Jarkko Kari

University of Turku | UTU · Department of Mathematics and Statistics

A d -dimensional configuration $$c:\mathbb {Z}^d\longrightarrow A$$ c : Z d ⟶ A is a coloring of the d -dimensional infinite grid by elements of a finite alphabet $$A\subseteq \mathbb {Z}$$ A ⊆ Z . The configuration c has an annihilator if a non-trivial linear combination of finitely many translations of c is the zero configuration. Writing c as a...

We study an abstract group of reversible Turing machines. In our model, each machine is interpreted as a homeomorphism over a space which represents a tape filled with symbols and a head carrying a state. These homeomorphisms can only modify the tape at a bounded distance around the head, change the state and move the head in a bounded way. We stud...

Given a set P of allowed \(n\times m\) rectangular patterns of colors, a coloring of the grid \(\mathbb {Z}^2\) is called valid if every \(n\times m\) pattern in the coloring is in P. It is known that if the number of allowed \(n\times m\) patterns is at most nm and if there exists a valid coloring of \(\mathbb {Z}^2\) then there exists a valid per...

In this paper we study colorings (or tilings) of the two-dimensional grid ${\mathbb {Z}}^{2}$ ℤ 2 . A coloring is said to be valid with respect to a set P of n × m rectangular patterns if all n × m sub-patterns of the coloring are in P . A coloring c is said to be of low complexity with respect to a rectangle if there exist $m,n\in \mathbb {N}$ m ,...

In [1] we construct aperiodic tile sets on the Baumslag-Solitar groups BS(m,n). Aperiodicity plays a central role in the undecidability of the classical domino problem on Z2, and analogously to this we state as a corollary of the main construction that the Domino problem is undecidable on all Baumslag-Solitar groups. In the present work we elaborat...

Nilpotent cellular automata have the simplest possible dynamics: all initial configurations lead in bounded time into the unique fixed point of the system. We investigate nilpotency in the setup of one-dimensional non-uniform cellular automata (NUCA) where different cells may use different local rules. There are infinitely many cells in NUCA but on...

In our article in MCU'2013 we state the the Domino problem is undecidable for all Baumslag-Solitar groups $BS(m,n)$, and claim that the proof is a direct adaptation of the construction of a weakly aperiodic subshift of finite type for $BS(m,n)$ given in the paper. In this addendum, we clarify this point and give a detailed proof of the undecidabili...

We study the problem of sequentializing a cellular automaton without introducing any intermediate states, and only performing reversible permutations on the tape. We give a decidable characterization of cellular automata which can be written as a single sweep of a bijective rule from left to right over an infinite tape. Such cellular automata are n...

Reversible computation allows computation to proceed not only in the standard, forward direction, but also backward, recovering past states. While reversible computation has attracted interest for its multiple applications, covering areas as different as low-power computing, simulation, robotics and debugging, such applications need to be supported...

Cellular automata are topological dynamical systems. We consider the problem of deciding whether two cellular automata are conjugate or not. We also consider deciding strong conjugacy, that is, conjugacy by a map that commutes with the shift maps. We show that the following two sets of pairs of one-dimensional one-sided cellular automata are recurs...

The rank of a word in a deterministic finite automaton is the size of the image of the whole state set under the mapping defined by this word. We study the length of shortest words of minimum rank in several classes of complete deterministic finite automata, namely, strongly connected and Eulerian automata. A conjecture bounding this length is know...

We study Nivat's conjecture on algebraic subshifts and prove that in some of them every low complexity configuration is periodic. This is the case in the Ledrappier subshift (the 3-dot system) and, more generally, in all two-dimensional algebraic subshifts over Fp defined by a polynomial without line polynomial factors in more than one direction. W...

A two-dimensional configuration is a coloring of the infinite grid \(\mathbb {Z}^2\) with finitely many colors. For a finite subset D of \(\mathbb {Z}^2\), the D-patterns of a configuration are the colored patterns of shape D that appear in the configuration. The number of distinct D-patterns of a configuration is a natural measure of its complexit...

Cellular automata are topological dynamical systems. We consider the problem of deciding whether two cellular automata are conjugate or not. We also consider deciding strong conjugacy, that is, conjugacy by a map that commutes with the shift maps. We show that the following two sets of pairs of one-dimensional one-sided cellular automata are recurs...

This article studies the complexity of the word problem in groups of automorphisms (or reversible cellular automata) of subshifts. We show in particular that for any computably enumerable Turing degree, there exists a (two-dimensional) subshift of finite type whose automorphism group contains a subgroup whose word problem has exactly this degree. I...

A two-dimensional configuration is a coloring of the infinite grid Z^2 with finitely many colors. For a finite subset D of Z^2, the D-patterns of a configuration are the colored patterns of shape D that appear in the configuration. The number of distinct D-patterns of a configuration is a natural measure of its complexity. A configuration is consid...

We investigate the tiling problem, also known as the domino problem, that asks whether the two-dimensional grid Z^2 can be colored in a way that avoids a given finite collection of forbidden local patterns. The problem is well-known to be undecidable in its full generality. We consider the low complexity setup where the number of allowed local patt...

This article studies the complexity of the word problem in groups of automorphisms of subshifts. We show in particular that for any Turing degree, there exists a subshift whose automorphism group contains a subgroup whose word problem has exactly this degree.

A cellular automaton is a dynamical system on an infinite array of cells defined by a local update rule that is applied simultaneously at all cells. By carefully choosing the update rule, the global dynamics can be made information preserving. In this case, the cellular automaton is called reversible. In this article, we explain fundamental classic...

We study Nivat's conjecture on algebraic subshifts and prove that in some of them every low complexity configuration is periodic. This is the case in the Ledrappier subshift (the 3-dot system) and, more generally, in all two-dimensional algebraic subshifts over $\mathbb{F}_p$ defined by a polynomial without line polynomial factors in more than one...

We study the problem of sequentializing a cellular automaton without introducing any intermediate states, and only performing reversible permutations on the tape. We give a decidable characterization of cellular automata which can be written as a single left-to-right sweep of a bijective rule from left to right over an infinite tape.

We prove that surjective ultimately right-expansive cellular automata over full shifts are chain-transitive. This immediately implies Boyle’s result that expansive cellular automata are chain-transitive. This means that the chain-recurrence assumption can be dropped from Nasu’s result that surjective ultimately right-expansive cellular automata wit...

We study the problem of sequentializing a cellular automaton without introducing any intermediate states, and only performing reversible permutations on the tape. We give a decidable characterization of cellular automata which can be written as a single left-to-right sweep of a bijective rule from left to right over an infinite tape.

Two cellular automata are strongly conjugate if there exists a shift-commuting conjugacy between them. We prove that the following two sets of pairs $(F,G)$ of one-dimensional one-sided cellular automata over a full shift are recursively inseparable: (i) pairs where $F$ has strictly larger topological entropy than $G$, and (ii) pairs that are stron...

We consider one-dimensional cellular automata $F_{p,q}$ which multiply numbers by $p/q$ in base $pq$ for relatively prime integers $p$ and $q$. By studying the structure of traces with respect to $F_{p,q}$ we show that for $p\geq 2q-1$ (and then as a simple corollary for $p>q>1$) there are arbitrarily small finite unions of intervals which contain...

This book constitutes the refereed proceedings of the 13th Conference on Computability in Europe, CiE 2017, held in Turku, Finland, in June 2017. The 24 revised full papers and 12 invited papers were carefully reviewed and selected from 69 submissions. The conference CiE 2016 has six special sessions, namly: algorithmics for biology; combinatorics...

It is well-known that the Toffoli gate and the negation gate together yield a universal gate set, in the sense that every permutation of $\{0,1\}^n$ can be implemented as a composition of these gates. Since every bit operation that does not use all of the bits performs an even permutation, we need to use at least one auxiliary bit to perform every...

It is well-known that the Toffoli gate and the negation gate together yield a universal gate set, in the sense that every permutation of \(\{0,1\}^n\) can be implemented as a composition of these gates. Since every bit operation that does not use all of the bits performs an even permutation, we need to use at least one auxiliary bit to perform ever...

We discuss cellular automata over arbitrary finitely generated groups. We call a cellular automaton post-surjective if for any pair of asymptotic configurations, every pre-image of one is asymptotic to a pre-image of the other. The well known dual concept is pre-injectivity: a cellular automaton is pre-injective if distinct asymptotic configuration...

We consider Turing machines as actions over configurations in \(\varSigma ^{\mathbb {Z}^d}\) which only change them locally around a marked position that can move and carry a particular state. In this setting we study the monoid of Turing machines and the group of reversible Turing machines. We also study two natural subgroups, namely the group of...

In this paper we prove the existence of quasiperiodic rhombic substitution tilings with 2n-fold rotational symmetry, for any n. The tilings are edge-to-edge and use ⌊n2⌋\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \se...

We study multidimensional configurations (infinite words) and subshifts of low pattern complexity using tools of algebraic geometry. We express the configuration as a multivariate formal power series over integers and investigate the setup when there is a non-trivial annihilating polynomial: a non-zero polynomial whose formal product with the power...

We consider Turing machines as actions over configurations in $\Sigma^{\mathbb{Z}^d}$ which only change them locally around a marked position that can move and carry a particular state. In this setting we study the monoid of Turing machines and the group of reversible Turing machines. We also study two natural subgroups, namely the group of finite-...

It is well-known that the Toffoli gate and the negation gate together yield a universal gate set, in the sense that every permutation of $\{0,1\}^n$ can be implemented as a composition of these gates. Since every bit operation that does not use all of the bits performs an even permutation, we need to use at least one auxiliary bit to perform every...

We study multidimensional configurations (infinite words) and subshifts of low pattern complexity using tools of algebraic geometry. We express the configuration as a multivariate formal power series over integers and investigate the setup when there is a non-trivial annihilating polynomial: a non-zero polynomial whose formal product with the power...

We apply linear algebra and algebraic geometry to study infinite multidimensional words of low pattern complexity. By low complexity we mean that for some finite shape, the number of distinct sub-patterns of that shape that occur in the word is not more than the size of the shape. We are interested in discovering global regularities and structures...

We discuss cellular automata over arbitrary finitely generated groups. We call a cellular automaton post-surjective if for any pair of asymptotic configurations, every preimage of one is asymptotic to a preimage of the other. The well known dual concept is pre-injectivity: a cellular automaton is pre-injective if distinct asymptotic configurations...

We study the message size complexity of recognizing, under the broadcast congested clique model, whether a fixed graph H appears in a given graph G as a minor, as a subgraph or as an induced subgraph. The n nodes of the input graph G are the players, and each player only knows the identities of its immediate neighbors. We are mostly interested in t...

We construct a one-dimensional reversible cellular automaton that is computationally universal in a rather strong sense while being highly non-sensitive to initial conditions as a dynamical system. The cellular automaton has no sensitive subsystems. The construction is based on a simulation of a reversible Turing machine, where a bouncing signal ac...

Delvenne, K\r{u}rka and Blondel have defined new notions of computational complexity for arbitrary symbolic systems, and shown examples of effective systems that are computationally universal in this sense. The notion is defined in terms of the trace function of the system, and aims to capture its dynamics. We present a Devaney-chaotic reversible c...

We study the decidability of some properties of self-affine sets specified by a graph-directed iterated function system (GIFS) with rational coefficients. We focus on topological properties and we prove that having empty interior is undecidable in dimension two. These results are obtained by studying a particular class of self-affine sets associate...

Reversible cellular automata are seen as microscopic physical models, and their states of macroscopic equilibrium are described using invariant probability measures. Characterizing all the invariant measures of a cellular automaton could be challenging. Nevertheless, we establish a connection between the invariance of Gibbs measures (used in statis...

Automata, Logic and Semantics International audience One of the first and most famous results of cellular automata theory, Moore's Garden-of-Eden theorem has been proven to hold if and only if the underlying group possesses the measure-theoretic properties suggested by von Neumann to be the obstacle to the Banach-Tarski paradox. We show that severa...

We exhibit a weakly aperiodic tile set for Baumslag-Solitar groups, and prove that the domino problem is undecidable on these groups. A consequence of our construction is the existence of an arecursive tile set on Baumslag-Solitar groups.

We discuss one-dimensional reversible cellular automata F ×3 and F ×3/2 that multiply numbers by 3 and 3/2, respectively, in base 6. They have the property that the orbits of all non-uniform 0-finite configurations contain as factors all finite words over the state alphabet {0,1,…,5}. Multiplication by 3/2 is conjectured to even have an orbit of 0-...

We survey results on decidability questions concerning cellular automata. Properties discussed include reversibility and surjectivity and their variants, time-symmetry and conservation laws, nilpotency and other properties of the limit set and the trace, properties chaoticity related such as sensitivity to initial conditions and mixing of the space...

The notion of reversibility has been intensively studied in the field of cellular automata (CA), for several reasons. However, a related notion found in physical theories has been so far neglected, not only in CA, but generally in discrete dynamical systems. This is the notion of time-symmetry, which refers to the inability of distinguishing betwee...

Cellular automata are mathematical models for massively parallel processing of information by a large number of identical, locally interconnected tiny processors on a regular grid. Extremely simple processors, or cells, are known to be able to generate together complex patterns. In this talk we consider the problem of designing a cellular automaton...

The density classification task asks to design a cellular automaton that converges to the uniform configuration that corresponds to the state that is in majority in the initial configuration. We investigate connections of this problem to state-conserving cellular automata. We propose a modified traffic CA that washes out finite islands in the same...

This chapter reviews some basic concepts and results of the theory of cellular automata (CA). Topics discussed include classical results from the 1960s, relations between various concepts of injectivity and surjectivity, and dynamical system concepts related to chaos in CA. Most results are reported without full proofs but sometimes examples are pr...

Multidimensional combinatorial substitutions are rules that replace symbols by finite patterns of symbols in Z^d. We focus on the case where the patterns are not necessarily rectangular, which requires a specific description of the way they are glued together in the image by a substitution. Two problems can arise when defining a substitution in suc...

Conservation laws in cellular automata (CA) are studied as an abstraction of the conservation laws observed in nature. In addition to the usual real-valued conservation laws we also consider more general group-valued and semigroup-valued conservation laws. The (algebraic) conservation laws in a CA form a hierarchy, based on the range of the interac...

One possible complexity measure for a cellular automaton is the size of its neighborhood. If a cellular automaton is reversible with a small neighborhood, the inverse automaton may need a much larger neighborhood. Our interest is to find good upper bounds for the size of this inverse neighborhood. It turns out that a linear algebra approach provide...

A careful analysis of an old undecidability proof reveals that periodicity and non-surjectivity of two-dimensional cellular automata are recursively inseparable properties. Analogously, Wang tile sets that admit tilings of arbitrarily long loops (and hence also infinite snakes) are recursively inseparable from the tile sets that admit no loops and...

We construct a cellular automaton (CA) with a sofic and mixing limit set and then construct a stable CA with the same limit set, showing there exist subshifts that can be limit sets of both stable and unstable CAs, answering a question raised by A. Maass [Ergodic Theory Dyn. Syst. 15, No. 4, 663–684 (1995; Zbl 0864.58030)].

Picture walking automata were introduced by M. Blum and C. Hewitt in 1967 as a generalization of one-dimensional two-way finite automata to recognize pictures, or two-dimensional words. Several variants have been investigated since then, including deterministic, non-deterministic and alternating transition rules; four-, three- and two-way movements...

geometrical computation involves drawing colored line segments (traces of signals) according to rules: signals with similar color are parallel and when they intersect, they are replaced according to their colors. Time and space are continuous and accumulations can be devised to unlimitedly accelerate a computation and provide, in a finite duration,...

International audience We discuss a close link between two seemingly different topics studied in the cellular automata literature: additive conservation laws and invariant probability measures. We provide an elementary proof of a simple correspondence between invariant full-support Bernoulli measures and interaction-free conserved quantities in the...

These local proceedings hold the papers of two catgeories: (a) Short, non-reviewed papers (b) Full papers

Nondeterministic finite automata with states and transitions labeled by real-valued weights have turned out to be powerful tools for the representation and compression of digital grayscale and color images. The addressing of pixels by input-sequences is extended to cover multi-resolution images. Encoding algorithms for such weighted finite automata...

Cellular automata are examples of discrete complex systems where non-trivial global behavior emerges from the local interaction of trivial components. Cellular automata have been studied, among other perspectives, as models of massively parallel computation tightly connected to the microscopic physics. Physics is time reversible and satisfies vario...

We investigate the continuity of the \omega-functions and real functions defined by weighted finite automata (WFA). We concentrate on the case of average preserving WFA. We show that every continuous \omega-function definable by some WFA can be defined by an average preserving WFA and then characterize minimal average preserving WFA whose \omega-fu...

Cellular automata (CA) are discrete, homogeneous dynamical systems. Non-surjective one-dimensional CA have finite words with no preimage (called orphans), pairs of different words starting and ending identically and having the same image (diamonds) and words with more/ fewer preimages than the average number (unbalanced words). Using a linear algeb...

Using the fact that the tiling problem of Wang tiles is undecidable even if the given tile set is deterministic by two opposite corners, it is shown that the question whether there exists a trajectory which belongs to the given open and closed set is undecidable for one-dimensional reversible cellular automata. This result holds even if the cellula...

We prove that for given morphisms g,h:{a 1 ,a 2 ,⋯,a n }→B * , it is decidable whether or not there exists a word w in the regular language a 1 * a 2 * ⋯a n * such that g(w)=h(w). In other words, we prove that the Post Correspondence Problem is decidable if the solutions are restricted to be from this special language. This yields a nice example of...

In this work, we have designed an efficient arithmetic coder for the non-linear bi-level image coder based on reversible cellular automata transform reported in the work of Cruz-Reyes and Kari (2008). The proposed approach relies on non-linear transform operations based on RmbCA, decomposing the image into four subimages (named LL, LH, HL, HH in le...

Spatially scalable image coding algorithms are mostly based on linear filtering techniques that give a multi-resolution representation of the data. Reversible cellular automata can be instead used as simpler, non-linear filter banks that give similar performance. In this paper, we investigate the use of reversible cellular automata for lossy to los...

Self-assembly, the process by which objects autonomously come together to form complex structures, is omnipresent in the physical world. Recent experiments in self-assembly demonstrate its potential for the parallel creation of a large number of nanostructures, including possibly computers. A systematic study of self-assembly as a mathematical proc...

We study the group-valued and semigroup-valued conserva- tion laws in cellular automata (CA). We provide examples to distin- guish between semigroup-valued, group-valued and real-valued conserva- tion laws. We prove that, even in one-dimensional case, it is undecidable if a CA has any non-trivial conservation law of each type. For a fixed range, ea...

. We give a new presentation of two results concerning synchronized automata. The first one gives a linear bound on the synchronization delay of complete local automata. The second one gives a cubic bound for the minimal length of a synchronizing pair in a complete synchronized unambiguous automaton. The proofs are based on results on unambiguous m...

Many properties of the dynamics of one-dimensional cellular automata are known to be undecidable. However, the undecidability proofs often rely on the undecidability of the nilpotency problem, and hence cannot be applied in the case the automaton is reversible. In this talk we review some recent approaches to prove dynamical properties of reversibl...

The problem of describing the dynamics of a conserved energy in a cellular automaton in terms of local movements of "particles" (quanta of that energy) has attracted some people's attention. The one-dimensional case was already solved by Fukś (2000) and Pivato (2002). For the two-dimensional cellular automata, we show that every (context-free) cons...

We investigate the decidability of the periodicity and the immortality problems in three models of reversible computation: reversible counter machines, reversible Turing machines and reversible one-dimensional cellular automata. Immortality and periodicity are properties that describe the behavior of the model starting from arbitrary initial config...

Real functions on the domain [0,1)n – often used to describe digital images – allow for different well-known types of binary operations. In this note, we recapitulate how weighted finite automata can be used in order to represent those functions and how certain binary operations are reflected in the theory of these automata. Different types of prod...

The tiling problem is the decision problem to determine if a given finite collection of Wang tiles admits a valid tiling of the plane. In this work we give a new proof of this fact based on tiling simulations of certain piecewise affine transformations. Similar proof is also shown to work in the hyperbolic plane, thus answering an open problem pose...

This article summarizes the theory and main techniques used in lossy data compression. Topics covered include lossy information theory rate-distortion theory, scalar and vector quantization, predictior-based coding and linear transformations, including KLT, DCT, and subband transformations. Also the JPEG image compression standard is discussed brie...

Glossary Definition of the Subject Introduction The Tiling Problem and Its Variants Undecidability in Cellular Automata Future Directions Acknowledgments Bibliography

A 1D Reversible Cellular Automata (RCA) with forward and backward radius- 1 2 neighborhoods is called Rectangular. It was previously conjectured that the conservation laws in 1D Rectangular RCA can be described as linear combinations of independent constant-speed flows to the right or to the left. This is indeed the case; so is a similar statement...

Conservation laws in physics are numerical invariants of the dynamics of a system. This article concerns conservation laws in a fictitious universe of a cellular automaton. We give an overview of the subject, with particular attention to problems of combinatorial flavor.

We give a new proof for the undecidability of the tiling problem. Then we show how the proof can be modified to demonstrate the undecidability of the tiling problem on the hyperbolic plane, thus answering an open problem posed by R.M.Robinson 1971 [6].

We give a new proof for the undecidability of the tiling problem. Then we show how the proof can be modified to demonstrate the undecidability of the tiling problem on the hyperbolic plane, thus answering an open problem posed by R.M. Robinson 1971 [6].

Reversible cellular automata (RCA) are models of massively parallel computation that preserve information. We generalize these systems by introducing the class of ωωbijective finite automata. It consists of those finite automata where for any bi-infinite word there exists a unique path labelled by that word. These systems are strictly included in t...