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Let $A\subseteq \mathbb{R}$ be Lebesgue-measurable and let $0<\alpha<1$ be its Hausdorff dimension.

For a given $0<\beta <\alpha$ can we find a subset $B\subset A$ with Hausdorff dimension $\beta$?

In case this is true, could you provide a reference for this statement?

Added: Actually I am happy if $A$ is compact.

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3 Answers 3

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First of all, $\dim_{H} (A) = \alpha$ iff $ H^k(A)=\infty$ for all $k<\beta$ and $H^k(A) = 0$ for all $k>\beta$. Then $H^\alpha(A) = \infty$ for all $\alpha \in (0,\beta)$.

If $A$ is closed then by Theorem 5.4 from [1] there is a compact $K\subset A$ such that $0<H^\alpha(K)<\infty$.

More generally, if $A$ is Souslin then by Theorem 5.6 from [1] again there is a compact $K\subset A$ such that $0<H^\alpha(K)<\infty$ (as @SeverinSchraven noted).


If $A$ is not Souslin then (at least assuming AC and CH) it may happen that $H^\beta(B) = 0$ for any $B\subset A$ and any $\beta \in (0, \alpha)$. In order to justify this claim let me add adopt the argument from a comment by fedja to a closely related post, where an interesting paper [2] is also mentioned.

Let $C$ denote the standard Cantor set in $[0,1]$ and take $\alpha = \dim_H(C) = \frac{\ln 2}{\ln 3}$. Take $\beta \in (0,\alpha)$.

Consider the family $\mathcal E$ of all $G_\delta$-subsets $E \subset C$ such that $H^\alpha(E)=0$. Then for any countable family $(E_i)_i\in \mathcal E$ the set $C \setminus \bigcup_{i=1}^\infty E_i$ has infinite $\beta$-dimensional Hausdorff measure and in particular is not empty.

Assuming AC+CH one can enumerate the family $\mathcal E$ using ordinals as $\mathcal E = \{E_\gamma : \gamma < \omega_1\}$, where $\omega_1$ is the first uncountable ordinal.

For each $\gamma < \omega_1$ take $x_\gamma \in C \setminus \bigcup_{\lambda < \gamma} E_\lambda$. Now we finally define the set $A:= \{x_\gamma : \gamma < \omega_1\}$.

If $H^\alpha(A)$ was zero then one could cover $A$ with $E_\gamma$ for some $\gamma$ (see e.g. Theorem 1.6 in [1]). This contradicts the definition of $A$, hence $H^\alpha(A)>0$.

Now if $B \subset A$ and $H^\beta(B)\in (0,\infty)$ then $H^\alpha(B)=0$ and then there exists $\gamma<\omega_1$ such that $B \subset E_\gamma$, hence $$ H^\beta(B) = H^\beta(B\cap E_\gamma) \le H^\beta(A\cap E_\gamma) = 0 $$ since the intersection $A\cap E_\gamma$ is at most countable for any $\gamma < \omega_1$.


References.

  1. The Geometry of Fractal Sets by K.J. Falconer (1985)

  2. Finding subsets of positive measure by Bjørn Kjos-Hanssen and Jan Reimann (2014) https://arxiv.org/abs/1408.1999

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  • $\begingroup$ That's exactly what I was looking for, thanks very much. $\endgroup$ Mar 15, 2019 at 22:01
  • $\begingroup$ Actually it is worth mentioning that in the same reference in Theorem 5.6, this is generalized to Souslin spaces. $\endgroup$ Mar 16, 2019 at 0:06
  • $\begingroup$ @SeverinSchraven Thank you, I have included this remark into my answer. $\endgroup$
    – Skeeve
    Mar 16, 2019 at 17:56
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The answer is yes under the additional assumption that the set is compact and I do not know what happens in the general case. The result is a consequence of the following one, see [1] and references therein.

Theorem. If a compact set $A\subset\mathbb{R}^n$ has non-$\sigma$-finite measure $\mathcal{H}^\beta$, then there exists a subset $B\subset A$ such that $0<\mathcal{H}^\beta(B)<\infty$.

[1] R.O. Davies, A theorem on the existence of non-σ-finite subsets. Mathematika 15 (1968), 60–62.

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    $\begingroup$ I actually have a question (which is the reason why I wrote that my answer is partial): in the OP it is assumed that $A$ is Lebesgue measurable. Strictly saying this does not imply that $A$ is Borel, so it is not immediate that the result we both eventually refer to can be used (if $A$ were Borel or at least Souslin then yes). $\endgroup$
    – Skeeve
    Mar 15, 2019 at 20:56
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    $\begingroup$ @Skeeve Good point. I do not know what happens in general, but perhaps the compact case is sufficient for the needs of OP. Let's hear from him. I will changes my answer to emphasize that it only answers the compact case. By the way you know a lot of geometric measure theory so you should be more active :) There are not too many of us. $\endgroup$ Mar 15, 2019 at 21:23
  • $\begingroup$ @PiotrHajlasz Indeed, the compact case is fine for me. Thanks for the answer. $\endgroup$ Mar 15, 2019 at 22:01
  • $\begingroup$ @PiotrHajlasz It seems to me that in the general case under certain axioms bad things might happen. I have updated my answer with some thoughts on that. By the way thank you, I will try to be more active :) Even though you probably overestimate my knowledge of GMT, I hope to learn it better in nice discussions on this site (in particular with you, of course!) $\endgroup$
    – Skeeve
    Mar 16, 2019 at 18:03
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The following is Corollary 7 of [1].

Theorem: For $X$ (an analytic subset of) a complete separable metric space, and $ s \in [0,\infty)$, the following is true about the Hausdorff measure $\mathcal{H}^s$: For every $ l < \mathcal{H}^s(X) $ there exists a compact subset $A \subset X$ such that $$ l < \mathcal{H}^s (A) < \infty \, . $$

[1] J. D. Howroyd, On dimension and on the existence of sets of finite positive Hausdorff measure, 1993

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