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Alexander Volberg

Publications and source records attributed to Alexander Volberg.

At least 55 records · Page 3Linked to original sources
arXiv2020

Bi-parameter Carleson embeddings with product weights

Coifman--Meyer multipliers represent a very important class of bi-linear singular operators, which were extensively studied and generalized. They have a natural multi-parameter counterpart. Decomposition of those operators into paraproducts, and, more generally to multi-parameter paraproducts is a staple of the theory. In this paper we consider weighted estimates for bi-parameter paraproducts that appear from such multipliers. Then we apply our harmonic analysis results to several complex variables. Namely, we show that a (weighted) Carleson embedding for a scale of Dirichlet spaces from the bi-torus to the bi-disc is equivalent to a simple ``box'' condition, for product weights on the bi-disc and arbitrary weights on the bi-torus. This gives a new simple necessary and sufficient condition for the embedding of the whole scale of weighted Dirichlet spaces of holomorphic functions on the bi-disc. This scale of Dirichlet spaces includes the classical Dirichlet space on the bi-disc. Our result is in contrast to the classical situation on the bi-disc considered by Chang and Fefferman, when a counterexample due to Carleson shows that the ``box'' condition does not suffice for the embedding to hold. But this was the embedding of bi-harmonic functions in bi-harmonic Hardy class. Our result can be viewed as a new and unexpected combinatorial property of all positive finite planar measures.

math.AP↗
arXiv2020

Carleson embedding on tri-tree and on tri-disc

We prove multi-parameter dyadic embedding theorem for Hardy operator on the multi-tree. We also show that for a large class of Dirichlet spaces in bi-disc and tri-disc this proves the embedding theorem of those Dirichlet spaces of holomorphic function on bi- and tri-disc. We completely describe the Carleson measures for such embeddings. The result below generalizes embedding result of \cite{AMPVZ} from bi-tree to tri-tree. One of our embedding description is similar to Carleson--Chang--Fefferman condition and involves dyadic open sets. On the other hand, the unusual feature of \cite{AMPVZ} was that embedding on bi-tree turned out to be equivalent to one box Carleson condition. This is in striking difference to works of Chang--Fefferman and well known Carleson quilt counterexample. We prove here the same unexpected result for the tri-tree. Finally, we explain the obstacle that prevents us from proving our results on polydiscs of dimension four and higher.

math.AP↗
arXiv2020

Rademacher type and Enflo type coincide

A nonlinear analogue of the Rademacher type of a Banach space was introduced in classical work of Enflo. The key feature of Enflo type is that its definition uses only the metric structure of the Banach space, while the definition of Rademacher type relies on its linear structure. We prove that Rademacher type and Enflo type coincide, settling a long-standing open problem in Banach space theory. The proof is based on a novel dimension-free analogue of Pisier's inequality on the discrete cube.

math.FA↗
arXiv2020

Dimension of the exceptional set in the Aronszajn-Donoghue theorem for finite rank perturbations

The classical Aronszajn-Donoghue theorem states that for a rank one perturbation of a self-adjoint operator (by a cyclic vector) the singular parts of the spectral measures of the original and perturbed operators are mutually singular. As simple direct sum type examples show, this result does not hold for finite rank perturbations. However, the set of exceptional perturbations is pretty small. Namely, for a family of rank $d$ perturbations $A_{\boldsymbolα} := A +\mathbf{B} \boldsymbolα \mathbf{B}^*$, $\mathbf{B}:\mathbb{C}^d\to \mathbf{H}$, with Ran$\,\mathbf{B}$ being cyclic for $A$, parametrized by $d\times d$ Hermitian matrices $\boldsymbolα$, the singular parts of the spectral measures of $A$ and $A_{\boldsymbolα}$ are mutually singular for all $\boldsymbolα$ except for a small exceptional set $E$. It was shown earlier by the first two authors that $E$ is a subset of measure zero of the space $\mathbf{H}(d)$ of $d\times d$ Hermitian matrices. In this paper we show that the set $E$ has small Hausdorff dimension, $\dim E \le \dim\mathbf{H}(d)-1 = d^2-1$.

math.FA↗
arXiv2019

Bi-parameter embedding and measures with restriction energy condition

Nicola Arcozzi, Pavel Mozolyako, Karl-Mikael Perfekt, and Giulia Sarfatti recently gave the proof of a bi-parameter Carleson embedding theorem. Their proof uses heavily the notion of capacity on bi-tree. In this note we give one more proof of a bi-parameter Carleson embedding theorem that avoids the use of bi-tree capacity. Unlike the proof on a simple tree (in a pervious paper of the authors) that used the Bellman function technique, the proof here is based on some rather subtle comparison of energies of measures on bi-tree.

math.CA↗
arXiv2019

Improving constant in end-point Poincaré inequality on Hamming cube

We improve the constant $\fracπ{2}$ in $L^1$-Poincaré inequality on Hamming cube. For Gaussian space the sharp constant in $L^1$ inequality is known, and it is $\sqrt{\fracπ{2}}$. For Hamming cube the sharp constant is not known, and $\sqrt{\fracπ{2}}$ gives an estimate from below for this sharp constant. On the other hand, L. Ben Efraim and F. Lust-Piquard have shown an estimate from above: $C_1\le \fracπ{2}$. There are at least two other independent proofs of the same estimate from above (we write down them in this note). Since those proofs are very different from the proof of Ben Efraim and Lust-Piquard but gave the same constant, that might have indicated that constant is sharp. But here we give a better estimate from above, showing that $C_1$ is strictly smaller than $\fracπ{2}$. It is still not clear whether $C_1> \sqrt{\fracπ{2}}$. We discuss this circle of questions and the computer experiments.

math.PR↗
arXiv2019

The sharp square function estimate with matrix weight

We prove the matrix $A_2$ conjecture for the dyadic square function, that is, a norm estimate of the matrix weighted square function, where the focus is on the sharp linear dependence on the matrix $A_2$ constant in the estimate. Moreover, we give a mixed estimate in terms of $A_2$ and $A_{\infty}$ constants. Key is a sparse domination of a process inspired by the integrated form of the matrix--weighted square function.

math.CA↗
arXiv2019

A comparison of box and Carleson conditions on bi-trees

In this note we give an example of measure satisfying the box condition on certain sub-bi-trees (see below) but not satisfying Carleson condition on those sub-bi-trees. This can be considered as a certain counterexample for two weight bi-parameter embedding of Carleson type. Our type of counterexample is impossible for a simple tree. In the case of a simple tree, the box condition, Carleson condition and two weight embedding are all equivalent. In the last section we show that bi-parameter box condition implies the bi-parameter capacitary estimate for dyadic rectangles.

math.CA↗
arXiv2019

Counting eigenvalues of Schrödinger operator with complex fast decreasing potential

We give a sharp estimate of the number of zeros of analytic functions in the unit disc belonging to analytic quasianalytic Carleman--Gevrey classes. As an application, we estimate the number of the eigenvalues for discrete Schrödinger operators with rapidly decreasing complex-valued potentials, and, more generally, for non-symmetric Jacobi matrices.

math.CA↗
arXiv2018

On the Sign Distributions of Hilbert Space Frames

We show that the positive and negative parts $ u_{k}^{\pm }$ of any frame in a real $ L^{2}$ space with respect to a continuous measure have both "infinite $ l^{2}$ masses": 1) always, $ \sum _{k}u_{k}^{\pm }(x)^{2}=\infty $ almost everywhere (in particular, there exist no positive frames, nor Riesz bases), but 2) $ \sum _{k=1}^{n}(u_{k}^{+}(x)-u_{k}^{-}(x))^{2}$ can grow "locally" as slow as we wish (for $ n\longrightarrow \infty $), and 3) it can happen that $ \sum _{k=1}^{n}u_{k}^{-}(x)^{2}=\, o(\sum _{k=1}^{n}u_{k}^{+}(x)^{2})$, and vice versa, as $ n\longrightarrow \infty $ on a set of positive measure. Property 1) for the case of an orthonormal basis in $ L^{2}(0,1)$ was settled earlier (V. Ya. Kozlov, 1948) using completely different (and more involved) arguments. Our elementary treatment includes also the case of unconditional bases in a variety of Banach spaces. For property 2), we show that, moreover, whatever is a monotone sequence $ ε_{k}>0$ satisfying $ \sum _{k}ε^{2}_{k}=\, \infty $ there exists an orthonormal basis $ (u_{k})_{k\, }$in $ L^{2}$ such that $ \vert u_{k}(x)\vert \leq \, A(x)ε_{k}$, $ 0<A(x)<\, \infty $.

math.FA↗
arXiv2018

The Sharp Constant in the Weak (1,1) Inequality for the Square Function: A New Proof

In this note we give a new proof of the sharp constant $C = e^{-1/2} + \int_0^1 e^{-x^2/2}\,dx$ in the weak (1, 1) inequality for the dyadic square function. The proof makes use of two Bellman functions $\mathbb{L}$ and $\mathbb{M}$ related to the problem, and relies on certain relationships between $\mathbb{L}$ and $\mathbb{M}$, as well as the boundary values of these functions, which we find explicitly. Moreover, these Bellman functions exhibit an interesting behavior: the boundary solution for $\mathbb{M}$ yields the optimal obstacle condition for $\mathbb{L}$, and vice versa.

math.CA↗
arXiv2018

Bellman function sitting on a tree

In this note we give a proof-by-formula of certain important embedding inequalities on dyadic tree. This is done with the help of Bellman function. We also consider the case of a bi-tree, where a different approach is explained.

math.CA↗
arXiv2018

Martingale transform and Square function: some weak and restricted weak sharp weighted estimates

Following the ideas of A. Lerner, F. Nazarov, S. Ombrosi from [12] we prove that there is a sequence of weights $w\in A^d_1$ such that $[w]^d_{A_1}\to \infty$, and martingale transforms $T$ such that with an absolute positive $c$ $\|T: L^1(w) \to L^{1, \infty}(w)\| \ge c [w]^d_{A_1}\log [w]^d_{A_1}$. We also show the existence of the sequence of weights (now in $A_2$) such that $[w]^d_{A_2}\to \infty$, and such that the following holds: $[w]_{A_2^d}\asymp \|M^d\|_{w^{-1}}^2$; $\|S_{w}: L^{2} (w) \to L^2(w^{-1})\| \ge c\, \|M^d\|_{w^{-1}}\sqrt{\log \|M^d\|_{w^{-1}}}$; $\|S_{w}: L^{2,1} (w) \to L^2(w^{-1})\| \ge c\, \|M^d\|_{w^{-1}}\sqrt{\log \|M^d\|_{w^{-1}}}; $ $\|S: L^2(w)\to L^{2, \infty}(w)\|=\|S_{w^{-1}}: L^{2}(w^{-1}) \to L^{2,\infty}(w)\|\le C\, \|M^d\|_{w^{-1}}\le C\, ([w]^d_{A_2})^{1/2}$. Finally, it is shown that for test functions of the form $χ_I$ the weak norm $\|S_w χ_I\|_{L^{2, \infty}} \le C\, [w]_{A_2^d}^{1/2}\|χ_I\|_w$.

math.AP↗
arXiv2018

Poincaré type and spectral gap inequalities with fractional Laplacians on Hamming cube

We prove here some dimension free Poincaré-type inequalities on Hamming cube for functions with different spectral properties and for fractional Laplacians. In this note the main attention is paid to estimates in $L^1$ norm on Hamming cube. We build the examples showing that our assumptions on spectral properties of functions cannot be dropped in general.

math.AP↗
arXiv2018

Square functions and the Hamming cube: Duality

For $1<p\leq 2$, any $n\geq 1$ and any $f:\{-1,1\}^{n} \to \mathbb{R}$, we obtain $(\mathbb{E} |\nabla f|^{p})^{1/p} \geq C(p)(\mathbb{E}|f|^{p} - |\mathbb{E}f|^{p})^{1/p}$ where $C(p)$ is the smallest positive zero of the confluent hypergeometric function ${}_{1}F_{1}(\frac{p}{2(1-p)}, \frac{1}{2}, \frac{x^{2}}{2})$. Our approach is based on a certain duality between the classical square function estimates on the Euclidean space and the gradient estimates on the Hamming cube.

math.AP↗
arXiv2017

Convex body domination and weighted estimates with matrix weights

We introduce the so called convex body valued sparse operators, which generalize the notion of sparse operators to the case of spaces of vector valued functions. We prove that Calderón--Zygmund operators as well as Haar shifts and paraproducts can be dominated by such operators. By estimating sparse operators we obtain weighted estimates with matrix weights. We get two weight $A_2$-$A_\infty$ estimates, that in the one weight case give us the estimate $$ \|T\|_{L^2(W)\to L^2 (W)} \le C [W]_{A_2}^{1/2} [W]_{A_\infty} \le C[W]_{A_2}^{3/2} $$ where $T$ is either a Calderon--Zygmund operator (with modulus of continuity satisfying the Dini condition), or a Haar shift or a paraproduct.

math.CA↗