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## Studia Mathematica

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## Sharp inequalities for Riesz transforms

### Volume 222 / 2014

Studia Mathematica 222 (2014), 1-18 MSC: Primary 42B20, 60G44; Secondary 46E30. DOI: 10.4064/sm222-1-1

#### Abstract

We establish the following sharp local estimate for the family $\{R_j\}_{j=1}^d$ of Riesz transforms on $\mathbb R^d$. For any Borel subset $A$ of $\mathbb R^d$ and any function $f:\mathbb R^d\to \mathbb R$, $$\int_A |R_jf(x)|\,d x\leq C_p\|f\|_{L^p(\mathbb R^d)}|A|^{1/q},\quad\ 1 < p < \infty.$$ Here $q=p/(p-1)$ is the harmonic conjugate to $p$, $$C_p=\bigg[\frac{2^{q+2}\varGamma(q+1)}{\pi^{q+1}}\sum_{k=0}^\infty \frac{(-1)^k}{(2k+1)^{q+1}}\bigg]^{1/q},\quad\ 1 < p < 2,$$ and $$C_p=\bigg[\frac{4\varGamma(q+1)}{\pi^{q}}\sum_{k=0}^\infty \frac{1}{(2k+1)^{q}}\bigg]^{1/q},\quad\ 2\leq p < \infty.$$ This enables us to determine the precise values of the weak-type constants for Riesz transforms for $1 < p < \infty$. The proof rests on appropriate martingale inequalities, which are of independent interest.

#### Authors

• Adam OsękowskiFaculty of Mathematics, Informatics and Mechanics
University of Warsaw
Banacha 2
02-097 Warszawa, Poland
e-mail

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