, orpoint-wise convergence of the sets {R ^k } toR. Such results have been used to construct and validate large classes of mathematical programming methods based on successive approximations of the problem functions. They are also directly applicable to parametric and sensitivity analysis and provide additional characterizations of optimality for large classes of nonlinear programming problems.     

The Zero Set of Fractional Brownian Motion Is a Salem Set

The existence of sets supporting a Borel measure such that its Fourier transform tends to zero at infinity can be traced back to the problem of uniqueness of trigonometric series, studied extensively by Cantor. Given \(\alpha \in (0, 1)\), Beurling asked if there exists a subset of the real line of Hausdorff dimension \(\alpha \) supporting a Borel measure whose Fourier transform converges to zero at infinity with rate \(\alpha /2\). Salem answered the question in the affirmative and such sets are now called Salem sets or rounded sets. Kahane showed that images of compact sets by fractional Brownian motion are Salem sets and this was recently extended to Gaussian random fields with stationary increments and to multi-parameter Brownian sheets. He asked if the level sets of fractional Brownian motion are also Salem sets and the problem has remained open since. This paper answers Kahane’s question in the affirmative. The argument is based on the study of oscillatory integrals with non-smooth amplitudes and new properties of the generalised Euler spiral which have independent interest.     

Borel equivalence relations between <Emphasis Type="Italic">ℓ</Emphasis><Subscript>1</Subscript> and <Emphasis Type="Italic">ℓ</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>

In this paper, we show that, for each p 〉 1, there are continuum many Borel equivalence relations between Rω/l1 and Rω/p ordered by ≤B which are pairwise Borel incomparable.     

On the analytic and Cauchy capacities

We give new sufficient conditions for a compact set E ? C to satisfy γ(E) = γ_c(E), where γ is the analytic capacity and γ_c is the Cauchy capacity. As a consequence, we provide examples of compact plane sets such that the above equality holds but the Ahlfors function is not the Cauchy transform of any complex Borel measure supported on the set.     

Removable singularities of solutions of linear uniformly elliptic second order equations

Let L be a uniformly elliptic linear second order differential operator in divergence form with bounded measurable real coefficients in a bounded domain G ? ?ⁿ (n ? 2). We define classes of continuous functions in G that contain generalized solutions of the equation L? = 0 and have the property that the compact sets removable for such solutions in these classes can be completely described in terms of Hausdorff measures.     

Iterative Averaging of Entropic Projections for Solving Stochastic Convex Feasibility Problems

The problem considered in this paper is that of finding a point which iscommon to almost all the members of a measurable family of closed convexsubsets of R₊₊ ⁿ , provided that such a point exists.The main results show that this problem can be solved by an iterative methodessentially based on averaging at each step the Bregman projections withrespect to f(x)=_i=1 ⁿx_i· ln x_i ofthe current iterate onto the given sets.     

Classification theory for non-elementary classes I: The number of uncountable models ofψ ∈L ω_1, ω. Part A

Assuming that 2^Nn < 2^Nn+1 forn < ω, we prove that everyψ ∈L _{ω_1, ω} has many non-isomorphic models of powerN _n for somen>0or has models in all cardinalities. We can conclude that every such Ψ has at least 2 _N 1 non-isomorphic uncountable models. As for the more vague problem of classification, restricting ourselves to the atomic models of some countableT (we can reduce general cases to this) we find a cutting line named “excellent”. Excellent classes are well understood and are parallel to totally transcendental theories, have models in all cardinals, have the amalgamation property, and satisfy the Los conjecture. For non-excellent classes we have a non-structure theorem, e.g., if they have an uncountable model then they have many non-isomorphic ones in someN _n (provided {ie212-7}).     

Some descriptive set theory and core models

We analyse the trees given by sharps for Π¹₂ sets via inner core models to give a canonical decomposition of such sets when a core model is Σ¹₃ absolute. This is by way of analogy with Solovay's analysis of Π¹₁ sets into ω₁ Borel sets — Borel in codes for wellorders. We find that Π¹₂ sets are also unions of ω₁ Borel sets — but in codes for mice and wellorders. We give an application of this technique in showing that if a core model, K, is Σ¹₃ absolute thenTheorem. Every real is in K iff every Π¹₃ set of reals contains a Π¹₃ singleton.     

Tight Gabor Frames Associated with Non-separable Lattices and the Hyperbolic Secant

We consider Gabor systems generated by a window given by the hyperbolic secant function. We show that such a system forms a Parseval frame for L ²(?) when the translations and modulations of the window are associated with certain non-separable lattices in ?² which we explicitly describe. We also study the more general problem of characterizing the positive Borel measures μ on ?²ⁿ with the property that the short-time Fourier transform defines an isometric embedding from L ²(? ⁿ ) to L _μ ² (?²ⁿ ) when the window belongs to the Schwartz class and, in particular, we characterize the extreme points of this set. In the case where the window is the hyperbolic secant function, we consider the situation where the measure is discrete with constant weights and supported on a non-separable lattice yielding a Parseval frame. We provide arithmetic conditions on the parameters defining the lattice characterizing when the associated measure is an extreme point.     

Sets of simple invariance

Let X be a space of smooth function on the unit circumference, in which there acts the invertible operator of the two-sided shift. A closed set E, Ec, is said to be a set of simple invariance for the space X if there exists a function, such that. It is established that the class of sets of simple invariance for the spaces coincides with the class of sets of zero measure, for the spaces C ⁿ , ⁿ , W _p ⁿ (p < ) with the class of nowhere dense closed sets, while for the space C with the class of sets satisfying the well-known Carleson condition. In addition, one considers the problem of describing the zeros of the functions f, possessing additional smoothness in comparison with X and satisfying the condition . Translated from Napiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 107, pp. 104–135, 1982.