Two new characterizations of carleson-newman Blaschke products

We present an extension of a result of Vasyunin by giving a characterization of finite products of interpolating Blaschke products B in terms of the minorization of B(z) by the distance of z to the zeros of B. We also characterize those Blaschke products that satisfy the hereditary weak embedding property.     

Approximation to linear functionals on H1 by point evaluations

The degree of approximation to a bounded linear functional on H¹ having a smooth kernel by finite sums of point evaluations is shown to depend on the smoothness of that kernel. Use is made of a Jackson-Bernstein type theorem concerning the approximation of continuous unimodular functions by finite Blaschke products.     

Bedrosian Identity in Blaschke Product Case

This paper offers a characterization of amplitude functions in L²(\mathbb R){L^2(\mathbb R)} satisfying the Bedrosian identity in the case that the phase functions are determined by the boundary value on the unit circle of finite Blaschke products.     

Maximal Blaschke products

We consider the classical problem of maximizing the derivative at a fixed point over the set of all bounded analytic functions in the unit disk with prescribed critical points. We show that the extremal function is essentially unique and always an indestructible Blaschke product. This result extends the Nehari–Schwarz Lemma and leads to a new class of Blaschke products called maximal Blaschke products. We establish a number of properties of maximal Blaschke products, which indicate that maximal Blaschke products constitute an appropriate infinite generalization of the class of finite Blaschke products.     

A Class of Surfaces with Flat Blaschke Metric and Their Characterization

We study locally strongly convex surfaces with complete flat Blaschke metric. We show how we can characterize all known examples by a tensorial condition involving the covariant derivative of the shape operator and the gradient of the Pick invariant.Mathematics Subject Classifications (2000): 53A15.     

Multiplication operators on the Bergman space via analytic continuation

In this paper, using the group-like property of local inverses of a finite Blaschke product ?, we will show that the largest C^?-algebra in the commutant of the multiplication operator M? by ? on the Bergman space is finite dimensional, and its dimension equals the number of connected components of the Riemann surface of ?−1°? over the unit disk. If the order of the Blaschke product ? is less than or equal to eight, then every C^?-algebra contained in the commutant of M? is abelian and hence the number of minimal reducing subspaces of M? equals the number of connected components of the Riemann surface of ?−1°? over the unit disk.     

Reducibility for a Class of Analytic Multipliers on the Dirichlet Space

It is proved that if \(\phi \) is a finite Blaschke product with four zeros, then \(M_\phi \) is reducible on the Dirichlet space with norm \(\Vert \ \Vert \) if and only if \(\phi =\phi _1\circ \phi _2\), where \(\phi _1, \phi _2\) are Blaschke products and \(\phi _2\) is equivalent to \(z^2\). Also, the same reducibility of \(M_\phi \) with finite Blaschke product \(\phi \) on the Dirichlet space under the equivalent norms \(\Vert \ \Vert _1\) and \(\Vert \ \Vert _0\) is given.     

General L p -harmonic Blaschke bodies

Lutwak introduced the harmonic Blaschke combination and the harmonic Blaschke body of a star body. Further, Feng and Wang introduced the concept of the L _p -harmonic Blaschke body of a star body. In this paper, we define the notion of general L _p -harmonic Blaschke bodies and establish some of its properties. In particular, we obtain the extreme values concerning the volume and the L _p -dual geominimal surface area of this new notion.     

Cluster sets of interpolating Blaschke products

We construct interpolating Balschke products whose radial cluster sets at a given point of the unit circle can be prescribed to be one of the following: the closed unit disk; an arbitrary closed arc on the unit circle; an arbitrary interval of the form [x, y], wherexy ≠ 0 and −1≤1x≤y≤1. We also show that there does not exist an interpolating Blaschke product having [0,y] or [x, 0] as a radial cluster set. On the other hand, there do exist finite products of interpolating Blaschke products that have [0, 1] as a radial cluster set. Research supported by the RIP-program Oberwolfach, 2002/2003.     

On the Blaschke isoparametric hypersurfaces in the unit sphere

Given an immersed submanifold x ： M^M → S^n in the unit sphere S^n without umbilics, a Blaschke eigenvalue of x is by definition an eigenvalue of the Blaschke tensor of x. x is called Blaschke isoparametric if its Mobius form vanishes identically and all of its Blaschke eigenvalues are constant. Then the classification of Blaschke isoparametric hypersurfaces is natural and interesting in the MSbius geometry of submanifolds. When n = 4, the corresponding classification theorem was given by the authors. In this paper, we are able to complete the corresponding classification for n = 5. In particular, we shall prove that all the Blaschke isoparametric hypersurfaces in S^5 with more than two distinct Blaschke eigenvalues are necessarily Mobius isoparametric.     

Classification of the Blaschke Isoparametric Hypersurfaces with Three Distinct Blaschke Eigenvalues

An immersed umbilic-free submanifold in the unit sphere is called Blaschke isoparametric if its Möbius form vanishes identically and all of its Blaschke eigenvalues are constant. Then the classification of Blaschke isoparametric hypersurfaces is natural and interesting in the Möbius geometry of submanifolds. In this paper, we give a classification of the Blaschke isoparametric hypersurfaces with three distinct Blaschke eigenvalues one of which is simple.     

Quotient Algebras of Toeplitz-Composition C^{*}-Algebras for Finite Blaschke Products

Let $R$ be a finite Blaschke product. We study the $C^*$ -algebra $\mathcal TC _R$ generated by both the composition operator $C_R$ and the Toeplitz operator $T_z$ on the Hardy space. We show that the simplicity of the quotient algebra $\mathcal OC _R$ by the ideal of the compact operators can be characterized by the dynamics near the Denjoy–Wolff point of $R$ if the degree of $R$ is at least two. Moreover we prove that the degree of finite Blaschke products is a complete isomorphism invariant for the class of $\mathcal OC _R$ such that $R$ is a finite Blaschke product of degree at least two and the Julia set of $R$ is the unit circle, using the Kirchberg–Phillips classification theorem.     

Kolmogorov diameters of entire functions of given growth

The paper establishes the decrease rate of the Kolmogorov diameters of entire functions from the space L ²(0, ∞) in terms of minimal Blaschke products on the singularity sets of Borel transforms. Besides, in C(K) the decrease rate of the Kolmogorov diameters is calculated for entire functions with given finite order.     

Finite Blaschke product interpolation on the closed unit disc

We show how to construct all finite Blaschke product solutions and the minimal scaled Blaschke product solution to the Nevanlinna-Pick interpolation problem in the open unit disc by solving eigenvalue problems of the interpolation data. Based on a result of Jones and Ruscheweyh we note that there always exists a finite Blaschke product of degree at most n−1 that maps n distinct points in the closed unit disc, of which at least one is on the unit circle, into n arbitrary points in the closed unit disc, provided that the points inside the unit circle form a positive semi-definite Pick matrix of full rank. Finally, we discuss a numerical limiting procedure.     

On Zeros of Certain Analytic Functions

Given a function s which is analytic and bounded by one in modulus in the open unit disk \mathbb D{{\mathbb D}} and given a finite Blaschke product J{\vartheta} of degree k, we relate the number of zeros of the function s-J{s-\vartheta} inside \mathbb D{{\mathbb D}} to the number of boundary zeros of special type of the same function.     

Immersed Hypersurfaces in the Unit Sphere S^m＋1 with Constant Blaschke Eigenvalues

For an immersed submanifold x ： M^m→ Sn in the unit sphere S^n without umbilics, an eigenvalue of the Blaschke tensor of x is called a Blaschke eigenvalue of x. It is interesting to determine all hypersurfaces in Sn with constant Blaschke eigenvalues. In this paper, we are able to classify all immersed hypersurfaces in S^m＋1 with vanishing MSbius form and constant Blaschke eigenvalues, in case （1） x has exact two distinct Blaschke eigenvalues, or （2） m = 3. With these classifications, some interesting examples are also presented.     

A complete classification of Blaschke parallel submanifolds with vanishing Mbius form

The Blaschke tensor and the Mbius form are two of the fundamental invariants in the Mobius geometry of submanifolds;an umbilic-free immersed submanifold in real space forms is called Blaschke parallel if its Blaschke tensor is parallel.We prove a theorem which,together with the known classification result for Mobius isotropic submanifolds,successfully establishes a complete classification of the Blaschke parallel submanifolds in S~n with vanishing Mobius form.Before doing so,a broad class of new examples of general codimensions is explicitly constructed.     

Inequalities for Rational Functions

A new hyperbolic area estimate for the level sets of finite Blaschke products is presented. The following inversion of the usual Sobolev embedding theorem is proved:

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Hereris a rational function of degreenwith poles outside . This estimate implies a new inverse theorem for rational approximation of analytic functions with respect to areaL^pnorm.     

Boundary Interpolation by Finite Blaschke Products

Given 2n distinct points z₁, z′₁, z₂, z′₂, ..., z_n, z′_n (in this order) on the unit circle, and n points w₁, ..., w_n on the unit circle, we show how to construct a Blaschke product B of degree n such that B(z_j) = w_j for all j and, in addition, B(z′_j) = B(z′_k) for all j and k. Modifying this example yields a Blaschke product of degree n - 1 that interpolates the z_j's to the w_j's. We present two methods for constructing our Blaschke products: one reminiscent of Lagrange's interpolation method and the second reminiscent of Newton's method. We show that locating the zeros of our Blaschke product is related to another fascinating problem in complex analysis: the Sendov Conjecture. We use this fact to obtain estimates on the location of the zeros of the Blaschke product.