Best Approximation in an Asymmetrically Weighted L1 Measure

Let f(x) be a given, real-valued, continuous function definedon an interval [a,b]of the real line. Given a set of m real-valued,continuous functions _j(x) defined on [a,b], a linear approximatingfunction can be formed with any real setA = {a₁, a₂,..., a_m}. We present results for determining A sothat F(A, x) is a best approximation to(x) when the measureof goodness of approximation is a weighted sum of |F(A, x)–f(x)|,the weights being positive constants, w, when F(A, x) f(x)and w2 otherwise (when w, = w2 = 1, the measure is the L₁, norm).The results are derived from a linear programming formulationof the problem. In particular, we give a theorem which shows when such bestapproximations interpolate the function at fixed ordinates whichare independent of f(x). We show how the fixed points can becalculated and we present numerical results to indicate thatthe theorem is quite robust.     

Robust tracking for MIMO nonlinear time-dependent mismatched uncertain systems: non-adaptive versus adaptive control

This paper considers the problem of robust output-tracking controlfor multi-input multi-output (MIMO) nonlinear systems in thepresence of mismatched time-dependent uncertainties. It firstgeneralizes the stable combined-variable perturbation method(SCVPM) which was proposed by Li et al. (1996) to derive a newrobust controller and its adaptive version for MIMO systemsto track a desired trajectory. It is shown that both controllersnot only stabilize the closed-loop systems but also guaranteethat the tracking errors remain in an O() neighbourhood of theorgin, where is a small design parameter of the controller.Moreover, the adaptive robust controller is only based on thenominal system, and no a priori data on the uncertainties areneeded. Therefore, it is more feasibly implemented than thenon-adaptive controller.     

Approximate parametric optimization of infinite-dimensional systems

We consider the problem of finding g M_n such that where M_n is the n-dimensional subspace of the complexHilbert space L²(0, ) spanned by an n-tuple of normalized eigenvectoesof the operator , corresponding to eigenvalues. The solution is g = P_nf and P_n denotesthe orthoprojector onto M_n. From Grabowski (1991) we know thatP_n can be expressed in terms of the Malmquist functions. Wegive an alternative approach, more convenient for applicationof the standard mathematical software. The problem of convergenceas n is discussed from both theoretical and numerical viewpoint.The reslts are illustrated by the problems of finding the optimaladjustment of the proportional controller stabilizing a distributedplant. Email: pgrab{at}ia.agh.edu.pl     

Robust H{infty} control of uncertain systems with time-varying state and control input delays

In this paper, a new method for robust H controller of uncertainsystems with time-varying state and control input delays isproposed. A sufficient condition, which is dependent on themaximum value of the time derivative of time-varying delay,is also presented for the existence of H controller. The proposedcontroller not only stabilizes closed-loop uncertain systemsbut also guarantees a prescribed H norm bound of the closed-looptransfer matrix from the disturbance to controlled output. Furthermore,we propose a new robust stability criterion to decide the allowedmaximum bound of plant uncertainty which still can be stabilizedby proposed robust controller. An example given illustratesthe availability of the proposed design method.     

Lech's Conjecture on Deformations of Singularities and Second Harrison Cohomology

Let B₀ be a local singularity of dimension d. Then we considerthe problem of Lech, whether for every deformation (A, m) (B,n) of B₀ the inequality between the Hilbert functions is true,and give a positive answer in the case that the formal versaldeformation of B₀ is a base change of an algebraic family (R,M) (S, N), where R is regular and dim S = dim R + d. So one would hope to lift versal deformations in that way. Thereare obstructions against this in certain second Harrison cohomologygroups.     

The gradient iteration for approximation in reproducing kernel Hilbert spaces

Consider the bounded linear operator, L: F Z, where Z R^N andF are Hilbert spaces defined on a common field X. L is madeup of a series of N bounded linear evaluation functionals, L_i:F R. By the Riesz representation theorem, there exist functionsk(x_i, ^·) F : L_if = f, k(x_i, ^·)_F. The functions,k(x_i, ^·), are known as reproducing kernels and F is areproducing kernel Hilbert space (RKHS). This is a natural frameworkfor approximating functions given a discrete set of observations.In this paper the computational aspects of characterizing suchapproximations are described and a gradient method presentedfor iterative solution. Such iterative solutions are desirablewhen N is large and the matrix computations involved in thebasic solution become infeasible. This is also exactly the casewhere the problem becomes ill-conditioned. An iterative approachto Tikhonov regularization is therefore also introduced. Unlikeiterative solutions for the more general Hilbert space setting,the proofs presented make use of the spectral representationof the kernel.     

Dimensions of Julia Sets of Hyperbolic Entire Functions

It is known that, if f is a hyperbolic rational function, thenthe Hausdorff, packing and box dimensions of the Julia set J(f)are equal. It is also known that there is a family of hyperbolictranscendental meromorphic functions with infinitely many polesfor which this result fails to be true. In this paper, new methodsare used to show that there is a family of hyperbolic transcendentalentire functions f_K, K N, such that the box and packing dimensionsof Jf_K are equal to two, even though as K the Hausdorff dimensionof Jf_K tends to one, the lowest possible value for the Hausdorffdimension of the Julia set of a transcendental entire function.2000 Mathematics Subject Classification 30D05, 37F10, 37F15,37F35, 37F50.     

Differentiability Properties of an Abstract Autonomous Composition Operator

The autonomous composition operator is the nonlinear map whichtakes a pair of functions into its composite function. The compositionoperator often appears in problems of nonlinear analysis andto analyse such problems it is often important to know whetherthe composition operator is continuous or differentiable. Afairly large number of papers in the literature have been devotedto the study of composition operators. For fullscale references,we refer the reader to the extensive monographs of Appell andZabrejko [1] and Runst and Sickel [8]. To exemplify a typicalsituation, we consider the semilinear Dirichlet boundary valueproblem where denotes a sufficiently regular bounded open subset ofR^N, and h₀ a map of R to R, and where u is the unknown of theproblem. We assume that we know that a certain function u₀ belongingto a certain function space X solves (1.1). Then if we wishto know whether by perturbing h₀ in a certain function space,say Y, the solutions u depend on h continuously, with differentiability,with analyticity or bifurcate, we could set G[h, u] u+h u,recast problem (1.1) into the abstract form G[h, u] (1.2) and study the solution set of equation (1.2) around the pair(h₀, u₀) by means of the implicit function theorem or by localbifurcation theorems in a Banach space setting.     

Recovery of residual stress in a vertically heterogeneous elastic medium

We study the problem of identifying residual stress within athin subsurface layer in an elastic medium occupying a region = {(x₁, x₂, x₃) , R³: 0 < x₃ < L, where L } in space,where all parameters depend only on the depth x₃. Under thetheoretical framework of linear elasticity with initial stress,the incremental elasticity tensor of each material point iswritten as a sum of two terms, namely the elasticity tensorand the acoustoelastic tensor, both of which are taken hereas isotropic functions of their arguments. By imposing impulsiveloads and measuring the displacements at the boundary x₃ = 0,we recover the residual stress and its gradient there. If theresidual stress has a diagonal form, we can recover the residualstress inside the subsurface layer.     

Bessel Functions: Monotonicity and Bounds

Monotonicity with respect to the order v of the magnitude ofgeneral Bessel functions C_v(x) = aJ_v(x)+bY_v(x) at positive stationarypoints of associated functions is derived. In particular, themagnitude of C_v at its positive stationary points is strictlydecreasing in v for all positive v. It follows that sup_x|J_v(x)|strictly decreases from 1 to 0 as v increases from 0 to . Themagnitude of x^1/2C_v(x) at its positive stationary points isstrictly increasing in v. It follows that sup_x|x^1/2J_v(x)| equals2/ for 0 v 1/2 and strictly increases to as v increases from1/2 to . It is shown that v^1/3sup_x|J_v(x)| strictly increases from 0 tob = 0.674885... as v increases from 0 to . Hence for all positivev and real x, where b is the best possible such constant. Furthermore, forall positive v and real x, where c = 0.7857468704... is the best possible such constant. Additionally, errors in work by Abramowitz and Stegun and byWatson are pointed out.     

Reduction of CalderON Commutators

Let A₁,..., A_n be Lipschitz functions on R such that A'₁,...,A'_nVMO. We show that on any bounded interval, the Calderóncommutator associated with the kernel (A₁(x)–A₁(y)) ...(A_n(x) – A_n(y))/(x–y) ⁿ¹ is a compact perturbationof , where H is the Hilberttransform. 1991 Mathematics Subject Classification 47B38, 47B47,47G10, 45E99.     

Isomorphisms of Group Algebras

Let G₁ and G₂ be locally compact groups. If T is an algebraisomorphism of L¹(G₁) onto L¹(G₂) with ||T|| (1+3), then G₁and G₂ are isomorphic. This improves on earlier results, and,in a certain sense, is best possible. However, the main conjecturethat the groups are isomorphic if ||T|| < 2 remains unsolvedexcept for abelian groups and for connected groups. Similarresults are given for the measure algebra M(G) and for the algebraC(G) of continuous functions when the group G is compact.     

Best Approximation with Coefficient Constraints

For given = (₁,..., _n) and ß = (ß₁,...,ß_n), with – _i < ß_i (i = 1, ...,n) and continuous functions u₁,...,u_n, set This paper is concerned with best approximating continuous functions,in the uniform norm, from U(; ß). We exactly characterizethe u₁,..., u_n for which the best approximant to every continuousfunction is unique. We also present a general theorem characterizingall best approximants. When (u₁,..., u_n) is a Descartes, ora weak Descartes, system on [0, 1], explicit characterizationsof the best approximants in terms of equioscillations are given.These results are applied to spline spaces. They are also usedto complete the characterizations in certain specific examplespreviously considered in the literature.     

Exceptional Functions and Normality

Yang proved in [10] that if f and f^(k) have no fix-points forevery fF, where F is a family of meromorphic functions in adomain G and k a fixed integer, then F is normal in G. In thispaper we prove normality for families F for which every fF omits₁ and f^(k) omits ₂, where ₁ and ₂ are analytic functions with. 1991 Mathematics SubjectClassification 30D35, 30D45.     

The sound field due to a periodic array of forced wave-bearing strips

A compressible fluid in a two-dimensional half-space (y >0) is bounded by a plane surface (y = 0) which is acousticallyhard except for a set of periodically arranged strips S_n givenby nd – a < x < nd + a, y = 0 with n = 0, 1, 2,....The velocity potential Re {(x, y)exp(–it)} satisfies theHelmholtz wave equation in the fluid region y>0, with /y= 0 on the plane y = 0, x S_n. The boundary condition on thepistons S_n is taken to have the form where the prescribed forcing function V(x) is the same on eachstrip, so that V(x + nd) = V(x), and the operators L and M arepolynomial functions of the second derivative ²/x². This boundarycondition includes the possibilities of an elastic plate, amembrane, or an impedance surface for S_n. When the separationdistance d is much greater than the strip width 2a and wavelength2/k, the problem is reduced to that of finding the potential_p due to a single piston S_o set in a rigid baffle, togetherwith a potential _c subject to a similar condition with forcingfunctions exp (ikx) in place of V(x). The problem is generalizedto allow for the possibility of a phased forcing function V(x),such that V(x + nd) = exp (ißnd)V(x), where ßis a given constant.     

Geometrical relations and plethysms in the Homfly skein of the annulus

Let _m be the closure of the Hecke algebra with m strings, H_m,in the oriented framed Homfly skein of the annulus, which providesthe natural parameter space for the Homfly satellite invariantsof a knot. The submodule ₊ spanned by the union _{m 0 m} isan algebra, isomorphic to the algebra of the symmetric functions.Turaev's geometrical basis for ₊ consists of monomials in closedm-braids A_m, the closure of the braid _m–1·...·₂₁.We collect and expand formulae relating elements expressed interms of symmetric functions to Turaev's basis. We reformulatethe formulae of Rosso and Jones for quantum sl(N) invariantsof cables in terms of plethysms of symmetric functions, anduse the connection between quantum sl(N) invariants and theskein ₊ to give a formula for the satellite of a cable as anelement of the Homfly skein ₊. We can then analyse the casewhere a cable is decorated by the pattern P_d which correspondsto a power sum in the symmetric function interpretation of ₊to get direct relations between the Homfly invariants of somediagrams decorated by power sums     

Sharp Distortion Estimate for Locally Schlicht Bloch Functions

Let B be the space of locally schlicht Bloch functions f whichare analytic in the unit disc with f(0) = f'(0) – 1 =0 satisfying 0 < |f'(z)|(1 – |z|²) 1. For each fixedz₀ we shall determine the shape of the set {logf'(z₀): fB},that is, we shall give the sharp distortion estimate for locallyschilcht Bloch functions.     

Minimal vanishing sums of roots of unity with large coefficients

A vanishing sum , where_n is a primitive nth root of unity and the a_is are non-negativeintegers is called minimal if the coefficient vector (a₀, ..., a_n–1) does not properly dominate the coefficient vectorof any other such non-zero sum. We show that for every c thereis a minimal vanishing sum of nth roots of unity with its greatestcoefficient equal to c, where n is of the form 3pq for odd primesp, q. This solves an open problem posed by Lenstra Jr.     

H{infty} filtering for linear continuous-time systems with sampled measurements

The problem of H filtering of continuous-time systems with sampledmeasurements has been investigated. Both finite-and infinite-horizoncases have been considered. It has bas been shown that the sampled-dataH filtering problem can be solved in terms of a differentialRiccati equation with finite discrete jumps. When the systemis time-invariant with zero initial condition, and an infinitehorizon is concerned, the solution to the above problem is equivalentto a solution to a discrete-time algebraic Riccati equation.     

On the 'pits effect' of Littlewood and Offord

Asymptotic behaviour of the entire functions , with real _n is studied. It turns out that the Phragmén–Lindelöfindicator of such a function is always non-negative, unlessf(z)=e^az. For a special choice of _n= n² with irrational , theindicator is constant and f has completely regular growth inthe sense of Levin and Pfluger. Similar functions of arbitraryorder are also considered.