# The effect of projections on fractal sets and measures in Banach spaces

OTT W, HUNT B, Kaloshin V. 2006. The effect of projections on fractal sets and measures in Banach spaces. Ergodic Theory and Dynamical Systems. 26(3), 869–891.

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*Journal Article*|

*Published*|

*English*

Author

OTT, WILLIAM;
HUNT, BRIAN;
Kaloshin, Vadim

^{IST Austria}^{}Abstract

We study the extent to which the Hausdorff dimension of a compact subset of an infinite-dimensional Banach space is affected by a typical mapping into a finite-dimensional space. It is possible that the dimension drops under all such mappings, but the amount by which it typically drops is controlled by the ‘thickness exponent’ of the set, which was defined by Hunt and Kaloshin (Nonlinearity12 (1999), 1263–1275). More precisely, let $X$ be a compact subset of a Banach space $B$ with thickness exponent $\tau$ and Hausdorff dimension $d$. Let $M$ be any subspace of the (locally) Lipschitz functions from $B$ to $\mathbb{R}^{m}$ that contains the space of bounded linear functions. We prove that for almost every (in the sense of prevalence) function $f \in M$, the Hausdorff dimension of $f(X)$ is at least $\min\{ m, d / (1 + \tau) \}$. We also prove an analogous result for a certain part of the dimension spectra of Borel probability measures supported on $X$. The factor $1 / (1 + \tau)$ can be improved to $1 / (1 + \tau / 2)$ if $B$ is a Hilbert space. Since dimension cannot increase under a (locally) Lipschitz function, these theorems become dimension preservation results when $\tau = 0$. We conjecture that many of the attractors associated with the evolution equations of mathematical physics have thickness exponent zero. We also discuss the sharpness of our results in the case $\tau > 0$.

Publishing Year

Date Published

2006-06-01

Journal Title

Ergodic Theory and Dynamical Systems

Volume

26

Issue

3

Page

869-891

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### Cite this

OTT W, HUNT B, Kaloshin V. The effect of projections on fractal sets and measures in Banach spaces.

*Ergodic Theory and Dynamical Systems*. 2006;26(3):869-891. doi:10.1017/s0143385705000714OTT, W., HUNT, B., & Kaloshin, V. (2006). The effect of projections on fractal sets and measures in Banach spaces.

*Ergodic Theory and Dynamical Systems*. Cambridge University Press. https://doi.org/10.1017/s0143385705000714OTT, WILLIAM, BRIAN HUNT, and Vadim Kaloshin. “The Effect of Projections on Fractal Sets and Measures in Banach Spaces.”

*Ergodic Theory and Dynamical Systems*. Cambridge University Press, 2006. https://doi.org/10.1017/s0143385705000714.W. OTT, B. HUNT, and V. Kaloshin, “The effect of projections on fractal sets and measures in Banach spaces,”

*Ergodic Theory and Dynamical Systems*, vol. 26, no. 3. Cambridge University Press, pp. 869–891, 2006.OTT, WILLIAM, et al. “The Effect of Projections on Fractal Sets and Measures in Banach Spaces.”

*Ergodic Theory and Dynamical Systems*, vol. 26, no. 3, Cambridge University Press, 2006, pp. 869–91, doi:10.1017/s0143385705000714.