Some remarks on extremal solutions of multivalued differential equations

In this paper, we consider extremal solutions of multivalued differential equations, i.e., solutions that steer to the boundary of the attainable set. Multivalued differential equations arise in a natural way from control systems governed by ordinary differential equations that have a variable control-constraint set. Extremal solutions of multi-valued differential equations are important in the study of the optimal control of such systems. We give conditions under which extremality of a solution at a certain time implies extremality of the solution at all previous times where it is defined. Necessary conditions for extremality are also obtained. We treat both the time-dependent case and the time-independent case.     

Extremal dilations and their associated extremal measures

If μ and λ are probability measures on a metrisable compact convex set with μ < λ in the Choquet sense, then the main object of this paper is the study of the extremal structure of the convex set of all dilations carrying μ to λ. The extremal dilations are characterized and the relationships between these dilations and the extremal measures they induce are investigated. Several examples of extremal dilations with special properties are given to illustrate their behavior. Also given is a systematic characterization of measures which are extreme in the convex set of all measures dominating μ in the Choquet ordering.     

On a class of extremal solutions of a moment problem for rational matrix-valued functions in the nondegenerate case II

The main theme of this paper is the discussion of a family of extremal solutions of a finite moment problem for rational matrix functions in the nondegenerate case. We will point out that each member of this family is extremal in several directions. Thereby, the investigations below continue the studies in Fritzsche et al. (in press) [1]. In doing so, an application of the theory of orthogonal rational matrix functions with respect to a nonnegative Hermitian matrix Borel measure on the unit circle is used to get some insights into the structure of the extremal solutions in question. In particular, we explain characterizations of these solutions in the whole solution set in terms of orthogonal rational matrix functions. We will also show that the associated Riesz-Herglotz transform of such a particular solution admits specific representations, where orthogonal rational matrix functions are involved.     

Regular and extremal solutions for difference equations with generalized phi-Laplacian

Non-oscillatory solutions for second-order difference equations with generalized phi-Laplacian are studied. Solutions are classified according to the asymptotic behaviour as regular or extremal solutions. Their existence and possible coexistence are investigated as well. In particular, the existence of infinitely many extremal solutions for equations with the discrete mean curvature operator is proved by means of an iterative method. This paper is completed by examples and some open problems.     

The number of extremal components of a rigid measure

The Littlewood-Richardson rule can be expressed in terms of measures, and the fact that the Littlewood-Richardson coefficient is one amounts to a rigidity property of some measure. We show that the number of extremal components of such a rigid measure can be related to easily calculated geometric data. We recover, in particular, a characterization of those extremal measures whose (appropriately defined) duals are extremal as well. This result is instrumental in writing explicit solutions of Schubert intersection problems in the rigid case.     

Resolving infeasibility in extremal algebras

Two extremal algebras =(B,) based on a linearly ordered set (B, ) are considered: in the maxmin algebra =max, = min and in the maxgroup algebra = max and is a group operation. If a system A x = b of linear equations over an extremal algebra is insolvable, then any subset of equations such that its omitting leads to a solvable subsystem is called a relieving set. We show that the problem of finding the minimum cardinality relieving set is NP-complete in the maxmin algebra already for bivalent systems, while it is polynomially solvable for bivalent systems in maxgroup algebra and also NP-complete for trivalent systems.     

On one extremal problem for positive series

The approximation properties of the spaces S _ϕ ^p introduced by Stepanets’ were studied in a series of works of Stepanets’ and his disciples. In these works, problems related to the determination of exact values of n-term approximations of q-ellipsoids in these spaces were reduced to some extremal problems for series with terms that are products of elements of two nonnegative sequences one of which is fixed and the other varies on a certain set. Since solutions of these extremal problems may be of independent interest, in the present work we develop a new method for finding these solutions that enables one to obtain the required result in a substantially shorter and more transparent way. __________ Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, No. 12, pp. 1677–1683, December, 2005.     

On the extremal sets of extremal quasiconformal mappings

The properties of the extremal sets of extremal quasiconformal mappings are discussed. It is proved that if an extremal Beltrami coefficient μ(z) is not uniquely extremal, then there exists an extremal Beltrami coefficient v(z) in its equivalent class and a compact subset E Δ with positive measure such that the essential upper bound of v(z) on E is less than the norm of [μ].     

On extremal solutions of inclusion problems with applications to game theory

A recursion principle, generalized iteration methods and the axiom of choice are applied to prove the existence of extremal fixed points of set-valued mappings in posets, extremal solutions of an inclusion problem, and extremal Nash equilibria for a normal-form game.     

More on a class of extremal solutions of a moment problem for rational matrix-valued functions in the nondegenerate case

The main subject of the paper is an in-depth analysis of Weyl matrix balls which are associated with a finite moment problem for rational matrix functions in the nondegenerate case. Thereby, the investigations tie in with preceding studies on a class of extremal solutions of the moment problem in question. We will point out that each member of this class is also extremal concerning the parameters of Weyl matrix balls. The considerations lead to characterizations of these particular solutions within the whole solution set of the problem. In doing so, an application of the theory of orthogonal rational matrix functions with respect to a nonnegative Hermitian matrix Borel measure on the unit circle is used to get that insight.     

Metric generalized inverse for linear manifolds and extremal solutions of linear inclusion in Banach spaces

Let X,Y be Banach spaces and M a linear manifold in X×Y={{x,y}∣x∈X,y∈Y}. The central problem which motivates many of the concepts and results of this paper is the problem of characterization and construction of all extremal solutions of a linear inclusion y∈M(x). First of all, concept of metric operator parts and metric generalized inverses for linear manifolds are introduced and investigated, and then, characterizations of the set of all extremal or least extremal solutions in terms of metric operator parts and metric generalized inverses of linear manifolds are given by the methods of geometry of Banach spaces. The principal tool in this paper is the generalized orthogonal decomposition theorem in Banach spaces.     

Boundedness of the extremal solution for some p-Laplacian problems

We investigate the regularity of extremal solutions to some p-Laplacian Dirichlet problems with strong nonlinearities. Under adequate assumptions we prove the smoothness of the extremal solutions for some classes of nonlinearities. Our results suggest that the extremal solution’s boundedness for some range of dimensions depends on the nonlinearity f.     

Capturing the multivariate extremal index: bounds and interconnections

The multivariate extremal index function is a direction specific extension of the well-known univariate extremal index. Since statistical inference on this function is difficult it is desirable to have a broad characterization of its attributes. We extend the set of common properties of the multivariate extremal index function and derive sharp bounds for the entire function given only marginal dependence. Our results correspond to certain restrictions on the two dependence functions defining the multivariate extremal index, which are opposed to Smith and Weissman’s (1996) conjecture on arbitrary dependence functions. We show further how another popular dependence measure, the extremal coefficient, is closely related to the multivariate extremal index. Thus, given the value of the former it turns out that the above bounds may be improved substantially. Conversely, we specify improved bounds for the extremal coefficient itself that capitalize on marginal dependence, thereby approximating two views of dependence that have frequently been treated separately. Our results are completed with example processes.      

Existence and controllability results for nondensely defined impulsive semilinear functional differential inclusions

In this paper, we shall establish sufficient conditions for the existence of integral solutions and extremal integral solutions for some nondensely defined impulsive semilinear functional differential inclusions in separable Banach spaces. We shall rely on a fixed point theorem for the sum of completely continuous and contraction operators. The question of controllability of these equations and the topological structure of the solutions set are considered too.     

On a class of extremal problems

In the paper we introduce a class of trigonometrical polynomial extremal problems depending on a continuous parameter 0≤r≤1. It turns out that the two border cases r=0 and r=1 are known problems investigated earlier by Kamae, Mendes-France, Ruzsa and the present author. We also introduce another set of extremal problems for measures with similar parametrization, and prove a duality relationship between the two type of extremal quantities. The proof relies on a minimax theorem proved earlier by the author. The known duality results are proved as corollaries. 1980 MS Classification. Primary 42A05; Secondary 46B25, 46N05.     

Boundary value problems of a class of nonlinear partial differential inclusions

This paper deals with the boundary value problems of nonlinear partial differential inclusions, driven by a negative Laplacian, and with the multivalued term which contains the gradient. It is proved the existence of solutions for the inclusions with the convex and nonconvex valued perturbations. The existence of extremal solutions and a strong relaxation theorem are also obtained.     

Problems and results in extremal combinatorics—II

Extremal Combinatorics is one of the central areas in Discrete Mathematics. It deals with problems that are often motivated by questions arising in other areas, including Theoretical Computer Science, Geometry and Game Theory. This paper contains a collection of problems and results in the area, including solutions or partial solutions to open problems suggested by various researchers. The topics considered here include questions in Extremal Graph Theory, Polyhedral Combinatorics and Probabilistic Combinatorics. This is not meant to be a comprehensive survey of the area, it is merely a collection of various extremal problems, which are hopefully interesting. The choice of the problems is inevitably biased, and as the title of the paper suggests, it is a sequel to a previous paper [N. Alon, Problems and results in extremal combinatorics—I, Discrete Math. 273 (2003), 31-53.] of the same flavor, and hopefully a predecessor of another related future paper. Each section of this paper is essentially self contained, and can be read separately.     

Discrete extremal problems

Precise and heuristic algorithms for solving various classes of discrete extremal problems are considered as are the relations between the class of discrete extremal problems and linear programming and are extremal problems from the point of view of the theory of polynomial completeness. A class of bottleneck optimization problems and stability in discrete extremal problems with a linear object function are also considered.S. P. Tarasov participated in the work of Secs. 3, 4, 5 and the bibliography.Translated from Itogi Nauki i Tekhniki. Teoriya Veroyatnostei, Matematicheskaya Statistika, Teoreticheskaya Kibernetika, Vol. 16, pp. 39–101, 1979.     

Monotone penalty approximation of extremal solutions for quasilinear noncoercive variational inequalities

This paper is about a monotone approximation scheme for extremal (least or greatest) solutions of the following variational inequality:$\text{u∈K:〈Au+F(u),v−u〉⩾0,}\text{∀v∈K,}$in the interval between some appropriately defined sub- and supersolutions. The variational inequality is approximated by a sequence of penalty equations. The extremal solutions of the penalty equations, constructed iteratively and forming a monotone sequence, are proved to converge to the corresponding solutions of the original inequality. We note that no monotoneity assumption on the lower-order term F is imposed.