Posted by: Silvio Capobianco | 2015/03/12

## More Evidence that Regularity Is Not Preserved Up to Infinity

Let $\mathcal{A} = \langle Q, \mathcal{N}, f \rangle$ be a one-dimensional cellular automaton and let $F = F_\mathcal{A}$ be its global transition function. The very construction of $F$ shows that it is continuous in the product topology, as $F(c)$ coincides with $F(e)$ on $U \subseteq \mathbb{Z}$ as soon as $c$ and $e$ coincide on $U + \mathcal{N}$: and also commutes with translations, as it is easy to see how the translate of the update is precisely the update of the translate. We will discuss this in more detail in a future post.

As the image of a continuous function of a compact metric space into itself, the set $X = F(Q^\mathbb{Z})$ is a closed subset of $Q^\mathbf{Z}$ is a subshift, that is, a closed and translation invariant subset of the full shift $Q^\mathbb{Z}$. As such (again, we will go into further details in another post) $X$ is entirely determinated by its language

$\mathcal{L}(x) = \{ w \in Q^\ast \mid \exists c \in X, k \in \mathbb{Z} : c_{[k:k+|w|-1]} = w \}$

that is, the set of words that appear as “chunks” of elements of $X$. Such language, which we call the language of $\mathcal{A}$, is always regular.

To see this, we suppose $\mathcal{N} = \{ k, \ldots, k+m-1 \}$ for some $k \in \mathbb{Z}$, which we always can: and construct the de Bruijn graph of order $m$ on $Q$. This graph has as its nodes the words of length $m-1$ over $Q$, and as its edges the words of length $m$ over $Q$: an edge $w$ boes from the node $x$ to the node $y$ if and only if there exist $a, b \in Q$ such that $xb = ay = w$: that is, each word of length $m$ joins its own longest proper prefix to its own longest proper suffix. If we label each edge $w = w_1 \cdots w_m$ with $f(w_1, \ldots, w_m)$, and consider all states as both initial and final, we obtain a nondeterministic finite automaton that accepts precisely the language of $\mathcal{A}$.

As a composition of cellular automata is clearly a cellular automaton, we may consider the languages of the iterates of $\mathcal{A}$, which are the languages of the subshifts of the form $F^n(Q^\mathbb{Z})$ for $n \geq 0$. Now, an arbitrary intersection of closed sets is closed: similarly, an arbitrary intersection of translation invariant sets is translation  invariant. This means that the set

$\Omega_\mathcal{A} = \bigcap_{n \geq 0} F^n(Q^\mathbb{Z})$

is a subshift: which we call the limit set of $\mathcal{A}$. Similarly, we call the language $\Lambda_\mathcal{A}$ of $\Omega_\mathcal{A}$ the limit language of $\mathcal{A}$.

The first question that comes to the mind, is whether the limit language is regular or not: after all, regularity of intersection is only ensured for finitely many regular languages at once. Indeed, the answer is negative: a very nice counterexample was constructed by Lyman Hurd and published in 1987 on Complex Systems.

The idea of Hurd’s counterexample is to use signals moving either left or right at unitary speed, and ummovable, elastic walls that make signals bounce if they arrive at the same time from opposite directions, otherwise just stopping them. Conflicts are resolved by having the signals stop if they would end up occupying the same spot or crossing each other.

So, let’s set $Q = \{ \ell, r, \Box, W \}$, meaning signal moving to the left, signal moving to the right, empty slot, and wall, respectively; $\mathcal{N} = \{ -2, -1, 0, +1, +2 \}$, which is needed to correctly manage collisions; and $f$ according to the following lookup table:

$\begin{array}{|l|l|l|} \hline \mathrm{Input} & \mathrm{Output} & \mathrm{Condition} \\ \hline \left( \cdot, r, \Box, x, \cdot \right) & r & x \neq \ell \\ \hline \left( \cdot, \cdot, r, \Box, x \right) & \Box & x \neq \ell \\ \hline \left( \cdot, y, \Box, \ell, \cdot \right) & \ell & y \neq r \\ \hline \left( y, \Box, \ell, \cdot, \cdot \right) & \Box & y \neq r \\ \hline \left( \cdot, \cdot, r, W, \ell \right) & \ell & \\ \hline \left( r, W, \ell, \cdot, \cdot \right) & r & \\ \hline \left( \cdot, \cdot, z, \cdot, \cdot \right) & z & \mathrm{otherwise} \\ \hline \end{array}$

Hurd’s next trick relies on the following observation. Suppose $L$ is an arbitrary language, and $R$ a regular language: as a finite intersection of regular languages is regular, if it happens that $L \cap R$ is not regular, the only possible reason is that $L$ was already not regular in the first place! This is precisely what Hurd does by setting

$R = \Box \Box \ell \Box^\ast W \Box^\ast r \Box \Box$

and showing that

$\Lambda_\mathcal{A} \cap R = \{ \Box \Box \ell \Box^n W \Box^n r \Box \Box \mid n \geq 0 \}$

which clearly does not respect the necessary condition given by the pumping lemma for regular languages.

First, every such word belongs not only to $R$ (as it is evident) but to $\Lambda_\mathcal{A}$ too. Indeed, if we extend the word to a configuration by setting all the other cells as blank, we may obtain an infinitely long backward history by observing that:

• A predecessor of $\cdots \Box \Box \ell \Box^n W \Box^n r \Box \Box \cdots$ for $n > 0$ is $\cdots \Box \Box \ell \Box^{n-1} W \Box^{n-1} r \Box \Box \cdots$
• A predecessor of $\cdots \Box \Box \ell W r \Box \Box \cdots$ is $\cdots \Box \Box r W \ell \Box \Box \cdots$
• A predecessor of $\cdots \Box \Box r \Box^n W \Box^n \ell \Box \Box \cdots$ for $n \geq 0$  is $\cdots \Box \Box r \Box^{n+1} W \Box^{n+1} \ell \Box \Box \cdots$

Next, let $\Box \Box \ell \Box^i W \Box^j r \Box \Box$ be an arbitrary word of $R$ which also belongs to $\Lambda_\mathcal{A}$. Can it be that $i > j$? No, it cannot, because by doing $i-1$ backward steps we necessarily arrive at $\Box \Box \ell \Box W r \Box \Box$, which is an orphan. Why so? Because the only ways to have a situation of the form $W r$, is either as unchanged from the beginning, or immediately after a bounce: but after a bounce, the situation is $\ell W r$, not $\Box W r$. Symmetrically, it cannot be $i < j$.

Can more be said? Indeed, Hurd himself provides an upgraded construction, adding new signals moving left and right at double speed: the limit language of the new cellular automaton is not context free. Finally, as a consequence of computational universality of cellular automata (the reader can easily see how to simulate arbitrary Turing machines by suitable one-dimensional cellular automata) Hurd proves that some limit languages are not recursively enumerable: this last proof is, as it can be expected, nonconstructive. On the other hand, the complement of the limit language, as a union of regular languages, is recursively enumerable.

Bibliography:

1. Lyman P. Hurd. Formal Language Characterizations of Cellular Automaton Limit Sets. Complex Systems 1 (1987), 69–80.
Posted by: Silvio Capobianco | 2014/05/15

## Random Settings in Cellular Automata Machines

In cellular automata machines such as the CAM8 and SIMP/STEP (which is CAM8’s software incarnation) memory is organized into planes: each plane represents a certain type of information. A single cell spans through planes.

Consider, for example, the HPP (Hardy, de Pazzis and Pomeau) lattice gas model. In HPP, gas particles travel along the lines of a square grid and collide in the nodes of the grid. The collision rule states that, if exactly two particle meet in a node, and they come from opposite direction, then they bounce with a $90^\circ$ angle; in all other cases, particles go straight ahead. In SIMP/STEP, we can construct the HPP lattice gas model by defining interaction as follows: Read More…

Posted by: Silvio Capobianco | 2013/06/27

## In a Garden of Eden

The Garden-of-Eden theorem was the first rigorous result of cellular automata theory. It thus seems just fair to me that, after introducing the blog in my previous post, and having given talk about it during the Theory Lunch at the Institute of Cybernetics where I am working, I write a post about it on this blog.

The Garden-of-Eden theorem arose from the studies by Edward F. Moore about self-reproduction in distributed structures, a theme already discussed by John von Neumann in his groundbreaking work where the expression “cellular automaton” first appeared. Read More…

Posted by: Silvio Capobianco | 2013/06/25

## Hello, World!

Hi All,

So here it is at last. The idea for such a blog was suggested to me very long ago, but until now my laziness had always proven unbeatable.

I have worked on cellular automata theory and practice since my tesi di laurea, then as a graduate student, and now as a full-time researcher. I am not one of the big names, but suspect I might have done one or two good things: I’ll work so that this blog becomes one of them.

There are many people whom I wish to thank:

My thesis advisor, Patrizia Mentrasti, who introduced me to cellular automata, and most importantly suggested that I get a doctorate—so setting in motion an unpredictable chain of events thad made my life much more rich and adventurous than it would have been possible otherwise.

Tommaso Toffoli, a master scientist and true volcano of ideas, who helped and guided me at several points during my career and also introduced me to Python, which became my favorite (imperative) programming language. The idea that I write this blog, is actually his.

Ted Bach, who wrote the SIMP/STEP software.

Francesca Fiorenzi, whose doctoral thesis has been a great source of inspiration for me.

Luca Aceto and Anna Ingólfsdóttir, my supervisors in Reykjavík, who always let me free to pursue my research.

Tarmo Uustalu, my supervisor in Tallinn, a great scientist, a strong manager, and a wonderful person.

And, in random order: Jarkko Kari, Pierre Guillon, Enrico Formenti, Andy Adamatzky, Lynette van Zijl, Anna Ławniczak, Bruno Di Stefano, Pedro Paulo Balbi de Oliveira, Tullio Ceccherini-Silberstein, Antonio Machí, Aldo de Luca, Flavio D’Alessandro, Giovanna Sontacchi, Deepa Iyengar, Maria Arinbjarnar, MohammadReza Mousavi, Jérémy Terrien, Anahí Gajardo, Jaan Penjam, Hellis Tamm, James Chapman, Keiko Nakata, Pavel Grigorenko, Wolfgang Jeltsch, Mohamed El-Zawawy, Heiko Herrmann, David Schryer, Andres Braunbrück, and all the other wonderful people I have met during these years.

And, last but not least: you, the readers of this blog.