Subordination by orthogonal martingales in $L^{p}$ and zeros of Laguerre polynomials

Some new Bellman functions and subordination by orthogonal martingales in $L^{p}, 1<p\le 2$

Given two martingales on the filtration generated by two dimensional Brownian motion, we want to estimate the $L^p$ norm of the subordinated one if we have some extra orthogonality property available. We construct several new Bellman functions, very different from Burkholder's function, and using them give an estimate of $L^p$ norm of a subordinated martingale, if the dominating martingale is orthogonal and $1<p\le 2$. We use Monge--Ampere equation to construct these Bellman functions.

Fourier multipliers and weak differential subordination of martingales in UMD Banach spaces

In this paper we introduce the notion of weak differential subordination for martingales and show that a Banach space $X$ is a UMD Banach space if and only if for all $p\in (1,\infty)$ and all purely discontinuous $X$-valued martingales $M$ and $N$ such that $N$ is weakly differentially subordinated to $M$, one has the estimate $\mathbb E \|N_{\infty}\|^p \leq C_p\mathbb E \|M_{\infty}\|^p$. As a corollary we derive the sharp estimate for the norms of a broad class of even Fourier multipliers, which includes e.g. the second order Riesz transforms.

Subordination by orthogonal martingales in $L^{p}, 1<p\le 2$

We are given two martingales on the filtration of the two dimensional Brownian motion. One is subordinated to another. We want to give an estimate of $L^p$-norm of a subordinated one via the same norm of a dominating one. In this setting this was done by Burkholder in \cite{Bu1}--\cite{Bu8}. If one of the martingales is orthogonal, the constant should drop. This was demonstrated in \cite{BaJ1}, when the orthogonality is attached to the subordinated martingale and when $2\le p<\infty$. This note contains an (almost obvious) observation that the same idea can be used in the case when the orthogo…

可预测从属、尖锐鞅不等式及其应用

Predictable subordination, sharp martingale inequalities and applications

本文提出一种新方法，在可预测从属下获得鞅的尖锐Lp估计，通过量化并补偿Bellman函数中连续与跳跃贡献的缺陷，显著改进了Hamming立方体和Z^n上Riesz向量的显式无维界。

Martingale decompositions and weak differential subordination in UMD Banach spaces

In this paper we consider Meyer-Yoeurp decompositions for UMD Banach space-valued martingales. Namely, we prove that $X$ is a UMD Banach space if and only if for any fixed $p\in (1,\infty)$, any $X$-valued $L^p$-martingale $M$ has a unique decomposition $M = M^d + M^c$ such that $M^d$ is a purely discontinuous martingale, $M^c$ is a continuous martingale, $M^c_0=0$ and \[ \mathbb E \|M^d_{\infty}\|^p + \mathbb E \|M^c_{\infty}\|^p\leq c_{p,X} \mathbb E \|M_{\infty}\|^p. \] An analogous assertion is shown for the Yoeurp decomposition of a purely discontinuous martingales into a sum of a quasi-l…

Differential subordination under change of law

We prove optimal ${L}^2$ bounds for a pair of Hilbert space valued differentially subordinate martingales under a change of law. The change of law is given by a process called a weight and sharpness in this context refers to the optimal growth with respect to the characteristic of the weight. The pair of martingales are adapted, uniformly integrable, and cadlag. Differential subordination is in the sense of Burkholder, defined through the use of the square bracket. In the scalar dyadic setting with underlying Lebesgue measure, this was proved by Wittwer, where homogeneity was heavily used. Rec…