# Domino tiling obtained from space-filling curves, is possible to predict basic properties?

Periodic and aperiodic domino tiling systems can be obtained by the following construction rules:

1. Draw a regular square grid n×n of n2 cells.

2. Select a space-filling curve that is consistent with squares: that do a recursive four-partition of the quadrilateral unit grid (refinement ratio 4 in the OGC jargon).

3. Draw a space-filling curve path through the center of the square cells, using the curve also as index i for each cell.

4. Merge neighboring cells to obtain a domino with index j=floor(i/2).
It results in a domino tiling of n2/2 dominoes.

Example:

In domino tiling problems the domino's relative orientation to its neighboring is important...
Is there a way to predict the basic properties of the domino tiling?
That is, for each space-filling curve:

• predict the type of tiling system, periodic or aperiodic;
• predic the fraction of dominoes that will be different orientation.

## Notes

The merge rule transforms the original curve into another fractal, so we cannot use directly the properties of the original fractal. The degenerated curve is like a "dual", we can deduce some general properties of the transformation that will be useful for predictions. For example original Morton curve have periodic Z-shape, and degenerated curve preserves the periodicity. Hilbert curve is aperiodic (the U-shape is rotated), and the degenerated curve preserves this aperiodic orientation.

Illustrating with more dominoes, from the same degenerated curves.

By empirical induction we can suppose that, for any n:

• Morton curve generates periodic domino tiling, all dominoes in the same orientation.
• Hilbert curve generates aperiodic domino tiling, 50% horizontal and 50% vertical.

PS: another interesting question is about fractal classification, can we use this construction rules to group isolated fractals in complementary ones? The "И-shape curve" is the complement (degenerated form) of the Z-shape curve; the Munkres's fractal (defined at "Theorem 44.1" in this book) is the complement of the Hilbert curve.

• Is it clear that the dominoes are always well defined? Apr 11, 2020 at 7:33
• "Draw a space-filling curve path through the center of the square cells, using the curve also as index i for each cell." None of the curves you have drawn are actually space-filling. They are just some approximations in some constructions of such curves. It's not clear every space-filling curve can be approximated this way, and then (as Wolfgang highlights) it's not clear that you can always pair up points into dominoes (in fact I expect it's not always the case) Apr 11, 2020 at 9:57
• Thanks @Wolfgang and Wojowu, sorry, I omitted the class of curves. Edited. It is sufficient to express a constraint in this terms? "space-filling curve that can be used in a recursive four-partition of the quadrilateral unit grid". Apr 11, 2020 at 11:16
• Jigsaw HAVE become very popular during the pandemic. Apr 11, 2020 at 20:43
• From "bounty expires", I deduce that my answer did not answer your question? Could you explain why? Apr 29, 2020 at 11:41

I'm not sure I understood the question and answer given by the asker, but I gather that they are interested in decidability questions about some substitutive subshifts.

As far as I can tell, the questions about which dominoes occur more often are in general solved by basic linear algebra; when you have a substitution giving the tiling you just compute some eigenvectors for its abelianization, not really on-topic for this site. In the case of the two example substitutions the solutions are already given by the asker; I don't know what "empirical induction" is, but the facts that horizontal and vertical dominoes appear with the same frequency in Hilbert, and all are horizontal in the other one, can be proved by induction. That said, let me concentrate on aperiodicity, which is more interesting.

I'll give some basic info about that: for any substitutive curve, the aperiodicity question about its dominofied version is decidable (if the class being implied is anything like I think). I didn't actually get what the degenerated stuff was about, but seems to be about some projections or factor images as well, so should be covered by the general theory. I also give a more classical style symbolic dynamics proof that the dominofication of the Hilbert curve is a minimal aperiodic subshift.

Aperiodicity is decidable: an automata theory proof

First, some definitions. Let $$m, n \in \mathbb{N}$$, $$A$$ a finite alphabet and $$\tau : A \to A^{m \times n}$$ a function. We interpret $$\tau$$ as replacing $$a \in A$$ by an $$m \times n$$ matrix, and we call such $$\tau$$ a ($$m$$-by-$$n$$)-substitution. We can apply $$\tau$$ to $$P \in A^{k \times \ell}$$ to obtain $$\tau(P) \in A^{km \times \ell n}$$, in the obvious way by replacing the individual elements of $$A$$ with $$m \times n$$ matrices.

If $$\tau$$ is a substitution, we say a (one-sided) infinite configuration $$x \in A^{\mathbb{N}^2}$$ is a $$\tau$$-periodic point if $$\tau^k(x) = x$$ for some $$k > 0$$, where we apply a substitution to an infinite configuration $$x$$ in an obvious way (the origin stays put, everything else blows up in the positive direction). We can find natural $$\tau$$-periodic points as follows: Start with a symbol $$a \in A$$, and keep applying $$\tau$$. The symbol $$\tau^n(a)|_{(0,0)}$$ evolves eventually periodically, say with some eventual period $$p$$. Then $$\tau^{p\ell}(a)$$ actually tends to a limit in an obvious sense (cellwise), and this limit is $$\tau$$-periodic as an infinite configuration (with period $$p$$).

Say $$x \in A^{\mathbb{N}^2}$$ is $$n$$-automatic if for all $$a \in A$$, the set $$\{v \in \mathbb{N}^2 \;|\; x_v = a\}$$ is $$n$$-automatic. A set of pairs of numbers $$N \subset \mathbb{N}^2$$ is $$n$$-automatic if the language $$L_n$$, of words $$w \in (\{0,1,...,n-1\}^2)^*$$ which evaluate to a pair of numbers in $$N$$ when you separately read off the $$n$$-ary numbers on the two tracks, is a regular language. Regular languages are very robust so I won't give the precise formulas, you can't really guess them wrong. We similarly define $$n$$-automatic configurations in $$A^{\mathbb{N}^d}$$ and subsets of $$\mathbb{N}^d$$.

The following is relatively easy to show, you can find it (or at least its one-dimensional version) in many references and books that discuss automata and substitutions.

Theorem. Let $$\tau : A \to A^{n \times n}$$ be a substitution and let $$x$$ be any $$\tau$$-periodic point. Then $$x$$ is $$n$$-automatic.

The proof is not very hard, the idea is the automaton just keeps track of the current symbol and when it reads a digit, that tells it where it moves in the substitutive image (and it looks up $$\tau$$ to see which symbol is there).

The following is obvious by basic closure properties of regular languages.

Lemma. Let $$x \in A^{\mathbb{N}^2}$$ be $$n$$-automatic, and $$\pi : A \to B$$ a function. Then $$\pi(x) \in B^{\mathbb{N}^2}$$, defined by $$\pi(x)_v = \pi(x_v)$$, is also $$n$$-automatic.

The following is classical, perhaps first invented by Büchi. There is an implementation called Walnut where you can directly input such statements. I get the impression the asker is into computers, so I'll leave it as an exercise to try this out (sometimes Walnut cracks very difficult problems, sometimes it gets stuck on very trivial things, it's all about whether the intermediate DFAs happen to have huge numbers of states, which is hard to predict). The decidability proof is not so hard, the idea is to do quantifier elimination by using the subset construction.

Theorem. Let $$x \in A^{\mathbb{N}^2}$$ be $$n$$-automatic, and let $$\phi$$ be any first-order formula (with constants and free variables) where quantifiers range over vectors in $$\mathbb{N}^2$$, and you have function sumbols for vector addition, and have a unary predicate for "$$x_v = a$$" for each $$a \in A$$, with the obvious interpretations. Then the set of solutions to $$\phi$$ (possible values for the free variables) are an $$n$$-automatic subset of $$\mathbb{N}^d$$, which can be computed effectively; if there are no free variables, whether $$\phi$$ is a true statement is decidable.

Now, the question of aperiodicity amounts to programming in first-order logic. We say $$x \in A^{\mathbb{N}^2}$$ is aperiodic if its stabilizer $$\{v \in \mathbb{N}^2 \;|\; v+x = x\}$$ is trivial, where $$v+x$$ denotes translation $$(v+x)_u = x_{v+u}$$. This is not to be confused with $$\tau$$-periodicity.

Lemma. A configuration $$x \in A^{\mathbb{N}^2}$$ is aperiodic if and only if it satisfies the following first-order statement (of the type in the previous theorem): $$\forall v \neq 0: \exists u: x_{u+v} \neq x_u.$$

Hopefully no need for a proof, since basically I just wrote the definition. This lemma and formula are about periodicity of a full $$\tau$$-periodic quarterplane, but you can modify the formula to talk about arbitrarily large periodic areas, or periodicity up to skipping some initial rows and columns, or many other things, it's just first-order formula programming. Now, I believe the following theorem solves a relatively general version of your question:

Theorem. For any substitution $$\tau : A \to A^{n \times n}$$, any $$\tau$$-periodic $$x \in A^{\mathbb{N}^2}$$, and any map $$\pi : A \to B$$, it is decidable whether the configuration $$\pi(x)$$ is aperiodic.

Proof: We have that $$x$$ is $$n$$-automatic, thus $$\pi(x)$$ is $$n$$-automatic, and it is decidable whether the formula of the previous lemma is true, thus it is decidable whether $$\pi(x)$$ is aperiodic. Square.

Now, we can solve your Hilbert curve question as follows:

Take as alphabet the symbols for the cardinal directions $$\{ N, E, W, S \}$$, which represent the different directions where a basic "$$U$$-shape/$$U$$-curve" of the Hilbert curve (by which I mean one of the length-$$4$$ segments from which the curve consists) may open. Take the Cartesian product with $$\{-1, 0, 1\}$$ as there are three different ways the curve may continue from the first and last point of the curve (either it continues straight or turns, and it never goes straight from both ends). You can then work out a substitution on $$12$$ symbols, which carries all the relevant information. (It's a reasonable coding, since it's easy to see that you can tell from a dominofied configuration which are the $$2$$-by-$$2$$ blocks coming from a basic $$U$$-shape.) The map $$\pi$$ takes $$\{N, E\}$$-type symbols to $$V$$ and $$\{W, E\}$$-type symbols to $$H$$ (ignoring the $$\{0, 1, 2\}$$ component).

For example, if $$-1$$ means "turn on the left", $$1$$ means "turn on the right" and $$0$$ means "turn on both ends", then $$\tau(N, 0) = \begin{pmatrix} (W, -1) & (E, 1) \\ (N, 1) & (N, -1) \end{pmatrix}.$$ This is the example you give, but I carry the additional information of how the curve would continue, and I do not yet do the final substitutions where you actually write the basic $$U$$-curves, and where you then substitute them for a pair of dominoes.

So there's an algorithm that solves whether the Hilbert curve domino thingie is aperiodic. I did not run it, and do not claim it's very efficient. Instead...

Aperiodicity of the Hilbert curve: a symbolic dynamics proof

I'll give a manual proof (I did check the combinatorics in Python, but it's my native language so that's much less work than Walnut, and I think it shouldn't be hard to work out on pencil-and-paper).

Now, let me describe another way, which is what people usually do in practice (because it's more fun to do this way when working manually), and then I can give a quick manual proof that the Hilbert curve gives you an aperiodic domino tiling. We are going to replace computation with more conceptual ideas, so we need more definitions.

A subshift is a subset $$X \subset A^{\mathbb{Z}^2}$$ which is topologically closed in the profinite (Cantor) topology, and is shift-invariant meaning $$\forall x \in X: \forall v \in \mathbb{Z}^2: v+x \in X$$. (I move to $$\mathbb{Z}^2$$-configurations form $$\mathbb{N}^2$$-configurations for convenience, but actually after the topologization everything will turn into questions about finite objects, so this will not matter much.)

If $$x \in A^{\mathbb{Z}^2}$$ and $$P \in A^{k \times \ell}$$, write $$P \sqsubset x$$ for $$P$$ appearing somewhere in $$x$$, i.e. $$\exists v \in \mathbb{Z^2}: (v+x)|_{k \times \ell} = P$$, where $$v+x$$ denotes translation $$(v+x)_u = x_{v+u}$$. Similarly we write $$P \sqsubset Q$$ for $$P$$ appearing somewhere in $$Q$$ when $$P \in A^{m \times n}$$ and $$Q \in A^{k \times \ell}$$, with the obvious meaning. The subshift generated by $$\tau$$ from $$a \in A$$ is the set of all $$x \in A^{\mathbb{Z}^2}$$ such that for any $$k, \ell$$ and any such that $$P \sqsubset x$$, there exists $$n$$ such that $$P \sqsubset \tau^n(a)$$.

We also reformulate $$\tau$$-periodicity for $$\mathbb{Z}^2$$-configurations. We say $$x \in A^{\mathbb{Z}^2}$$ is a good $$\tau$$-periodic point if there exists $$R \in A^{2 \times 2}$$ such that $$R$$ appears in $$\tau^n(a)$$ for some $$a \in A$$, $$n \in \mathbb{N}$$, and $$x$$ is the limit of $$R$$ obtained by taking the limits $$\tau^p(b)$$ of the four symbols in $$R$$ separately, for some $$p \in \mathbb{N}$$, i.e. each of them separately expands into its own direction. Such $$\tau$$-periodic configurations again exist by the pigeonhole principle since the set of $$2$$-by-$$2$$ patterns is finite.

If $$\tau$$ is a substitution, write $$M_\tau$$ for the $$|A|$$-by-$$|A|$$ matrix where $$(M_\tau)_{a,b} = |\{k \;|\; \tau(a)_k = b\}|$$, i.e. rows tell you how many of each symbol appears in each $$\tau$$-image. We say a matrix $$M$$ is primitive if there exists $$n$$ such that $$M^n$$ has only positive entries. We have that $$M_\tau$$ is primitive if and only if $$b \sqsubset \tau^n(a)$$ for any choice $$a, b \in A$$, and we then also say $$\tau$$ is primitive.

The following lemma can be found in any reference that discusses substitutions (at least its one-dimensional version, but it's exactly the same in two dimensions since our substitution is rectangle-shaped).

Let $$\tau$$ be a substitution such that $$M_\tau$$ is primitive. Then the subshift $$X$$ generated from $$a$$ does not depend on the choice of $$a$$, and the orbit-closure of every good $$\tau$$-periodic point is $$X$$. The subshift $$X$$ is minimal, i.e. the orbit-closure of every point in $$X$$ is $$X$$.

By this lemma, and the easily proved fact that the Hilbert curve substitution is primitive, we don't really have to worry about whether things are one-sided or two-sided: minimality means that either every configuration $$x \in X$$ satisfies $$v+x = x$$ for some $$v \in \mathbb{Z}^2 \setminus \{(0,0)\}$$ independent of $$x$$, or for all $$v \in \mathbb{Z}^2 \setminus \{(0,0)\}$$ there exists $$m$$ such that the $$v$$-periodic is locally broken in all patterns of size $$m$$-by-$$m$$ that appear when you iterate this substitution.

A morphism from subshift $$X$$ to subshift $$Y$$ is a continuous function $$\pi : X \to Y$$ which commutes with the shift maps, i.e. $$\pi(v+x) = v + \pi(x)$$ for all $$x \in X$$, $$v \in \mathbb{Z}^2$$. It is easy to see that being aperiodic is preserved under a morphism.

Now, we proceed as follows: Let $$X$$ be the subshift generated by the Hilbert curve substitution $$\tau$$.

1. Define the map $$\pi : \{N,E,W,S\} \times \{-1,0,1\} \to \{N,E,W,S\}$$, which drops the information about how the curve continues, let the image be $$Y$$, so we have a morphism $$\pi : X \to Y$$.
2. Show that $$Y$$ is aperiodic.
3. Show that the map $$\pi' : \{N,E,W,S\} \to \{H,V\}$$, which extracts the dominoes, is an isomorphism onto its image, denote it as $$\pi' : Y \to Z$$.
4. Now, $$Z$$ has to also be aperiodic, because the inverse $$(\pi')^{-1} : Z \to Y$$ would preserve any period.

We first show that $$Y$$ is aperiodic. To see this, we observe that actually we could have in the first place defined the Hilbert curve substitution $$\tau$$ without using the curve information (I just included it to keep it as close to the original description as possible; I didn't check whether $$\pi$$ is an isomorphism): $$Y$$ is the subshift given by the substitution $$\tau'$$ obtained from $$\tau$$ that ignores the curve information completely (again $$\tau'$$ is primitive so $$Y$$ is minimal).

I proved the aperiodicity as follows: the pattern $$\begin{pmatrix} N & W & E & N \\ S & W & E & S \end{pmatrix}$$ can only appear in only even positions of substituted images of patterns (both coordinates even), as you can see by analyzing $$(\tau')^n(R)$$ for small $$n$$ and $$2$$-by-$$2$$ patterns $$R$$ ($$n = 3$$ is enough). Since $$\tau'$$ is injective, you can "use a local rule to detect whether a given $$2$$-by-$$2$$ block you see is actually a substituted image of a symbol, or whether it appears between two such images, in a unique way" (one says the subshift recognizable in the sense of Mossé).

Let me not define that because I don't know a particularly neat way to do that, but you can find this in any reference that discusses recognizability and substitutions. Instead, let me intuitively explain what we do with this local rule: once you can figure out which $$2$$-by-$$2$$ blocks come from symbols, in fact by the substitutive nature of the subshift the preimages of these blocks form a configuration of the same subshift $$Y$$. So you can iterate the local rule, and what you get once you forget all but the "phases" is a continuous shift-invariant map $$\phi : Y \to I^2$$, where $$I$$ the $$2$$-adic integers, and where $$\mathbb{Z}^2$$ acts on $$I^2 = I \times I$$ by translation ($$\mathbb{Z} \leq I$$ is a dense subgroup; the dynamical system $$I$$ is usually called the $$2$$-adic odometer). Since $$I^2$$ is a torsion-free group, $$I^2$$ under the translation action of $$\mathbb{Z}^2$$ has only aperiodic points, and thus so must $$Y$$ (morphisms between general dynamical systems also preserve periods).

Finally, we need to show that map $$\pi' : Y \to Z$$, is an isomorphism. For this, we argue similarly as above: A short case analysis shows that the pattern $$\pi'(\tau'(\begin{pmatrix} W & E \\ N & N \end{pmatrix})) = \begin{pmatrix} V & H & H & V \\ V & H & H & V \\ H & H & H & H \\ V & V & V & V \end{pmatrix}$$ only appears in even positions (both coordinates even), among the patterns $$\pi'(\tau'^n(a)$$ for any $$n$$ and $$a \in A$$. This again gives us a unique $$2$$-by-$$2$$ phase. We observe that $$\pi'$$ is injective on the $$\tau'(A)$$ so we can deduce the preimage configuration completely, proving $$\pi' : Y \to Z$$ is actually an isomorphism.

So as discussed earlier, the subshift $$Z$$ which is the dominofication of the Hilbert curve subshift, must be aperiodic (i.e. every configuration is aperiodic in it). It is also minimal as it is a morphic image of $$Y$$.

• Hi Ville Salo, thanks for the detailed and complete answer (!). As I commented before, I will use next weekend to read and discuss it. What I can say before are just my doubts, to get a better interpretation of your statements, and explain something of my question... 1. "interested in decidability questions", yes; 2. "questions about which dominoes occur more often", you answerd the subquestion, by confirming is a proof by induction, and thanks to the clues showing how to formalize. ...3... .... I will continue later. Apr 29, 2020 at 12:36
• 3. "I didn't actually get what the degenerated stuff was about", it is only my jargon to refer to the "transformated Hilbert curve", that maybe have another name like "Munkres curve". I liked your jargon, "dominofication of the Hilbert curve" Apr 29, 2020 at 12:46
• Oh, the degenerated curve is the curve along the dominoes of course. I don't know how I missed that. As dynamical systems, the original curve and the degenerated one are presumably the same, i.e. you can locally deduce one from the other. Apr 29, 2020 at 13:18
• On my cellphone app I keep seeing a comment here by the asker (one starting with an inverted exclamation mark), and then it disappears again, very strange. It's gone again. May 1, 2020 at 14:41
• (sorry deleted bad English and write again - not my language) Only checking my interpretatios... At the end, at the last proof, the $\pi'$ map transforms "Hilbert level 1" in to "Hilbert level 3". It is not a "dominofication", but we can suppose that the union of adjacent cells will be U-shape-horizontal or U-shape-vertial in a dominofication. In this case the dominoes of the "Degenerated Hilbert level 2.5" where each U-shape was transformated into 2 dominoes... And your matrix predicts correctly the vertial and horizontal dominoes. May 1, 2020 at 14:42

## This is not an answer...

This is a solution sketch in a Wiki, please edit here (!) to enhance. Informal clues and perhaps some starting point for good answers.

### All space-filling curves that satisfies the constraints

The step2 (second construction rule of the question) is a big constraint for "all" space-filling curves:

• it is to be used in a space partitioning: divides a space into non-overlapping regions, each receiving a region label;

• only the "split into 4 regions" partition is valid.

So, seems that no other curve exists (!), only the following 3 types, but one is not valid for domino generation:

### Labeling suggestion

This section is also for reviewing the description of the problem, not by mathematical theory, but by naming cells.

The step4 (merge rule) was expressed by indexes   $$j = \lceil{i/2} \rceil$$

The merge rule transforms the original curve into another fractal, so we cannot use directly all properties of the original fractal.

Geometrically it is a recursive 4-partition of the cells, them, the natural label is like Natural Number expressed by base 4... For each curve of hierarchical level L we have a grid of $$4^L$$ square cells labeled by a numeric code of L digits.

But it is not a number because it need to preserve hierarchy: "0" and "00" are distinct labels.
Notice also that the dominoes arise at the intermediate levels, L=½, L=1½, L=2½ etc. The dominoes of "half levels" with $$L>½$$ and index $$j = \lceil{i/2} \rceil$$ from $$i=0...4^{L+½}$$ can reuse the labels of level $$L-½$$ concatenated with a letter:

• L=0;   no grid, the square region to partitioned.

• L=0.5; 4/2=2 dominoes;   labels: G H.

• L=1;   grid of n=41=4;   labels: 0 1 2 3.

• L=1.5; 16/2=8 dominoes;   labels: 0G 0H 1G 1H 2G 2H 3G 3H.

• L=2;   grid of n=42=16;   labels: 00 01 02 03 10 11 12 13 20 21 ... 33.

• L=2.5; 64/2=32 dominoes;   labels: 00G 00H 01G 01H 02G 02H 10G 10H ... 33G 33H.

• L=3;   grid of n=43=64;   labels: 000 001 002 ... 333.

Any set of labels can be ordered by the lexicographical order of its alphabet: G,H,0,1,2,3.

Note: the illustrations above were obtained with Sfc4q, where you can play with more variants.

### Some label-region relationships

The labels above are codes (not numbers but strings of characters), but its syntax is subject of some mathematical analysis. We can express cell label and cell spatial relationships by some algebra...

Suppose that the union of labels has the semantic of geometrical union, "∪", of grid cells. The union of two squares is a domino.
The "merging rule", that is the transformation $$j = \lceil{i/2} \rceil$$, from integer level L to "half level" L+½,
can be translated into a label transformation:

• Level 1 to level 0.5, L1→L½ :   01 = G23 = H

• Level 2 to level 1.5, L2→L1½0001 = 0G0203 = 0H;   etc.

• Level 3 to level 2.5, L3→L2½000001 = 00G002003 = 00H;   etc.

Summarizing: the $$j = \lceil{i/2} \rceil$$ transformation always can be expressed by simple syntactic rule,
"preserve prefix and concatenate a letter"
that preserves parent cell label's prefix and preserves geometrical region of the parent cell.

For example, as we can see at illustrations, the code 10 at L2 and its position are preserved at levels L2½ (as 10G and 10H) and L3 (as 100, 101, 102 and 103). Same for code 21, preserved at L2½ (21G and 21H) and L3 (210, 211, 212 and 213).

Of course, it not solves the problem, only perhaps simplify demonstrations. Some other properties of the label transformation, valid for any fractal, Morton or Hilbert:

• In any transformation L → L-½  the union of any distinct cells, $$x \cup y$$, has labels with "even last digit" in $$x$$ (0, 2, 00, ...) and "odd last digit" in $$y$$ (1, 3, 01, ...).

• The transformation L1→L½ is not representative for induction
(we must to start induction proofs by the transform L2→L1½)

## Looking for references

There are some mathematical scholar article about the fractals and domino tiling?
Seems not easy to find... For example this illustration is a cut from Goodman-Strauss (2016)

There are no citation of Hilbert curve relationship, but maybe we can find some proof that it is a "infinite aperiodic".

It is interesting also to check references proofing that

• in the Morton curve, the "Z"-shape is periodic, for any level L.
• ...

### Clues for Morton curve

Seems easy to proof (if no reference to cite) that

the "Z"-shape is periodic, for any level L.

Proofs about the complementary fractal can use induction:

• L2 generates 100% of (periodic) horizontal dominoes.
• ... Any level L generates 100% of horizontal dominoes.
All pairs even-odd labels are horizontal, in any level.

### Clues for Hilbert curve

Seems easy to proof (if no reference to cite) that suppose the properties P1 and P2 bellow are based on true assertions:

P1. Hilbert curve have infinite aperiodic "U"-shape distribution.

P2. Hilbert curve have constant (rotated) "U"-shape distribution, for any integer level L>1: 50% are "⊐"-shape or "⊏"-shape, 50% are "⊓"-shape or "U"-shape.

After the "merge rule" transformation the property P2 results in a regular distribution of dominoes of different orientation:

• "⊐" and "⊏" generates 2 horizontal dominoes;
• "⊓" and "⊔" generates 2 vertical dominoes.