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Eric T. Sawyer

Publications and source records attributed to Eric T. Sawyer.

At least 37 records · Page 2Linked to original sources
arXiv2020

Continuity of infinitely degenerate weak solutions via the trace method

In 1971 Fediĭ proved the remarkable theorem that the linear second order partial differential operator in the plane with coefficients 1 and f^2 is hypoelliptic provided that f is smooth, vanishes at the origin and is positive otherwise. Variants of this result, with hypoellipticity replaced by continuity of weak solutions, were recently given by the authors, together with Cristian Rios and Ruipeng Shen, to infinitely degenerate elliptic divergence form equations where the nonnegative matrix A(x,u) has bounded measurable coefficients with trace roughly 1 and determinant comparable to f, and where F=ln(1/f) is essentially doubling. However, in the plane, these variants assumed additional geometric constraints on f, something not required in Fediĭ's theorem. In this paper we in particular remove these additional geometric constraints in the plane for homogeneous equations with F essentially doubling.

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arXiv2020

A two weight inequality for Calderón-Zygmund operators on spaces of homogeneous type with applications

Let $(X,d,μ)$ be a space of homogeneous type in the sense of Coifman and Weiss, i.e. $d$ is a quasi metric on $X$ and $μ$ is a positive measure satisfying the doubling condition. Suppose that $u$ and $v$ are two locally finite positive Borel measures on $(X,d,μ)$. Subject to the pair of weights satisfying a side condition, we characterize the boundedness of a Calderón--Zygmund operator $T$ from $L^{2}(u)$ to $L^{2}(v)$ in terms of the $A_{2}$ condition and two testing conditions. For every cube $B\subset X$, we have the following testing conditions, with $\mathbf{1}_{B}$ taken as the indicator of $B$ \begin{equation*} \Vert T(u\mathbf{1}_{B})\Vert _{L^{2}(B, v)}\leq \mathcal{T}\Vert 1_{B}\Vert _{L^{2}(u)}, \end{equation*} \begin{equation*} \Vert T^{\ast }(v\mathbf{1}_{B})\Vert _{L^{2}(B, u)}\leq \mathcal{T}\Vert 1_{B}\Vert _{L^{2}(v)}. \end{equation*} The proof uses stopping cubes and corona decompositions originating in work of Nazarov, Treil and Volberg, along with the pivotal side condition.

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arXiv2019

A restricted weak type inequality with application to a Tp theorem and optimal cancellation conditions for CZO's

This paper continues the investigation begun in arXiv:1906.05602 of extending the T1 theorem of David and Journé, and optimal cancellation conditions, to more general weight pairs. The main additional tool developed here is a two weight restricted weak type inequality. Assuming σ and ω are locally finite positive Borel measures, with one of them an A infinity weight, we show that the two weight restricted weak type inequality for an α-fractional Calderón-Zygmund singular integral T holds if and (provided T is elliptic) only if the classical fractional Muckenhoupt condition holds. In the case α = 0, boundedness of T on unweighted L2 is assumed as well. Applications are then given to a Tp theorem for CZO's with doubling weights when one is A infinity, and then to optimal cancellation conditions for CZO's in terms of polynomial testing in similar situations.

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arXiv2019

T1 testing implies Tp polynomial testing: optimal cancellation conditions for CZO's

This paper is the third in an investigation begun in arXiv:1906.05602 and arXiv:1907.07571 of extending the T1 theorem of David and Journé, and optimal cancellation conditions, to more general weight pairs. The main result here is that the familiar T1 testing conditions over indicators of cubes, together with the one-tailed A2 conditions, imply polynomial testing. Analogous results for fractional singular integrals hold as well. Applications include a T1 theorem for fractional CZO's T in the case of doubling measures when one of the weights is A infinity, and then to optimal cancellation conditions for such CZO's in similar situations.

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arXiv2019

Control of the Bilinear Indicator Cube Testing property

We show that the α-fractional Bilinear Indicator/Cube Testing Constant arising in arXiv:1906.05602 is controlled by the classical fractional Muckenhoupt constant, provided the product measure σ x ω is diagonally reverse doubling (in particular if it is reverse doubling) with exponent exceeding 2(n-α). Moreover, this control is sharp within the class of diagonally reverse doubling product measures. When combined with the main results in arXiv:1906.05602, 1907.07571 and 1907.10734, the above control of BICT_{T^{α}} for α>0 yields a two weight T1 theorem for doubling weights with appropriate diagonal reverse doubling, i.e. the norm inequality for T^{α} is controlled by cube testing constants and the α-fractional one-tailed Muckenhoupt constants (without any energy assumptions), and also yields a corresponding cancellation condition theorem for the kernel of T^{α}, both of which hold for arbitrary α-fractional Calderón-Zygmund operators T^{α}. We do not know if the analogous result for BICT_{H}(σ,ω) holds for the Hilbert transform H in case α=0, but we show that BICT_{H^{dy}}(σ,ω) is not controlled by the Muckenhoupt condition for the dyadic Hilbert transform H^{dy} and doubling weights σ,ω.

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arXiv2019

Restricted testing for positive operators

We prove that for certain positive operators $T$, such as the Hardy-Littlewood maximal function and fractional integrals, there is a constant $D>1$, depending only on the dimension $n$, such that the two weight norm inequality \begin{equation*} \int_{\mathbb{R}^{n}}T\left( fσ\right) ^{2}dω\leq C\int_{\mathbb{ R}^{n}}f^{2}dσ\end{equation*} holds for all $f\geq 0$ if and only if the (fractional) $A_{2}$ condition holds, and the restricted testing condition \begin{equation*} \int_{Q}T\left( 1_{Q}σ\right) ^{2}dω\leq C\left\ | Q\right\ |_{σ} \end{equation*} holds for all cubes $Q$ satisfying $\left\ | 2Q\right\ |_{σ}\leq D\left\ | Q\right\ |_{σ}$. If $T$ is linear, we require as well that the dual restricted testing condition \begin{equation*} \int_{Q}T^{\ast }\left( 1_{Q}ω\right) ^{2}dσ\leq C\left\ | Q\right\ |_{ω} \end{equation*} holds for all cubes $Q$ satisfying $\left\ | 2Q\right\ |_{ω}\leq D\left\ | Q\right\ |_{ω}$.

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arXiv2019

A two weight local Tb theorem for the Hilbert transform

We obtain a two weight local Tb theorem for any elliptic and gradient elliptic fractional singular integral operator T on the real line, and any pair of locally finite positive Borel measures on the line. This includes the Hilbert transform and in a sense improves on the T1 theorem by the authors and M. Lacey.

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arXiv2019

Weighted Alpert Wavelets

In this paper we construct a wavelet basis in weighted L^2 of Euclidean space possessing vanishing moments of a fixed order for a general locally finite positive Borel measure. The approach is based on a clever construction of Alpert in the case of Lebesgue measure that is appropriately modified to handle the general measures considered here. We then use this new wavelet basis to study a two-weight inequality for a general Calderón-Zygmund operator on the real line and show that under suitable natural conditions, including a weaker energy condition, the operator is bounded from one weighted L^2 space to another if certain stronger testing conditions hold on polynomials. An example is provided showing that this result is logically different than existing results in the literature.

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arXiv2018

Restricted testing for the Hardy-Littlewood maximal function

We answer a special case of a question of T. Hytonen regarding the two weight norm inequality for the maximal function M in the affirmative, namely that there is a constant D > 1, depending only on dimension n, such that the two weight norm inequality for M holds if and only if the A_2 condition holds, and the testing condition holds for all doubling cubes.

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arXiv2018

Weighted inequalities for product fractional integrals

We investigate one and two weight norm inequalities for product fractional integrals. We show that in the one weight case, most of the 1 parameter theory carries over to the 2 parameter setting. However, in the two weight case, apart from the trivial case of product weights, the rectangle characteristic never controls the operator norm without side conditions. The Stein-Weiss extension of the classical Hardy-Littlewood-Sobolev inequality carries over to the setting of 2 parameters with nonproduct power weights using a sandwiching technique.

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arXiv2018

Two Weight Inequalities for the Cauchy Transform from $ \mathbb{R}$ to $ \mathbb{C}_+$

We characterize those pairs of weights $ σ$ on $ \mathbb{R}$ and $ τ$ on $ \mathbb{C}_+$ for which the Cauchy transform $\mathsf{C}_σ f (z) \equiv \int_{\mathbb{R}} \frac {f(x)} {x-z} \; σ(dx)$, $ z\in \mathbb{C}_+$, is bounded from $L ^2(\mathbb{R};σ)$ to $L ^{2}(\mathbb{C}_+; τ)$. The characterization is in terms of an $A_2$ condition on the pair of weights and testing conditions for the transform, extending the recent solution of the two weight inequality for the Hilbert transform. As corollaries of this result we derive (1) a characterization of embedding measures for the model space $K_\vartheta$, for arbitrary inner function $ \vartheta $, and (2) a characterization of the (essential) norm of composition operators mapping $K_\vartheta$ into a general class of Hardy and Bergman spaces.

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arXiv2018

Energy conditions and twisted localizations of operators

We show that the energy conditions are not necessary for boundedness of fractional Riesz transforms in dimension at least 2. We also give a weak converse, namely that the energy conditions are necessary for boundedness of families of twisted localizations of fractional singular integrals having the positive gradient property.

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arXiv2017

Failure of necessity of the energy condition

We give an example of a pair of weights (u,v) on the line, and an elliptic convolution singular integral operator H on the line, such that H_u is bounded from L^2(u) to L^2(v), yet the measure pair (u,v) fails to satisfy the backward energy condition. The key to the construction is that the kernel K of H has flat spots where d/dx K(x) = 0. Conversely, we show that if H is gradient elliptic, i.e. d/dx K(x) =< c < 0, then the energy conditions are necessary for boundedness of H, and by our theorem in arXiv:1603.04332v2, the T1 theorem holds for H.

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arXiv2017

The Corona Problem for Kernel Multiplier Algebras

We prove an alternate Toeplitz corona theorem for the algebras of pointwise kernel multipliers of Besov-Sobolev spaces on the unit ball in $\mathbb{C}^{n}$, and for the algebra of bounded analytic functions on certain strictly pseudoconvex domains and polydiscs in higher dimensions as well. This alternate Toeplitz corona theorem extends to more general Hilbert function spaces where it does not require the complete Pick property. Instead, the kernel functions $k_{x}\left(y\right)$ of certain Hilbert function spaces $\mathcal{H}$ are assumed to be invertible multipliers on $\mathcal{H}$, and then we continue a research thread begun by Agler and McCarthy in 1999, and continued by Amar in 2003, and most recently by Trent and Wick in 2009. In dimension $n=1$ we prove the corona theorem for the kernel multiplier algebras of Besov-Sobolev Banach spaces in the unit disk, extending the result for Hilbert spaces $H^\infty\cap Q_p$ by A. Nicolau and J. Xiao.

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arXiv2016

A Good-Lambda Lemma, two weight T1 theorems without weak boundedness, and a two weight accretive global Tb theorem

The weak boundedness property associated with a standard alpha-fractional Calderon-Zygmund operator and a weight pair is good-lambda controlled by the testing conditions and the Muckenhoupt and energy side conditions. As a consequence, assuming the side conditions, we can eliminate the weak boundedness property from Theorem 1 of arXiv:1603.04332v2 to obtain that T is bounded if and only if the testing conditions hold for T and its dual (an earlier instance of this type of conclusion appears in Lacey and Wick arXiv:1312.6163v3). As a corollary we give a simple derivation of a two weight accretive global Tb theorem from a related T1 theorem. The role of two different parameterizations of the family of dyadic grids, by scale and by translation, is highlighted in simultaneously exploiting both goodness and NTV surgery with families of grids that are common to both measures.

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arXiv2016

A two weight fractional singular integral theorem with side conditions, energy and k-energy dispersed

This paper is a sequel to our paper Rev. Mat. Iberoam. 32 (2016), no. 1, 79-174. Let T be a standard fractional Calderon Zygmund operator. Assume appropriate Muckenhoupt and quasienergy side conditions. Then we show that T is bounded from one weighted space to another if the quasicube testing conditions hold for T and its dual, and if the quasiweak boundedness property holds for T. Conversely, if T is bounded, then the quasitesting conditions hold, and the quasiweak boundedness condition holds. If the vector of fractional Riesz transforms (or more generally a strongly elliptic vector of transforms) is bounded, then the appropriate Muckenhoupt conditions hold. We do not know if our quasienergy conditions are necessary in higher dimensions, except for certain situations in which one of the measures is one-dimensional as in arXiv:1310.4820 and arXiv:1505.07822v4, and for certain side conditions placed on the measures such as doubling and k-energy dispersed, which when k=n-1 is similar to the condition of uniformly full dimension in Lacey and Wick arXiv:1312.6163v3.

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