Abstract of a paper by Tuomas Hytonen, Sean Li, and Assaf Naor
This is an announcement for the paper "Quantitative affine approximation for UMD targets" by Tuomas Hytonen, Sean Li, and Assaf Naor. Abstract: It is shown here that if $(Y,\|\cdot\|_Y)$ is a Banach space in which martingale differences are unconditional (a UMD Banach space) then there exists $c=c(Y)\in (0,\infty)$ with the following property. For every $n\in \mathbb{N}$ and $\varepsilon\in (0,1/2]$, if $(X,\|\cdot\|_X)$ is an $n$-dimensional normed space with unit ball $B_X$ and $f:B_X\to Y$ is a $1$-Lipschitz function then there exists an affine mapping $\Lambda:X\to Y$ and a sub-ball $B^*=y+\rho B_X\subseteq B_X$ of radius $\rho\ge \exp(-(1/\varepsilon)^{cn})$ such that $\|f(x)-\Lambda(x)\|_Y\le \varepsilon \rho$ for all $x\in B^*$. This estimate on the macroscopic scale of affine approximability of vector-valued Lipschitz functions is an asymptotic improvement (as $n\to \infty$) over the best previously known bound even when $X$ is $\mathbb{R}^n$ equipped with the Euclidean norm and $Y$ is a Hilbert space. Archive classification: math.FA math.MG Submitted from: naor@math.princeton.edu The paper may be downloaded from the archive by web browser from URL http://front.math.ucdavis.edu/1510.00276 or http://arXiv.org/abs/1510.00276
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