arXiv subjects
Publications and source records attributed to Alexander Borichev.
We study the zeros of functions in the Dirichlet space. Using extremal functions, we produce a necessary and sufficient condition for a sequence of points in the unit disk to be a zero set of the classical Dirichlet space. This Shapiro-Shields type condition involves kernels of the harmonic Dirichlet space associated with measures depending on the zero sequence.
We derive lower bounds in best rational approximation of given degree to finite Blaschke products, in the Hardy space $H^2$ of the unit disk. We first consider approximation to $z^N$, and then move on to more general Blaschke products whose zeros are bounded away from the circle. The latter case depends on Fourier coefficients estimates for Blaschke products which are of independent interest.
We establish the asymptotic sharpness of a Nikolskii type inequality proved by A. Baranov and R. Zarouf for rational functions $f$ in the Wiener algebra of absolutely convergent Fourier series, with at most $n$ poles, all lying outside the dilated disc $\frac{1}{\lambda}\mathbb{D}$, where $\mathbb{D}$ denotes the open unit disc and $\lambda\in[0,1)$ is fixed. More precisely, this inequality tells that the Wiener norm of such functions is bounded by their $H^{2}$-norm -- i.e., their norm in the Hardy space of the disc -- times a factor of order $\sqrt{\frac{n}{1-\lambda}}$. In this paper, we construct explicit test functions showing that this bound cannot be improved in general: the inequality is asymptotically sharp as $n\to\infty$, up to a universal constant, for every fixed $\lambda\in[0,1)$.
We consider the commutativity problem for the Berezin transform on weighted Fock spaces. Given a real number $m>0$, for every $\alpha >0$ we denote by $B_{\alpha}$ the Berezin transform associated to the measure $\mu_{m}^{\alpha}$ with density proportional to $e^{-\alpha |z|^m}$ with respect to Lebesgue measure on the complex plane and normalized so that $\mu_{\phi}^{\alpha}(\mathbb C)=1$. We show that the commutativity relation $B_{\alpha}B_{\beta}f=B_{\beta}B_{\alpha}f$ holds for all $f\in L^{\infty}(\mathbb C)$ and $\alpha,\beta> 0$ if and only if $m=2$.
We consider inner functions $\Theta$ with the zero set $\mathcal Z(\Theta)$ such that the quotient algebra $H^\infty / \Theta H^\infty$ satisfies the Strong Invertibility Property (SIP), that is for every $\varepsilon>0$ there exists $\delta>0$ such that the conditions $f \in H^\infty$, $\|[f]\|_{H^\infty/ \Theta H^\infty}=1$, $\inf_{\mathcal Z(\Theta)} |f| \ge 1-\delta$ imply that $[f]$ is invertible in $H^\infty / \Theta H^\infty$ and $\| 1/ [f] \|_{H^\infty/ \Theta H^\infty}\le 1+\varepsilon$. We prove that the SIP is equivalent to the maximal asymptotic growth of $\Theta $ away from its zero set. We also describe inner functions satisfying the SIP in terms of the narrowness of their sublevel sets and relate the SIP to the Weak Embedding Property introduced by P.Gorkin, R.Mortini, and N.Nikolski as well as to inner functions whose Frostman shifts are Carleson--Newman Blaschke products. We finally study divisors of inner functions satisfying the SIP. We describe geometrically the zero set of inner functions such that all its divisors satisfy the SIP. We also prove that a closed subset $E$ of the unit circle is of finite entropy if and only if any singular inner function associated to a singular measure supported on $E$ is a divisor of an inner function satisfying the SIP.
We establish a relation between the approximation in $L^2[-\pi,\pi]$ by exponentials with the set of frequencies of Beurling--Malliavin density less than $1$ and the meromorphic interpolation at $\mathbb Z$. Furthermore, we show that typical $L^2[-\pi,\pi]$ functions admit such an approximation.
We prove here the concentration of measure inequality for Lipschitz function on the Hamming cube with values in any Banach spaces of finite cotype.
Given a finite Blaschke product $B$ we prove asymptotically sharp estimates on the $\ell^{\infty}$-norm of the sequence of the Fourier coefficients of $B^{n}$ as $n$ tends to $\infty$. We provide constructive examples which show that our estimates are sharp. As an application we construct a sequence of $n\times n$ invertible matrices $T$ with arbitrary spectrum in the unit disk and such that the quantity $|\det{T}|\cdot\|T^{-1}\|\cdot\|T\|^{1-n}$ grows as a power of $n$. This is motivated by Schäffer's question on norms of inverses.
We study the indivisible intervals and the monotonicity of the growth of the exponential type in the chains of de Branges subspaces in terms of the spectral measure. We prove that for spectral measures supported on $\mathbb Z$, there exist at most two subspaces of the same type, which then bound an indivisible interval. Furthermore, in this case, we study possible locations of the indivisible intervals.
We give a negative answer to the Newman--Shapiro problem on weighted approximation for entire functions formulated in 1966 and motivated by the theory of operators on the Fock space. There exists a function in the Fock space such that its exponential multiples do not approximate some entire multiples in the space. Furthermore, we establish several positive results under different restrictions on the function in question.
We compute asymptotic formulas for the $k^{\rm th}$ Fourier coefficients of $b_λ^n$, where $b_λ(z)=\frac{z-λ}{1-λz}$ is the Blaschke factor associated to $λ\in\mathbb{D}$, $k\in[0,\infty)$ and $n$ is a large integer. We distinguish several regions of different asymptotic behavior of those coefficients in terms of $k$ and $n$. Given $β\in((1-λ)/(1+λ),(1+λ)/(1-λ))$ their decay is oscillatory for $k\in[βn,n/β]$. Given $α\in(0,(1-λ)/(1+λ))$ their decay is exponential for $k\in[0,nα]\cup[n/α,\infty).$ Airy-type behavior is happening near the $k$-transition points $n(1-λ)/(1+λ)$ and $n(1+λ)/(1-λ)$. The asymptotic formulas for the $k^{\rm th}$ Fourier coefficients of $b_λ^{n}$ are derived using standard tools of asymptotic analysis of Laplace-type integrals. More precisely, the integral defining the $k^{\rm th}$ Fourier coefficient of $b_λ^n$ is perfectly suited for an application of the method of stationary phase when $k\in\left(n(1-λ)/(1+λ),n(1+λ)/(1-λ)\right)$ and requires the use of the method of the steepest descent when $k\notin[n(1-λ)/(1+λ),n(1+λ)/(1-λ)]$. Uniform versions of those standard methods are required when $k$ approaches one of the boundaries $n(1-λ)/(1+λ),$ $n(1+λ)/(1-λ)$. As an application, we construct strongly annular functions with Taylor coefficients satisfying sharp summation properties.
We study the distribution of zeroes of power series with infinite radius of convergence. The coefficients of the series have the form $\xi(n)a(n)$, where $a$ is a smooth sequence of positive numbers, and $\xi$ is a sequence of complex-valued multipliers having binary correlations and no gaps in the spectrum. We show that under certain assumptions on the smoothness of the sequence $a$ and on the binary correlations of the multipliers $\xi$, the zeroes of the power series are equidistributed with respect to a radial measure defined by the sequence $a$. We apply our approach to several examples of the sequence $\xi$: (i) IID sequences, (ii) sequences $e(\alpha n^2)$ with Diophantine $\alpha$, (iii) random multiplicative sequences, (iv) the Golay--Rudin--Shapiro sequence, (v) the indicator function of the square-free integers, (vi) the Thue--Morse sequence.
We study the weighted Fock spaces in one and several complex variables. We evaluate the dimension of these spaces in terms of the weight function extending and completing earlier results by Rozenblum-Shirokov and Shigekawa.
We solve Chui's conjecture on the simplest fractions (i.e., sums of Cauchy kernels with unit coefficients) in weighted (Hilbert) Bergman spaces. Namely, for a wide class of weights, we prove that for every $N$, the simplest fractions with $N$ poles on the unit circle have minimal norm if and only if the poles are equispaced on the circle. We find sharp asymptotics of these norms. Furthermore, we describe the closure of the simplest fractions in weighted Bergman spaces, using an $L^2$ version of Thompson's theorem on dominated approximation by simplest fractions.
We study the upper and the lower densities of complete and minimal Gabor Gaussians systems. In contrast to the classical lattice case when they are both equal to $1$, we prove that the lower density may reach $0$ while the upper density may vary at least from $\frac{1}π$ to $e$. In the case when the upper density exceeds $1$, we establish a sharp inequality relating the upper and the lower densities.
In this part, we prove several quantitative results concerning with the Szego minimum problem for classes of measure on the unit circle concentrated on small subsets. As a by-product, we refute one conjecture of Nevai. This note can be read independently from the first one.
The classical Szego polynomial approximation theorem states that the polynomials are dense in the space $L^2(ρ)$, where $ρ$ is a measure on the unit circle, if and only if the logarithmic integral of the measure $ρ$ diverges. In this note we give a quantitative version of Szego's theorem in the special case when the divergence of the logarithmic integral is caused by deep zeroes of the measure $ρ$ on a sufficiently rare subset of the circle.
Given a sequence $\xi\colon \mathbb Z_+ \to \mathbb C$, we find a simple spectral condition which guarantees the angular equidistribution of the zeroes of the Taylor series \[ F_\xi (z) = \sum_{n\ge 0} \xi (n) \frac{z^n}{n!}\,. \] This condition yields practically all known instances of random and pseudo-random sequences $\xi$ with this property (due to Nassif, Littlewood, Chen-Littlewood, Levin, Eremenko-Ostrovskii, Kabluchko-Zaporozhets, Borichev-Nishry-Sodin), and provides several new ones. Among them are Besicovitch almost periodic sequences and multiplicative random sequences. It also conditionally yields that the M\"obius function $\mu$ has this property assuming "the binary Chowla conjecture".