## The Kolakoski-Cantor set

A 0-1 sequence can be interpreted as a point in the interval [0,1]. But this makes the long-term behavior of the sequence practically invisible due to limited resolution of our screens (and eyes). To make it visible, we can also plot the points obtained by shifting the binary sequence to the left (Bernoulli shift, which also goes by many other names). The resulting orbit  is often dense in the interval, which doesn’t really help us visualize any patterns. But sometimes we get an interesting complex structure.

The vertical axis here is the time parameter, the number of dyadic shifts. The 0-1 sequence being visualized is the Kolakoski sequence in its binary form, with 0 and 1 instead of 1 and 2. By definition, the n-th run of equal digits in this sequence has length ${x_n+1}$. In particular, 000 and 111 never occur, which contributes to the blank spots near 0 and 1.

Although the sequence is not periodic, the set is quite stable in time; it does not make a visible difference whether one plots the first 10,000 shifts, or 10,000,000. The apparent symmetry about 1/2 is related to the open problem of whether the Kolakoski sequence is mirror invariant, meaning that together with any finite word (such as 0010) it also contains its complement (that would be 1101).

There are infinitely many forbidden words apart from 000 and 111 (and the words containing those). For example, 01010 cannot occur because it has 3 consecutive runs of length 1, which implies having 000 elsewhere in the sequence. For the same reason, 001100 is forbidden. This goes on forever: 00100100 is forbidden because it implies having 10101, etc.

The number of distinct words of length n in the Kolakoski sequence is bounded by a power of n (see F. M. Dekking, What is the long range order in the Kolakoski sequence?). Hence, the set pictured above is covered by ${O(n^p)}$ intervals of length ${2^{-n}}$, which implies it (and even its closure) is zero-dimensional in any fractal sense (has Minkowski dimension 0).

The set KC apparently does not have any isolated points; this is also an open problem, of recurrence (whether every word that appears in the sequence has to appear infinitely many times). Assuming this is so, the closure of the orbit is a totally disconnected compact set without isolated points, i.e., a Cantor set. It is not self-similar (not surprising, given it’s zero-dimensional), but its relation to the Bernoulli shift implies a structure resembling self-similarity:

Applying the transformations ${x\mapsto x/2}$ and ${x\mapsto (1+x)/2}$ yields two disjoint smaller copies that cover the original set, but with some spare parts left. The leftover bits exist because not every word in the sequence can be preceded by both 0 and 1.

Applying the transformations ${x\mapsto 2x}$ and ${x\mapsto 2x-1}$ yields two larger copies that cover the original set. There are no extra parts within the interval [0,1] but there is an overlap between the two copies.

The number ${c = \inf KC\approx 0.146778684766479}$ appears several times in the structure of the set: for instance, the central gap is ${((1-c)/2, (1+c)/2)}$, the second-largest gap on the left has the left endpoint ${(1-c)/4}$, etc. The Inverse Symbolic Calculator has not found anything about this number. Its binary expansion begins with 0.001 001 011 001 001 101 001 001 101 100… which one can recognize as the smallest binary number that can be written without doing anything three times in a row. (Can’t have 000; also can’t have 001 three times in a row; and 001 010 is not allowed because it contains 01010, three runs of length 1. Hence, the number begins with 001 001 011.) This number is obviously irrational, but other than that…

In conclusion, the Python code used to plot KC.

import numpy as np
import matplotlib.pyplot as plt
n = 1000000
a = np.zeros(n, dtype=int)
j = 0
same = False
for i in range(1, n):
if same:
a[i] = a[i-1]
same = False
else:
a[i] = 1 - a[i-1]
j += 1
same = bool(a[j])
v = np.array([1/2**k for k in range(60, 0, -1)])
b = np.convolve(a, v, mode='valid')
plt.plot(b, np.arange(np.size(b)), '.', ms=2)
plt.show()

## Infinite beatitude of non-existence: a journey into Nothingland

In the novella Flatland by Edwin A. Abbott, the Sphere leads the Square “downward to the lowest depth of existence, even to the realm of Pointland, the Abyss of No dimensions”:

I caught these words, “Infinite beatitude of existence! It is; and there is nothing else beside It.” […] “It fills all Space,” continued the little soliloquizing Creature, “and what It fills, It is. What It thinks, that It utters; and what It utters, that It hears; and It itself is Thinker, Utterer, Hearer, Thought, Word, Audition; it is the One, and yet the All in All. Ah, the happiness, ah, the happiness of Being!”

Indeed, Pointland (a one-point space) is zero-dimensional by every concept of dimension that I know of. Yet there is something smaller: Nothingland — empty space, ${\varnothing}$ — whose non-existent inhabitants must be perpetually enjoying the happiness of Non-Being.

What is the dimension of Nothingland?

In topology, the empty set has dimension ${-1}$. This fits the inductive definition of topological dimension, which is the smallest number ${d}$ such that the space can be minced by removing a subset of dimension ${\le d-1}$. (Let’s say a space has been minced if what’s left has no connected subsets other than points.)

Thus, a nonempty finite (or countable) set has dimension ${0}$: it’s minced already, so we remove nothing, a set of dimension ${-1}$. A line or a curve is one-dimensional: they can be minced by removing a zero-dimensional subset, like rational numbers.

The Flatland itself can be minced by removing a one-dimensional subset (e.g., circles with rational radius and rational coordinates of the center), so it is two-dimensional. And so on.

The convention ${\mathrm{dim}\,\varnothing = -1}$, helpful in the definition, gets in the way later. For example, the topological dimension is subadditive under products: ${\mathrm{dim}\,(A\times B)\le \mathrm{dim}\,A + \mathrm{dim}\,B}$ … unless both ${A}$ and ${B}$ are empty, because then ${-1\le -2}$ is false. So the case ${A=B=\varnothing}$ must be excluded from the product theorem. We would not have to do this if ${\mathrm{dim}\,\varnothing }$ was defined to be ${-\infty}$.

Next, consider the Hausdorff dimension. Its definition is not inductive, but one has to introduce other concepts first. First, define the ${d}$-dimensional premeasure on scale ${\delta>0}$:

$\displaystyle \mathcal H^d_\delta (X) = \inf \sum_j (\mathrm{diam}\,{U_j})^{d}$

where the infimum is taken over all covers of ${X}$ by nonempty subsets ${U_j}$ with ${\mathrm{diam}\,{U_j}\le \delta}$. Requiring ${U_j}$ to be nonempty avoids the need to define the diameter of Nothingland, which would be another story. The empty space can be covered by empty family of nonempty subsets. The sum of empty set of numbers is ${0}$, and so ${\mathcal H^d_\delta (\varnothing) = 0}$.

Then we define the ${d}$-dimensional Hausdorff measure:

$\displaystyle \mathcal H^d (X) = \lim_{\delta\rightarrow0} \mathcal H^d_\delta (X)$

and finally,

$\displaystyle \mathrm{dim}_H (X) = \inf \{ d \colon \mathcal H^d (X)=0\}$

If in this last infimum we require ${d>0}$, the result is ${\mathrm{dim}_H (\varnothing) =0}$. But why make this restriction? The ${d}$-dimensional pre-measures and measures make sense for all real ${d}$. It’s just that for nonempty ${X}$, we are raising some small (or even zero) numbers to negative power, getting something large as a result. Consequently, every nonempty space has ${\mathcal H^d = \infty}$ for all ${d < 0}$.

But ${\mathcal H^d_\delta (\varnothing) = 0}$, from the sum of empty collection of numbers being zero. Hence, ${\mathcal H^d (\varnothing) = 0}$ for all real ${d}$, and this leads to ${\mathrm{dim}_H\,\varnothing = -\infty}$.

To have ${\mathrm{dim}_H\,\varnothing = -\infty}$ is also convenient because the Hausdorff dimension is superadditive under products: ${\mathrm{dim}_H\,(A\times B)\ge \mathrm{dim}_H\,A + \mathrm{dim}_H\,B}$. This inequality was proved for general metric spaces as recently as 1995, by John Howroyd. If we don’t have ${\mathrm{dim}_H\,\varnothing = -\infty}$, then both factors ${A}$ and ${B}$ must be assumed nonempty.

So… should Nothingland have topological dimension ${-1}$ and Hausdorff dimension ${-\infty}$? But that would violate the inequality ${\mathrm{dim} (X)\le \mathrm{dim}_H (X)}$ which holds for every other separable metric space. In fact, for such spaces the topological dimension is simply the infimum of the Hausdorff dimension over all metrics compatible with the topology.

I am inclined to let the dimension of Nothingland be ${-\infty}$ for every concept of dimension.

## Fractal-ish monotone functions

There are several ways to construct a strictly increasing continuous function which has zero derivative almost everywhere. I like the explicit construction given by R. Salem, On some singular monotonic functions which are strictly increasing (1943).

Here is a streamlined version of the construction.

Fix ${\lambda\in (0,1/2)}$ (on the above picture ${\lambda=1/4}$). Let ${f_0(x)=x}$, and inductively define ${f_{n+1}}$ so that

1. ${f_{n+1} (x) = f_n(x)}$ when ${x\in 2^{-n}\mathbb Z}$.
2. If ${x\in 2^{-n}\mathbb Z}$, let ${f_{n+1}(x+2^{-n-1}) =\lambda f_n(x) + (1-\lambda) f_n(x+2^{-n})}$.
3. Now that ${f_{n+1}}$ has been defined on ${2^{-n-1}\mathbb Z}$, extend it to ${\mathbb R}$ by linear interpolation.
4. Let ${f=\lim f_n}$.

Since ${f(x+1)=f(x)+1}$ by construction, it suffices to understand the behavior of ${f}$ on ${[0,1]}$.

Each ${f_n}$ is piecewise linear and increasing. At each step of the construction, every line segment of ${f_n}$ (say, with slope ${s}$) is replaced by two segments, with slopes ${2(1-\lambda)s}$ and ${2\lambda s}$. Since ${\lambda f_n(x+2^{-n-1})}$. Hence, ${f_{n+1}\ge f_n}$.

Since ${f(x)=f_n(x)}$ when ${x\in 2^{-n}\mathbb Z}$, it is easy to understand ${f}$ by considering its values at dyadic rationals and using monotonicity. This is how one can see that:

• The difference of values of ${f}$ at consecutive points of ${2^{-n}\mathbb Z}$ is at most ${(1-r)^{n}}$. Therefore, ${f}$ is Hölder continuous with exponent ${\alpha = - \frac{\log (1-r)}{\log 2}}$.
• The difference of values of ${f}$ at consecutive points of ${2^{-n}\mathbb Z}$ is at least ${r^{n}}$. Therefore, ${f}$ is strictly increasing, and its inverse is Hölder continuous with exponent ${\alpha = - \frac{\log r}{\log 2}}$.

It remains to check that ${f'=0}$ almost everywhere. Since ${f}$ is monotone, it is differentiable almost everywhere. Let ${x}$ be a point of differentiability (and not a dyadic rational, though this is automatic). For each ${n}$ there is ${x_n\in 2^{-n}\mathbb Z}$ such that ${x_n < x< x_n+2^{-n}}$. Let ${s_n = 2^{n} (f_n(x_n+2^{-n})-f_n(x_n))}$; this is the slope of ${f_n}$ on the ${2^{-n}}$-dyadic interval containing ${x}$. Since ${f'(x)}$ exists, we must have ${f'(x) = \lim_{n\rightarrow\infty} s_n}$. On the other hand, the ratio of consecutive terms of this sequence, ${s_{n+1}/s_n}$, is always either ${2 (1-\lambda )}$ or ${2\lambda}$. Such a sequence cannot have a finite nonzero limit. Thus ${f'(x)=0}$.

Here is another ${f}$, with ${\lambda=1/8}$.

By making ${\lambda}$ very small, and being more careful with the analysis of ${f'}$, one can make the Hausdorff dimension of the complement of ${\{x \colon f'(x)=0\}}$ arbitrarily small.

An interesting modification of Salem’s function was introduced by Tukia in Hausdorff dimension and quasisymmetric mappings (1989). For the functions considered above, the one-sided derivatives at every dyadic rational are zero and infinity, which is a rather non-symmetric state of affair. In particular, these functions are not quasisymmetric. But Tukia showed that if one alternates between ${\lambda}$ and ${1-\lambda}$ at every step, the resulting homeomorphism of ${\mathbb R}$ becomes quasisymmetric. Here is the picture of alternating construction with ${\lambda=1/4}$; preliminary stages of construction are in green.

This is quite similar to how the introduction of alternating signs turns Takagi curve (blancmange curve) into a quasiarc (i.e., a curve without cusps); see Sweetened and flavored dessert made from gelatinous or starchy ingredients and milk. But the fractal curves in this post are relatively mild-mannered: they are rectifiable (and thus, not really fractal).

Here is the simple Scilab code I used for the above plots.

r = 1/4
t = [0 1]
f = t
for i = 1:10
t = [t, t(1:$-1)+2^(-i)] f = [f, r*f(1:$-1)+(1-r)*f(2:\$)]
[t, ind] = gsort(t,'g','i')
f = f(ind)
end
plot(t,f)


To have preliminary stages shown as well, move plot(t,f) into the loop. For Tukia’s alternating version, insert the line r = 1-r into the loop.

## “Full dimensional sets without given patterns” by Péter Maga

The footnote says that the paper is a part of the authors’ Master’s thesis at Eötvös Loránd University. Well, they write neat Master’s theses over in Budapest. The paper is available online as a preprint and in final version (by subscription). I rewrote my Zentralblatt review into something more resembling a blog post.

In analogy with additive number theory, I am going to define additive geometric measure theory as the search for arithmetic/geometric patterns in sufficiently large subsets of $\mathbb R^n$. Given two sets $A$ and $P$ in $\mathbb R^n$, one says that $A$ contains $P$ as a pattern if there exists a similarity $\phi\colon \mathbb R^n\to\mathbb R^n$ such that $\phi(P)\subset A$. (A similarity is a map that multiplies all distances by the same nonzero factor.)

It is not hard to see (via Lebesgue density theorem) that a set of positive measure contains every finite set as a pattern. Erdős conjectured that finiteness is essential here: for any infinite pattern $P$ there is a positive-measure set $A$ which avoids it. This conjecture of Erdős is still open. The paper investigates the opposite direction: the patterns are finite, but the sets which contain (or avoid them) are smaller. There is nothing interesting about 1- and 2-point patterns, so the first case of interest is a 3-point pattern.

Tamás Keleti proved that for every three-point subset $P\subset \mathbb R$ there exists a compact set $A\subset \mathbb R$ of Hausdorff dimension $1$ which avoids $P$ as a pattern (actually, his sets can avoid countably many such patterns at once). One of the results of the present article achieves the same goal in $\mathbb R^2$: for any three-point subset $P\subset \mathbb R^2$ there exists a compact set $A\subset \mathbb R^2$ of full Hausdorff dimension (that is, $2$) which does not contain $P$ as a pattern. The higher dimensional version remains open.

The two dimensional case is special because the triangle-avoiding property can be related to the arithmetics of complex numbers. Given a set $A\subset \mathbb C$, let $\mathcal T(A)=\left\{\frac{z-x}{y-x} \colon x,y,z\in A, x\ne y\right\}$ be its divided difference set (compare to the usual difference set $\mathcal D(A)=\left\{{y-x} \colon x,y\in A\right\}$). Up to a similarity transformation, every nondegenerate triangle can be encoded by a complex number $\zeta\ne 0$. Actually, $\zeta$ is not unique: the numbers $\zeta, \zeta^{-1}, 1-\zeta, (1-\zeta)^{-1}, 1-\zeta^{-1}, (1-\zeta^{-1})^{-1}$ represent the same triangular pattern. According to the old book Automorphic Forms by Lester Ford, this incarnation of $D_3$ is called the group of anharmonic ratios, or maybe just the anharmonic group. I don’t remember seeing the term in more recent literature, but this does not say much. Anyway, one can take the triangle $\{z\colon \mathrm{Im}\, z>0, |z|>1, |z-1|>1\}$ as a fundamental domain for this group.

The author shows that if $A\subset \mathbb R$ is compact and $\mathrm{dim} A=1$, then
$\mathcal T(A)$ is dense in $\mathbb R$. He asks whether the divided difference set of every compact 2-dimensional subset $A\subset \mathbb C$ is dense in $\mathbb C$. Or maybe even $\mathrm{dim}\, A>1$ is enough to make $\mathcal T(A)$ dense. Of course $\mathrm{dim}\, A = 1$ would not be enough since $\mathcal T(\mathbb R)=\mathbb R$.

Some of the results of this paper were later put into a larger framework by András Máthé in Sets of large dimension not containing polynomial configurations.