# Isometrically-invariant measures and dilation of the Cantor set

Let $C$ be the Cantor middle-thirds set. Let $\mu$ be a finitely-additive isometrically-invariant measure on all subsets of $\mathbb R$. Then $\mu(3C)=2\mu(C)$, where $aB = \{ ax : x \in B \}$. Thus if $a$ is a power of $3$, $\mu(aC) = a^{\log_3 2} \mu(C)$.

Question 1: Is it the case for all $a\in (0,\infty)$ that $\mu(aC)=a^{\log_3 2} \mu(C)$, if $\mu$ is a finitely-additive isometrically-invariant measure on $\mathcal P\mathbb R$?

The remaining questions are predicated on an affirmative answer, though currently I'm suspecting a negative answer to 1.

Question 2: Is this still true if isometric-invariance is replaced by translation-invariance?

Question 3: Has there been any work on using the the relationship between $\mu(aC)$ and $\mu(C)$ for invariant measures $\mu$ on $\mathbb R^n$ (perhaps not defined on all of the powerset, but just on the Borel sets) to define a dimension and comparing it to Hausdorff and box dimensions?

• Interesting question. For Question 3 a problem is that many sets are given zero or non-$\sigma$-finite mass for any translation-invariant Borel measure on $\mathbb{R}$ (for example, the set of Liouville numbers, see bolyai.cs.elte.hu/analysis/keleti/papers/liouville.pdf). – Pablo Shmerkin Sep 27 '13 at 14:11
• Thanks for the reference. It's good that I specified the measures to be finitely additive, since then by supramenability of the isometries of $\mathbb R$ every subset of $\mathbb R$ has an invariant measure that normalizes it. – Alexander Pruss Sep 27 '13 at 16:07

Negative to the first question, so the others are moot.

Let $G_1$ be the isometries of $\mathbb R$.

First note that no finite number of translates (or reflections, but that doesn't add anything) of $(1/2)C$ covers $C$. This can be seen by playing around with base three expansions.

Let $I$ be the ideal in $\mathcal P(\mathbb R)$ generated by $(1/2)C$ and its translates. Let $\mathcal B$ be the quotient boolean algebra $\mathcal P(\mathbb R)/I$. Because $C$ isn't covered by translates of $(1/2)C$, $[C]\ne 0$. The action of $G_1$ on $\mathbb R$ induces an action of $G_1$ on $\mathcal B$. Since $G_1$ is supramenable (i.e., doesn't have nonempty paradoxical subsets), it follows from a theorem of Mycielski ("Finitely additive invariant measures. I", Colloq. Math. 42 (1979), 309–318) that there is a finitely additive measure $\mu_1$ on $\mathcal B$ invariant under $G_1$ with $\mu_1([C])=1$. Let $\mu(A)=\mu_1([A])$ for $A\subseteq\mathbb R$. Then $\mu(C)=1$ and $\mu((1/2)C)=0$ and the answer to Question 1 is negative for $a=1/2$.

This uses the Axiom of Choice.

This is too long for a comment. The following measure (defined on Borel sets) might be a counterexample to Question 1: let $\mathcal{I}_N$ be the collection of all left-closed, right-open intervals of length $3^{-k}$ for some $k\in\mathbb{N}, k\ge N$, and define $$\mu(A) = \lim_{N\to\infty} \inf\left\{ \sum_i |I_i|^{\log_3 2}: A\subset \bigcup_i I_i, I_i\in\mathcal{I}_N \right\}.$$

This is well-known to be a Borel-regular measure on the Borel $\sigma$-algebra (is constructed using the "Type 2" method from Rogers' classical book "Hausdorff measures"), and it's obviously isometry-invariant. It is clear that $\mu(C)=1$. Since we are covering only with intervals of length $3^{-k}$, it seems to me the scaling law should break down ($\mu(aC)$ should be strictly larger than $a^{\log_3 2}$ for $1<a<3$). I can't prove this at the moment though.

Regarding Question 3, in addition to my comment another problem is that for some sets $E$ there exist two (again, Borel) isometry-invariant measures $\mu,\nu$ which give $E$ positive and finite mass, and distinct numbers $s,t>0$ such that $\mu(a E)=a^s \mu(E)$ and $\nu(a E)=a^t(E)$ for all $a>0$. For example, we may choose a set $E$ of different Hausdorff and packing dimensions, and which has positive and finite Hausdorff/packing measure in their respective dimensions. It is not immediately clear to me if it is possible to get 3 or more different scaling exponents.

Essentially the approach in Question 3 is implicitly used in the definition of Hausdorff and packing dimensions, but it only works if one restricts the measures to the family Hausdorff and packing measures, respectively.

I know very little about finitely additive measures on $\mathcal{P}(\mathbb{R})$ so I don't have much to say on the actual questions.

• An invariant f.a. measure on the Borel sets (or on any other algebra of sets) will extend to an invariant f.a. measure on all subsets of $\mathbb R$ (it's somewhere in Wagon's BT Paradox book), so if this works for the Borel sets, it answers the question in the negative. – Alexander Pruss Sep 28 '13 at 1:56
• You can probably get an arbitrarily large finite number of different measure-based "dimensions" by using the trick in my answer. Let $C_1,C_2,...,C_n$ be self-similar Cantor sets of different dimensions, rescaled in such a way that no one of the sets can be covered by finitely many translates of the others. I don't have a proof that this can be done, but it wouldn't surprise me. I hereby conjecture it. Then for each $i$ there will be a f.a. invariant measure on $\mathcal P\mathbb R$ that's nonzero on $C_i$ but zero on each $C_j$, $j\ne i$. – Alexander Pruss Sep 28 '13 at 13:02