Simplices of Maximal Volume or Minimal Total Edge Length in Hyperbolic Space

In this paper, we are mainly concerned with n-dimensional simplicesin hyperbolic space Hⁿ. We will also consider simplices withideal vertices, and we suggest that the reader keeps the Poincaréunit ball model of hyperbolic space in mind, in which the sphereat infinity Hⁿ() corresponds to the bounding sphere of radius1. It is known that all hyperbolic simplices (even the idealones) have finite volume. However, explicit calculation of theirvolume is generally a very difficult problem (see, for example,[1] or [16]). Our first theorem states that, amongst all simplicesin a closed geodesic ball, the simplex of maximal volume isregular. We call a simplex regular if every permutation of itsvertices can be realized by an isometry of Hⁿ. A correspondingresult for simplices in the sphere has been proved by Böröczky[4].     

Covering a Ball with Smaller Equal Balls in ℝn

We give an explicit upper bound of the minimal number _T,n of balls of radius 1/2 which form a covering of a ball of radius T > 1/2 in ⁿ, n \geq 2. The asymptotic estimates of _T,n we deduce when n is large are improved further by recent results of Böröczky, Jr. and Wintsche on the asymptotic estimates of the minimal numberof equal balls of ⁿ covering the sphere S^n-1. The optimality of the asymptotic estimates is discussed.     

Formal neighbourhoods, combinatory Böhm trees, and untyped normalization by evaluation

We prove the correctness of an algorithm for normalizing untyped combinator terms by evaluation. The algorithm is written in the functional programming language Haskell, and we prove that it lazily computes the combinatory Böhm tree of the term. The notion of combinatory Böhm tree is analogous to the usual notion of Böhm tree for the untyped lambda calculus. It is defined operationally by repeated head reduction of terms to head normal forms. We use formal neighbourhoods to characterize finite, partial information about data, and define a Böhm tree as a filter of such formal neighbourhoods. We also define formal topology style denotational semantics of a fragment of Haskell following Martin-Löf, and let each closed program denote a filter of formal neighbourhoods. We prove that the denotation of the output of our algorithm is the Böhm tree of the input term. The key construction in the proof is a “glueing” relation between terms and semantic neighbourhoods which is defined by induction on the latter. This relation is related to the glueing relation which was earlier used for proving the correctness of normalization by evaluation algorithm for typed combinatory logic.     

A characterization of duality through section/projection correspondence in the finite dimensional setting

In this paper, we show that the well-known duality operation in the context of convex bodies in Rn is completely characterized by its property of interchanging sections with projections. Our results are compared to results by Böröczky-Schneider and Artstein-Milman, who showed that in many cases, the property of order reversing is sufficient to determine a duality operation, up to obvious linear modifications. In fact, we provide another result that recovers a known characterization of duality by the property of order reversing, and up to a mild condition, also a characterization of duality by interchanging sections by projections.     

Solution of nonlinear heat transfer problems by variational imbedding

Summary In this paper attention has been drawn to the variational imbedding method for solving a broad class of nonlinear heat transfer problems. Specific illustrations considered in this paper include the variable thermal properties heat conduction problem, the wedge flow problem, and the well known Bénard problem with variable properties, all of which are not amenable to analytical solutions by conventional techniques.

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Zusammenfassung In dieser Arbeit wird auf eine Klasse von nichtlinearen Wärmeleitungsproblemen hingewiesen, die durch variational imbedding gelöst werden können. Die behandelten Beispiele umfassen das Wärmeleitungsproblem mit veränderlichen thermischen Eigenschaften, das Problem der Keilströmung, und das bekannte Bénard-Problem mit veränderlichen Stoffwerten; alle diese Probleme können nicht mit konventionellen analytischen Methoden behandelt werden.

     

The colourful feasibility problem

We study a colourful generalization of the linear programming feasibility problem, comparing the algorithms introduced by Bárány and Onn with new methods. This is a challenging problem on the borderline of tractability, its complexity is an open question. We perform benchmarking on generic and ill-conditioned problems, as well as recently introduced highly structured problems. We show that some algorithms can lead to cycling or slow convergence and we provide extensive numerical experiments which show that others perform much better than predicted by complexity arguments. We conclude that the most efficient method is a proposed multi-update algorithm.     

Pattern selection in the Bénard problem for a viscoelastic fluid

We discuss pattern selection in the Bénard problem for a viscoelastic fluid. Double periodicity of the solutions with respect to a hexagonal lattice is assumed. Both steady and oscillatory onsets of instability are considered. For steady onset, we find that the rolls are the only bifurcating solution which can be stable.

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Zusammenfassung Wir diskutieren Musterauswahl beim Bénardproblem für eine viskoelastische Flüssigkeit. Es wird angenommen, daß die Lösungen doppelt periodisch hinsichtlich eines hexagonalen Gitters sind. Es werden sowohl Instabilitäten untersucht, die zu stationären Strömungen führen, als auch solche, die zu zeitlich periodischen Strömungen führen. Im stationären Falle stellt sich heraus, daß Rollen die einzige Verzweigungslösung sind, die stabil Scin kann.

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Dedicated to Prof. Klaus Kirchgässner on the occasion of his sixtieth birthday     

Ergodic properties of the Erdös measure, the entropy of the goldenshift, and related problems

We define a two-sided analog of the Erdös measure on the space of two-sided expansions with respect to the powers of the golden ratio, or, equivalently, the Erdös measure on the 2-torus. We construct the transformation (goldenshift) preserving both Erdös and Lebesgue measures on that is the induced automorphism with respect to the ordinary shift (or the corresponding Fibonacci toral automorphism) and proves to be Bernoulli with respect to both measures in question. This provides a direct way to obtain formulas for the entropy dimension of the Erdös measure on the interval, its entropy in the sense of Garsia-Alexander-Zagier and some other results. Besides, we study central measures on the Fibonacci graph, the dynamics of expansions and related questions.With 11 PiguresTo the memory of Paul ErdösSupported in part by the INTAS grant 93-0570. The first author was supported by the French foundation PRO MATHEMATICA. The first author expresses his gratitude to l'Institut de Mathématiques de Luminy for support during his stay in Marseille in 1996-97. The second author is grateful to the University of Stony Brook for support during his visit in February–March 1996 and to the Institute for Advanced studies of Hebrew University for support during his being there in 1997     

Variance bounds, with an application to norm bounds for commutators

Murthy and Sethi [M.N. Murthy, V.K. Sethi, Sankhya Ser. B 27 (1965) 201-210] gave a sharp upper bound on the variance of a real random variable in terms of the range of values of that variable. We generalise this bound to the complex case and, more importantly, to the matrix case. In doing so, we make contact with several geometrical and matrix analytical concepts, such as the numerical range, and introduce the new concept of radius of a matrix.We also give a new and simplified proof for a sharp upper bound on the Frobenius norm of commutators recently proven by Böttcher and Wenzel [A. Böttcher, D. Wenzel, The Frobenius norm and the commutator, Linear Algebra Appl. 429 (2008) 1864-1885] and point out that at the heart of this proof lies exactly the matrix version of the variance we have introduced. As an immediate application of our variance bounds we obtain stronger versions of Böttcher and Wenzel’s upper bound.     

Multinomial logistic regression algorithm

The lower bound principle (introduced in Böhning and Lindsay 1988, Ann. Inst. Statist. Math., 40, 641–663), Böhning (1989, Biometrika, 76, 375–383) consists of replacing the second derivative matrix by a global lower bound in the Loewner ordering. This bound is used in the Newton-Raphson iteration instead of the Hessian matrix leading to a monotonically converging sequence of iterates. Here, we apply this principle to the multinomial logistic regression model, where it becomes specifically attractive.Supplement to Monotonicity of quadratic-approximation algorithms by Böhning and Lindsay (1988). Ann. Inst. Statist. Math., 40, 641–663.This research was supported by the German Research Foundation.     

On the modulus of continuity for spectral measures in substitution dynamics

The paper gives first quantitative estimates on the modulus of continuity of the spectral measure for weak mixing suspension flows over substitution automorphisms, which yield information about the “fractal” structure of these measures. The main results are, first, a Hölder estimate for the spectral measure of almost all suspension flows with a piecewise constant roof function; second, a log-Hölder estimate for self-similar suspension flows; and, third, a Hölder asymptotic expansion of the spectral measure at zero for such flows. Our second result implies log-Hölder estimates for the spectral measures of translation flows along stable foliations of pseudo-Anosov automorphisms. A key technical tool in the proof of the second result is an “arithmetic-Diophantine” proposition, which has other applications. In Appendix A this proposition is used to derive new decay estimates for the Fourier transforms of Bernoulli convolutions.     

Spectral averaging, perturbation of singular spectra, and localization

A spectral averaging theorem is proved for one-parameter families of self-adjoint operators using the method of differential inequalities. This theorem is used to establish the absolute continuity of the averaged spectral measure with respect to Lebesgue measure. This is an important step in controlling the singular continuous spectrum of the family for almost all values of the parameter. The main application is to the problem of localization for certain families of random Schrödinger operators. Localization for a family of random Schrödinger operators is established employing these results and a multi-scale analysis.

     

On the statistical solutions of Vlasov-Poisson equations in two dimensions

We investigate the problem of the uniqueness of the statistical solutions of the Vlasov-Poisson equations for an electron plasma in a two dimensional torus.

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Zusammenfassung Wir untersuchen die Frage nach der Eindeutigkeit statistischer Lösungen der Vlasov-Poisson-Gleichung für eln Eiektronenplasma im räumlich zweidimensionalen periodischen Fall.

     

Weighted exponential approximation and non-classical orthogonal spectral measures

A long-standing open problem in harmonic analysis is: given a non-negative measure μ onR, find the infimal width of frequencies needed to approximate any function inL²(μ). We consider this problem in the “perturbative regime”, and characterize asymptotic smallness of perturbations of measures which do not change that infimal width. Then we apply this result to show that there are no local restrictions on the structure of orthogonal spectral measures of one-dimensional Schrödinger operators on a finite interval. This answers a question raised by V.A. Marchenko.     

The boundary regularity of non-linear parabolic systems II

This is the second part of a work aimed at establishing that for solutions to Cauchy–Dirichlet problems involving general non-linear systems of parabolic type, almost every parabolic boundary point is a Hölder continuity point for the spatial gradient of solutions. Here we establish higher fractional differentiability of solutions up to the boundary. Based on the necessary and sufficient condition for regular boundary points from the first part of Bögelein et al. (in this issue)[7] we achieve dimension estimates for the boundary singular set and eventually the almost everywhere regularity of solutions at the boundary.     

Symbolic computation, Bäcklund transformation and analytic N-soliton-like solution for a nonlinear Schrödinger equation with nonuniformity term from certain space/laboratory plasmas

With symbolic computation, a bilinear Bäcklund transformation is presented for a nonlinear Schrödinger equation with nonuniformity term from certain space/laboratory plasmas, and correspondingly the one-soliton-like solution is derived from the Bäcklund transformation. Simultaneously, the N-soliton-like solution in double Wronskian form is also given. Besides, the authors verify that the (N−1)- and N-soliton-like solutions satisfy the Bäcklund transformation. The results obtained in this paper might be valuable for the study of the nonuniform media.     

On a Pseudorandom Property of Binary Sequences

C. Mauduit and A. Sárközy proposed the use of well-distribution measure and correlation measure as measures of pseudorandomness of finite binary sequences. In this paper we will introduce and study a further measure of pseudorandomness: the symmetry measure. First we will give upper and lower bounds for the symmetry measure. We will also show that there exists a sequence for which each of the well-distribution, correlation and symmetry measures are small. Finally we will compare these measures of pseudorandomness.     

The design of corporate tax structures

We consider the corporate tax structuring problem (TaxSP), a combinatorial optimization problem faced by firms with multinational operations. The problem objective is nonlinear and involves the minimization of the firm's overall tax payments i.e. the maximization of shareholder returns. We give a dynamic programming (DP) formulation of this problem including all existing schemes of tax-relief and income-pooling. We apply state space relaxation and state space descent to the DP recursions and obtain an upper bound to the value of optimal TaxSP solutions. This bound is imbedded in a B&B tree search to provide another exact solution procedure. Computational results from DP and B&B are given for problems up to 22 subsidiaries. For larger size TaxSPs we develop a heuristic referred to as the Bionomic Algorithm (BA). This heuristic is also used to provide an initial lower bound to the B&B algorithm. We test the performance of BA firstly against the exact solutions of TaxSPs solvable by the B&B algorithm and secondly against results obtained for large-size TaxSPs by Simulated Annealing (SA) and Genetic Algorithms (GA). We report results for problems of up to 150 subsidiaries, including some real-world problems for corporations based in the US and the UK. Support for this work was provided by the IST Framework 5 Programme of the European Union, Contract IST2000-29405, Eurosignal ProjectMathematics Subject Classification (2000): 90C39, 91B28     

On the number of faces of centrally-symmetric simplicial polytopes

I. Bárány and L. Lovász [Acta Math. Acad. Sci. Hung.40, 323–329 (1982)] showed that ad-dimensional centrally-symmetric simplicial polytopeP has at least 2 ^d facets, and conjectured a lower bound for the numberf _i ofi-dimensional faces ofP in terms ofd and the numberf ₀ =2n of vertices. Define integers A. Björner conjectured (unpublished) that (which generalizes the result of Bárány-Lovász sincef _d–1 = h _i ), and more strongly that , which is easily seen to imply the conjecture of Bárány-Lovász. In this paper the conjectures of Björner are proved.Partially supported by NSF grant MCS-8104855. The research was performed when the author was a Sherman Fairchild Distinguished Scholar at Caltech.     

On the Sylvester–Gallai and the orchard problem for pseudoline arrangements

We study a non-trivial extreme case of the orchard problem for 12 pseudolines and we provide a complete classification of pseudoline arrangements having 19 triple points and 9 double points. We have also classified those that can be realized with straight lines. They include new examples different from the known example of Böröczky. Since Melchior’s inequality also holds for arrangements of pseudolines, we are able to deduce that some combinatorial point-line configurations cannot be realized using pseudolines. In particular, this gives a negative answer to one of Grünbaum’s problems. We formulate some open problems which involve our new examples of line arrangements.