$$\varepsilon $$ -Isometric Dimension Reduction for Incompressible Subsets of $$\ell _p$$

Alexandros Eskenazis1,2
1CNRS, Institut de Mathématiques de Jussieu, Sorbonne Université, Paris, France
2Trinity College, University of Cambridge, Cambridge, UK

Tóm tắt

Fix $$p\in [1,\infty )$$ , $$K\in (0,\infty )$$ , and a probability measure $$\mu $$ . We prove that for every $$n\in \mathbb {N}$$ , $$\varepsilon \in (0,1)$$ , and $$x_1,\ldots ,x_n\in L_p(\mu )$$ with $$\big \Vert \max _{i\in \{1,\ldots ,n\}} |x_i| \big \Vert _{L_p(\mu )} \le K$$ , there exist $$d\le \frac{32e^2 (2K)^{2p}\log n}{\varepsilon ^2}$$ and vectors $$y_1,\ldots , y_n \in \ell _p^d$$ such that $$\begin{aligned} {\forall }\,\,i,j\in \{1,\ldots ,n\}, \quad \Vert x_i-x_j\Vert ^p_{L_p(\mu )}-\varepsilon\le & {} \Vert y_i-y_j\Vert _{\ell _p^d}^p\le \Vert x_i-x_j\Vert ^p_{L_p(\mu )}+\varepsilon . \end{aligned}$$ Moreover, the argument implies the existence of a greedy algorithm which outputs $$\{y_i\}_{i=1}^n$$ after receiving $$\{x_i\}_{i=1}^n$$ as input. The proof relies on a derandomized version of Maurey’s empirical method (1981) combined with a combinatorial idea of Ball (1990) and a suitable change of measure. Motivated by the above embedding, we introduce the notion of $$\varepsilon $$ -isometric dimension reduction of the unit ball $${\textbf {B}}_E$$ of a normed space $$(E,\Vert \cdot \Vert _E)$$ and we prove that $${\textbf {B}}_{\ell _p}$$ does not admit $$\varepsilon $$ -isometric dimension reduction by linear operators for any value of $$p\ne 2$$ .

Tài liệu tham khảo

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