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RANDOM POINTS IN ISOTROPIC UNCONDITIONAL CONVEX BODIES

Published online by Cambridge University Press:  08 December 2005

A. GIANNOPOULOS
Affiliation:
Department of Mathematics, University of Athens, Panepistimiopolis, 157 84 Athens, Greeceapgiannop@math.uoa.gr
M. HARTZOULAKI
Affiliation:
Department of Mathematics, University of Missouri, Columbia, MO 65211-4100, USAmariann@math.missouri.edu
A. TSOLOMITIS
Affiliation:
Department of Mathematics, University of the Aegean, Karlovassi, 832 00 Samos, Greeceatsol@aegean.gr
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Abstract

The paper considers three questions about independent random points uniformly distributed in isotropic symmetric convex bodies $K, T_1,\ldots, T_s$. (a) Let $\varepsilon\,{\in}\, (0,1)$ and let $x_1,\ldots, x_N$ be chosen from K. Is it true that if $N\,{\geq}\, C(\varepsilon )n\log n$, then \[\left\| I-\frac{1}{NL_K^2}\sum_{i=1}^Nx_i\otimes x_i\right\|<\varepsilon\] with probability greater than $1\,{-}\,\varepsilon $? (b) Let $x_i$ be chosen from $T_i$. Is it true that the unconditional norm \[\|{\bf t}\|=\int_{T_1}\!{\ldots}\int_{T_s}\left\|\sum_{i=1}^st_ix_i\right\|_K\,dx_s\ldots dx_1\] is well comparable to the Euclidean norm in ${\mathbb R}^s$? (c) Let $x_1,\ldots, x_N$ be chosen from K. Let ${\mathbb E}\,(K,N):={\mathbb E}\,|{\rm conv}\{ x_1,\ldots, x_N\}|^{1/n}$ be the expected volume radius of their convex hull. Is it true that ${\mathbb E}\,(K,N)\,{\simeq}\, {\mathbb E}\,(B(n),N)$ for all N, where $B(n)$ is the Euclidean ball of volume 1?

It is proved that the answers to these questions are affirmative if there is a restriction to the class of unconditional convex bodies. The main tools come from recent work of Bobkov and Nazarov. Some observations about the general case are also included.

Type
Notes and Papers
Copyright
The London Mathematical Society 2005

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Footnotes

This research was partially supported by the EPEAEK II program Pythagoras II.