Extension of functions preserving the modulus of continuity

The problem of the extension of a real-valued function from a subset of a metric space to the entire space is treated. An extension operator preserving the modulus of continuity of a function is proposed and its properties are studied. An application to the problem of the trace of a locally Lipschitz function on a compact subset of a metric space is given. Translated fromMatematicheskie Zametki, Vol. 61, No. 2, pp. 236–245, February, 1997. Translated by N. K. Kulman     

Operator norm localization property of metric spaces under finite decomposition complexity

The notions of operator norm localization property and finite decomposition complexity were recently introduced in metric geometry to study the coarse Novikov conjecture and the stable Borel conjecture. In this paper we show that a metric space X has weak finite decomposition complexity with respect to the operator norm localization property if and only if X itself has the operator norm localization property. It follows that any metric space with finite decomposition complexity has the operator norm localization property. In particular, we obtain an alternative way to prove a very recent result by E. Guentner, R. Tessera and G. Yu that all countable linear groups have the operator norm localization property.     

Polynomial interpolation of operators

Necessary and sufficient conditions for the solvability of the polynomial operator interpolation problem in an arbitrary vector space are obtained (for the existence of a Hermite-type operator polynomial, conditions are obtained in a Hilbert space). Interpolational operator formulas describing the whole set of interpolants in these spaces as well as a subset of those polynomials preserving operator polynomials of the corresponding degree are constructed. In the metric of a measure space of operators, an accuracy estimate is obtained and a theorem on the convergence of interpolational operator processes is proved for polynomial operators. Applications of the operator interpolation to the solution of some problems are described. Bibliography: 134 titles. This paper is a continuation of the work published inObchyslyuval'na ta Prykladna Maternatyka, No. 78 (1994). The numeration of chapters, assertions, and formulas is continued. Translated fromObchyslyuval'na ta Prykladna Matematyka, No. 79, 1995, pp 10–116.     

Elliptic equations,principal eigenvalue and dependence on the domain

We consider a general second order uniformly elliptic differential operator L and also the set θ of all open sets (not necessarily smooth) in the unit ball of ?ⁿ. We define a metric d in this space (up to an equivalence relation ～) that makes the space (θ/ ～,d) a complete metric space. We show that the principal eigenvalue and eigenfunction of L are continuous with the metric d. Similar results are obtained for the solutions of the equation Lv = ?.     

Linear Volterra operators in some metric spaces

We apply our definition of Volterra operator on abstract spaces to some problems arising in metric spaces. In contrast to those known before, our definition requires only the existence of a σ-algebra on a metric space. Note that, being applied to such spaces, the new definition substantially extends the classes of operators of an evolutionary nature. It also allows one to relate different properties of the Volterra-type operators. In particular, the problem of quasi-nilpotentness studied traditionally in the Banach spaces only (since it requires equality to zero of the spectral radius of an operator) allows interpretation in complete locally convex spaces. Apparently, the question on preserving the Volterra property by a conjugate operator is posed for the first time. It should be mentioned that, generally speaking, the dual space to a Frechét (i.e., complete locally convex) space is not a Frechét space. __________ Translated from Trudy Seminara imeni I. G. Petrovskogo, No. 26, pp. 222–239, 2007.     

Generalized resolvents of closed dissipative operators in a Hilbert space with aF metric

Generalized resolvents of a closedF-dissipative operator in a Hilbert space with aF metric are defined and studied; in particular, a formula of generalized resolvents of such an operator is obtained.Translated from Matematicheskie Zametki, Vol. 20, No. 2, pp. 261–272, August, 1976.In conclusion the author expresses his gratitude to A. V. Shtraus for his scientific guidance.     

Spectral analysis of doublyJ-nonexpanding operators

For a contracting operator in a space with an indefinite metric (i.e., for a doublyJ-nonexpanding operator) a characteristic operator-function is defined. On the basis of a detailed investigation of the properties of regular dilatations and characteristic functions of doublyJ-nonexpanding operators, a spectral analysis of these operators is carried out.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 45, No. 3, pp. 384–388, March, 1993.     

A Unified Approach to Shifts Induced by Right Invertible Operators

In this paper, shifts for a right invertible operator D induced by an analytic function acting in a linear complete metric space are considered. The case when these shifts coincide with the operator - valued function on a set which contains the set of all D-polynomials and the set of all exponentials is studied. It is shown that in this case these shifts are R-shifts and D-shifts (cf. [1], [10]).     

A maximumb-matching problem arising from median location models with applications to the roommates problem

We consider maximumb-matching problems where the nodes of the graph represent points in a metric space, and the weight of an edge is the distance between the respective pair of points. We show that if the space is either the rectilinear plane, or the metric space induced by a tree network, then theb-matching problem is the dual of the (single) median location problem with respect to the given set of points. This result does not hold for the Euclidean plane. However, we show that in this case theb-matching problem is the dual of a median location problem with respect to the given set of points, in some extended metric space. We then extend this latter result to any geodesic metric in the plane. The above results imply that the respective fractionalb-matching problems have integer optimal solutions. We use these duality results to prove the nonemptiness of the core of a cooperative game defined on the roommate problem corresponding to the above matching model. © 1998 The Mathematical Programming Society, Inc. Published by Elsevier Science B.V.Corresponding author.     

Quantized Gromov-Hausdorff distance

A quantized metric space is a matrix order unit space equipped with an operator space version of Rieffel's Lip-norm. We develop for quantized metric spaces an operator space version of quantum Gromov-Hausdorff distance. We show that two quantized metric spaces are completely isometric if and only if their quantized Gromov-Hausdorff distance is zero. We establish a completeness theorem. As applications, we show that a quantized metric space with 1-exact underlying matrix order unit space is a limit of matrix algebras with respect to quantized Gromov-Hausdorff distance, and that matrix algebras converge naturally to the sphere for quantized Gromov-Hausdorff distance.     

Grüss-type and Ostrowski-type inequalities in approximation theory

We discuss the Grüss inequalities on spaces of continuous functions defined on a compact metric space. Using the least concave majorant of the modulus of continuity, we obtain the Grüss inequality for the functional L(f) = H(f; x), where H:C[a, b] → C[a, b] is a positive linear operator and x ∈ [a, b] is fixed. We apply this inequality in the case of known operators, e.g., the Bernstein operator, the Hermite–Fejér interpolation operator, and convolution-type operators. Moreover, we deduce Grüss-type inequalities using the Cauchy mean-value theorem, thus generalizing results of Chebyshev and Ostrowski. The Grüss inequality on a compact metric space for more than two functions is given, and an analogous Ostrowski-type inequality is obtained. The latter, in turn, leads to one further version of the Grüss inequality. In the appendix, we prove a new result concerning the absolute first-order moments of the classic Hermite–Fejér operator.     

A smoothing operator polynomial in the Hilbert spaceH λ

A mapping and an operator polynomial are considered as elements of the space H(λ). The polynomial is proved to be smoothing for the mapping in the metric of the space H(λ), that is, to be a solution to the corresponding extremum problem. Bibliography:4 titles. Translated fromObchyslyuval’na ta Prykladna Matematyka, No. 77, 1993, pp. 27–34.     

Nuclearity of a nonnegative definite integral kernel on a separable metric space

The aim of this paper is to characterize the nuclearity of an integral operator, defined by a continuous non-negative definite square integrable kernel on a separable metric space, in terms of the integrability of the trace of the kernel function. Nuclearity here plays a role forU-statistics.     

Gradient-constrained minimum networks (II). Labelled or locally minimal Steiner points

A gradient-constrained minimum network T is a minimum length network, spanning a given set of nodes N in space with edges whose gradients are all no more than an upper bound m. The nodes in T but not in N are referred to as Steiner points. Such networks occur in the underground mining industry where the typical maximal gradient is about 1:7 (≈ 0.14). Because of the gradient constraint the lengths of edges in T are measured by a special metric, called the gradient metric. An edge in T is labelled as a b-edge, or an m-edge, or an f-edge if the gradient between its endpoints is greater than, or equal to, or less than m respectively. The set of edge labels at a Steiner point is called its labelling. A Steiner point s with a given labelling is called labelled minimal if T cannot be shortened by a label-preserving perturbation of s. Furthermore, s is called locally minimal if T cannot be shortened by any perturbation of s even if its labelling is not preserved. In this paper we study the properties of labelled minimal Steiner points, as well as the necessary and sufficient conditions for Steiner points to be locally minimal. It is shown that, with the exception of one labelling, a labelled minimal Steiner point is necessarily unique with respect to its adjacent nodes, and that the locally minimal Steiner point is always unique, even though the gradient metric is not strictly convex.     

广义正交分解定理与Tseng度量广义逆

本文将Banach空间中广义正交分解定理从线性子空间拓广至非线性集—太阳集,分别给出了一算子为度量投影算子和一度量投影算子为有界线性算子的充要条件;得到了判别Banach空间中子空间广义正交可补的充要条件;建立了王玉文和季大琴(2000年)新近引入的Banach空间中的线性算子的Tseng度量广义逆存在的特征刻划条件;这些工作本质地把王玉文等人的新近结果从自反空间拓广至非自反空间的情形.     

Generic power convergence of operators in banach spaces

Let K be a closed convex subset of a Banach space X. We consider complete metric spaces of self-mappings of K which are nonexpansive with respect to a convex function on X. We prove that the iterates of a generic operator in these spaces converge strongly. In some cases the limits do not depend on the initial points and are the unique fixed point of the operator.     

A study of a general system of operator equations in $${\varvec{}}$$-metric spaces via the vector approach in fixed point theory

In this paper, we will present some coupled fixed point theorems on a metric space endowed with two b-metrics. The approach is based on the application of a fixed point theorem for an appropriate operator on the Cartesian product of the given spaces. An application to a boundary value problem for a system of second order differential equations is also presented.     

A pathological lattice of invariant subspaces

An explicit example of a Hilbert space operator whose lattice of invariant subspaces (under the metric topology “gap between subspaces”) contains an inaccessible point which is not isolated is constructed; the component of that inaccessible point is not arcwise connected and, moreover, no ball (of sufficiently small radius) about the point is connected.     

Unitarizable representations and fixed points of groups of biholomorphic transformations of operator balls

We show that the open unit ball of the space of operators from a finite-dimensional Hilbert space into a separable Hilbert space (we call it “operator ball”) has a restricted form of normal structure if we endow it with a hyperbolic metric (which is an analogue of the standard hyperbolic metric on the unit disc in the complex plane). We use this result to get a fixed point theorem for groups of biholomorphic automorphisms of the operator ball. The fixed point theorem is used to show that a bounded representation in a separable Hilbert space which has an invariant indefinite quadratic form with finitely many negative squares is unitarizable (equivalent to a unitary representation). We apply this result to find dual pairs of invariant subspaces in Pontryagin spaces. In Appendix A we present results of Itai Shafrir about hyperbolic metrics on the operator ball.