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We are lifting classical combinatorial problems from single instances to regular sets of instances. The task of finding a positive instance of the combinatorial problem P in a potentially infinite given regular set is equivalent to the so called in t Reg -problem of P, which asks for a given DFA A, whether the intersection of P with L ( A ) is non-empty. The in t Reg -problem generalizes the idea of considering multiple instances at once and connects classical combinatorial problems with the field of automata theory. While the question of decidability of the in t Reg -problem has been answered positively for several NP and even PSPACE -complete problems, we are presenting some natural problems even from LOGSPACE with an undecidable in t Reg -problem. We also discuss alphabet sizes and different encoding-schemes elaborating the boundary between problem-variants with a decidable respectively undecidable in t Reg -problem.

The locality of a graph problem is the smallest distance $T$ such that each node can choose its own part of the solution based on its radius-$T$ neighborhood. In many settings, a graph problem can be solved efficiently with a distributed or parallel algorithm if and only if it has a small locality. In this work we seek to automate the study of solvability and locality: given the description of a graph problem $\Pi$, we would like to determine if $\Pi$ is solvable and what is the asymptotic locality of $\Pi$ as a function of the size of the graph. Put otherwise, we seek to automatically synthesize efficient distributed and parallel algorithms for solving $\Pi$. We focus on locally checkable graph problems; these are problems in which a solution is globally feasible if it looks feasible in all constant-radius neighborhoods. Prior work on such problems has brought primarily bad news: questions related to locality are undecidable in general, and even if we focus on the case of labeled paths and cycles, determining locality is $\mathsf{PSPACE}$-hard (Balliu et al., PODC 2019). We complement prior negative results with efficient algorithms for the cases of unlabeled paths and cycles and, as an extension, for rooted trees. We introduce a new automata-theoretic perspective for studying locally checkable graph problems. We represent a locally checkable problem $\Pi$ as a nondeterministic finite automaton $\mathcal{M}$ over a unary alphabet. We identify polynomial-time-computable properties of the automaton $\mathcal{M}$ that near-completely capture the solvability and locality of $\Pi$ in cycles and paths, with the exception of one specific case that is $\mbox{co-$\mathsf{NP}$}$-complete.

Regularities in strings arise in various areas of science, including coding and automata theory, formal language theory, combinatorics, molecular biology and many others. A common notion to describe regularity in a string T is a cover, which is a string C for which every letter of T lies within some occurrence of C. The alignment of the cover repetitions in the given text is called a tiling. In many applications finding exact repetitions is not sufficient, due to the presence of errors. In this paper, we use a new approach for handling errors in coverable phenomena and define the approximate cover problem (ACP), in which we are given a text that is a sequence of some cover repetitions with possible mismatch errors, and we seek a string that covers the text with the minimum number of errors. We first show that the ACP is NP -hard, by studying the cover-length relaxation of the ACP, in which the requested length of the approximate cover is also given with the input string. We show that this relaxation is already NP -hard. We also study another two relaxations of the ACP, which we call the partial-tiling relaxation of the ACP and the full-tiling relaxation of the ACP, in which a tiling of the requested cover is also given with the input string. A given full tiling retains all the occurrences of the cover before the errors, while in a partial tiling there can be additional occurrences of the cover that are not marked by the tiling. We show that the partial-tiling relaxation has a polynomial time complexity and give experimental evidence that the full-tiling also has polynomial time complexity. The study of these relaxations, besides shedding another light on the complexity of the ACP, also involves a deep understanding of the properties of covers, yielding some key lemmas and observations that may be helpful for a future study of regularities in the presence of errors.

Linear sets are the building blocks of semilinear sets, which are in turn closely connected to automata theory and formal languages. Prior work has investigated the learnability of linear sets and semilinear sets in three models – Valiant's PAC-learning model, Gold's learning in the limit model, and Angluin's query learning model. This paper considers teacher–learner models of learning families of linear sets, in which a benevolent teacher presents a set of labelled examples to the learner. First, we study the classical teaching model, in which a teacher must successfully teach any consistent learner. Second, we will apply a generalisation of the recently introduced recursive teaching model to several infinite classes of linear sets, and show that thus the maximum sample complexity of teaching these classes can be drastically reduced compared to classical teaching. To this end, a major focus of the paper will be on determining two relevant teaching parameters, the teaching dimension and recursive teaching dimension, for various families of linear sets.

Recently, the minimum number of reticulation events that is required to simultaneously embed a collection P of rooted binary phylogenetic trees into a so-called temporal network has been characterized in terms of cherry-picking sequences. Such a sequence is a particular ordering on the leaves of the trees in P. However, it is well-known that not all collections of phylogenetic trees have a cherry-picking sequence. In this paper, we show that the problem of deciding whether or not P has a cherry-picking sequence is NP-complete for when P contains at least eight rooted binary phylogenetic trees. Moreover, we use automata theory to show that the problem can be solved in polynomial time if the number of trees in P and the number of cherries in each such tree are bounded by a constant., Accepted for publication in Theoretical Computer Science

One-way multi-head finite automata are considered towards their ability to perform reversible computations. It is shown that, for every number k ≥ 1 of heads, there are problems which can be solved by one-way k-head finite automata, but not by any one-way reversible k-head finite automaton. Additionally, a proper head hierarchy is obtained for one-way reversible multi-head finite automata. Closure properties and decidability problems are investigated as well. It turns out that one-way reversible finite automata with two heads are still a powerful model, since almost all commonly studied problems are not even semidecidable. Finally, descriptional complexity aspects are studied and non-recursive trade-offs are shown.

In this survey, we will discuss current uses of finite-state information in several statistical natural language processing tasks. To this end, we will review standard approaches in tokenization, part-of-speech tagging, and parsing, and illustrate the utility of finite-state information and technology in these areas. The particular problems were chosen to allow a natural progression from simple prediction to structured prediction. We aim for a sufficiently formal presentation suitable for readers with a background in automata theory that allows to appreciate the contribution of finite-state approaches, but we will not discuss practical issues outside the core ideas. We provide instructive examples and pointers into the relevant literature for all constructions. We close with an outlook on finite-state technology in statistical machine translation.

Finite languages are an important sub-regular language family, which were intensively studied during the last two decades in particular from a descriptional complexity perspective. An important contribution to the theory of finite languages are the deterministic and the recently introduced nondeterministic finite cover automata (DFCAs and NFCAs, respectively) as an alternative representation of finite languages by ordinary finite automata. We compare these two types of cover automata from a descriptional complexity point of view, showing that these devices have a lot in common with ordinary finite automata. In particular, we study how to adapt lower bound techniques for nondeterministic finite automata to NFCAs such as, e.g., the biclique edge cover technique, solving an open problem from the literature. Moreover, the trade-off of conversions between DFCAs and NFCAs as well as between finite cover automata and ordinary finite automata are investigated. Finally, we present some results on the average size of finite cover automata.

We present a disambiguation algorithm for weighted automata. The algorithm admits two main stages: a pre-disambiguation stage followed by a transition removal stage. We give a detailed description of the algorithm and the proof of its correctness. The algorithm is not applicable to all weighted automata but we prove sufficient conditions for its applicability in the case of the tropical semiring by introducing the weak twins property . In particular, the algorithm can be used with any weighted automaton over the tropical semiring for which the weighted determinization algorithm terminates and with any acyclic weighted automaton over an arbitrary weakly left divisible cancellative and commutative semiring. While disambiguation can sometimes be achieved using weighted determinization, our disambiguation algorithm in some cases can return a result that is exponentially smaller than any equivalent deterministic automaton. We also present some empirical evidence of the space benefits of disambiguation over determinization in speech recognition and machine translation applications.

We first outline main steps and achievements along Bertoni's research path in quantum finite automata theory from the very basic definitions of the models of quantum finite automata throughout the investigation of their computational and descriptional power. Next, we choose to focus on Bertoni's studies on quantum finite automata descriptional complexity. In particular, we expand on a statistical framework for the synthesis of succinct quantum finite automata, discussing its adaptation to the case of multiperiodic events and languages. We then improve such a framework to obtain even more succinct quantum finite automata for some multiperiodic languages. Finally, we introduce some promise problems for multiperiodic inputs, showing that even on this class of problems the descriptional power of quantum finite automata greatly outperforms that of equivalent classical finite automata. We build small size Monte Carlo quantum finite automata for multiperiodic languages.We analyze the modular architecture of such Monte Carlo quantum finite automata.We solve promise problems on multiperiodic inputs by small quantum finite automata.

In this paper, we pioneer a study of parameterized automata constructions for languages related to the design of parameterized algorithms. We focus on the k- Distinct language L k ( Σ ) ⊆ Σ k , defined as the set of words of length k over an alphabet Σ whose symbols are all distinct. This language is implicitly related to several breakthrough techniques developed during the last two decades, to design parameterized algorithms for fundamental problems such as k- Path and r- Dimensional k- Matching . Building upon the color coding, divide-and-color and narrow sieves techniques, we obtain the following automata constructions for L k ( Σ ) . We develop non-deterministic automata (NFAs) of sizes 4 k + o ( k ) ⋅ n O ( 1 ) and ( 2 e ) k + o ( k ) ⋅ n O ( 1 ) , where the latter satisfies a ‘bounded ambiguity’ property relevant to approximate counting, as well as a non-deterministic xor automaton (NXA) of size 2 k ⋅ n O ( 1 ) , where n = | Σ | . We show that our constructions can be used to develop both deterministic and randomized algorithms for k- Path , r- Dimensional k -Matching and Module Motif in a natural manner, considering also their approximate counting variants. Our framework is modular and consists of two parts: designing an automaton for k- Distinct , and designing a problem specific automaton, as well as an algorithm for deciding whether the intersection automaton's language is empty, or for counting the number of accepting paths in it.

In this paper we study structure and representations of cyclic automata. Corresponding to Green's equivalences in semigroup theory, we introduce three binary relations say L , R and H on cyclic automata. An automaton is said to be strict if L is an equivalence on the set of states. Some properties of these relations are established for giving characterizations of three subclasses of strict automata. Also, we provide representations of strict automata by representing the states as vectors and describing the state transitions in terms of matrix operations. These results generalize and extend Ito's representations of strongly connected automata.

One aspect of DNA Computing is the possibility of using DNA molecules for solving some "complicated" computational problems. In this context, the DNA code word design problem assumes a fundamental role: given a problem encoded in DNA strands and biochemical processes, the final computation is a concatenation of the input DNA strands that must allow us to recover the solution of the given problem in terms of the input (unique decipherability). Thus the initial set of DNA strands must be a code. In addition, it should satisfy some restrictions, called here DNA properties, in order to prevent them from interacting in undesirable ways. So a new interest towards the design of efficient algorithms for testing whether a language X is a code, has arisen from (wet) DNA Computing, but, as far as we know, only when X is a finite set. In this paper we provide an algorithm for testing whether an infinite but regular set of words is a code that avoids some DNA properties among unwanted intermolecular and intramolecular hybridizations.

The study of some close connections between the combinatorial properties of words and the performance of the automata minimization process constitutes the main focus of this paper. These relationships have been, in fact, the basis of the study of the tightness and the extremal cases of Hopcroft's algorithm, that is, up to now, the most efficient minimization method for deterministic finite state automata. Recently, increasing attention has been paid to another minimization method that, unlike the approach proposed by Hopcroft, is not based on refinement of the set of states of the automaton, but on automata operations such as determinization and reverse, and is also applicable to non-deterministic finite automata. However, even for deterministic automata, it was proved that the method incurs, almost surely, in an explosion of the number of the states. Very recently, some polynomial variants of Brzozowski's method have been introduced. In this paper, by using some combinatorial properties of words, we analyze the performance of one of such algorithms when applied to a particular infinite family of automata, called standard word automata, constructed by using standard sturmian words. In particular, ? ( n log ? n ) is the worst case time complexity when the algorithm is applied to the infinite family of word automata associated to Fibonacci words representing also the worst case of Hopcroft's minimization algorithm.

In a recent work, Gandhi, Khoussainov, and Liu 7] introduced and studied a generalized model of finite automata able to work over arbitrary structures. As one relevant area of research for this model the authors identify studying such automata over particular structures such as real and algebraically closed fields.In this paper we start investigations into this direction. We prove several structural results about sets accepted by such automata, and analyze decidability as well as complexity of several classical questions about automata in the new framework. Our results show quite a diverse picture when compared to the well known results for finite automata over finite alphabets.

The existing theory of graphical limit sets for cellular automata relies on algebraic structures and applies only to certain classes of cellular automata that possess this structure. We extend this theory to general cellular automata using topological methods. The starting point is the observation that the rescaled space-time diagrams, intersected with an appropriately chosen compact set, form sequences in a compact, metric space. They necessarily possess converging subsequences. In the present paper we define graphical limit sets as the collection of the accumulation points. The main result is that for a large class of cellular automata the graphical limit set defined in this way carries a group structure, which is either the trivial group consisting of one element only, or is homeomorphic to S 1 . The well known self-similar, graphical limit sets are representatives of the second class.

We study sampling schemes for executions of automata and programs, which can be used, based on Shannon information theory, to measure how much information flows from one variable to another. We show that the information rates of periodically sampled executions of a nondeterministic finite automaton and of a reversal-bounded nondeterministic multicounter machine are computable. Finally, through experiments, we use Lempel-Ziv compression algorithm to approximate information leakage in programs.

We present alternative relatively simple and self-contained proofs of decidability of the emptiness problems for one-register alternating finite-memory automata and one-register alternating finite-memory tree automata.

Monoid-matrix type automata are introduced and studied in this paper. We give a characterization of the cyclic monoid-matrix type automata and the regular monoid-matrix type automata. Also, we provide a method to determine the structures of canonical S@?-automata (canonical C-automata, respectively) whose endomorphism monoids are isomorphic to a given finite meet semilattice with the greatest element (Clifford monoid, respectively).

To overcome the sky-rocketing verification cost of embedded software, symbolic model-checking technology of dense-time automata has been proposed as an automated solution. The construction of a timed precondition is a central component in the technology. The general formulation for a timed precondition operator needs to check the continuity of time progress and usually results in high complexity in the construction. However, when the state space characterized by the path condition is convex, we can use a more efficient convex timed precondition operator. In this work, we discuss the concept of time-convexity that allows us to relax the restrictions on the application of the convex timed precondition operator in place of the general one. We present examples in model-checking that engender a non-time-convex space of time progress. Nevertheless, we have also identified a class of timed computation tree logic (TCTL) formulas that only characterize time-convex state spaces. This class includes several important types of TCTL formulas, including some timed inevitabilities with deadlines. We then present a new formulation for the efficient evaluation of general timed inevitabilities with non-time-convex path conditions. The new formulation also leads to a new technique for the approximate evaluation of timed inevitabilities with better precision. Finally, we report our implementation and experiments.

We continue here the investigation of P automata, in their non-extended case, a class of devices which characterize non-universal family of languages. First, a recent conjecture is confirmed: any recursively enumerable language is obtained from a language recognized by a P automaton, to which an initial (arbitrarily large) string is added. Then, we discuss possibilities of extending P automata, following suggestions from string finite automata. For instance, automata with a memory (corresponding to push-down automata) are considered and their power is briefly investigated, as well as some closure properties of the family of languages recognized by P automata. In the context, a brief survey of results about P and dP automata (a distributed version of P automata) is provided, and several further research topics are formulated.

In this paper we investigate the minimality problem of DFAs by varying the set of final states. In other words, we are interested on how the choice of the final states can affect the minimality of the automata. The state-pair graph is a useful tool to investigate such a problem. The choice of a set of final states for the automaton A defines a coloring of the closed components of the state-pair graph and the minimality of A corresponds to a property of these colored components. A particular attention is devoted to the analysis of some extremal cases such as, for example, the automata that are minimal for any choice of the subset of final states F from the state set Q of the automaton (uniformly minimal automata), the automata that are minimal for any proper subset F of Q (almost uniformly minimal automata) and the automata that are never minimal, under any assignment of final states (never-minimal automata). More generally, we seek to characterize those families of automata and show that some of them are related to well-known objects in a different context (e.g. multiple-entry automata and Fischer covers of irreducible sofic shifts in Symbolic Dynamics). Next, we study the complexity of the related decision problems and show, in some cases, how to derive a polynomial algorithm. Finally, we pay particular attention to the relationship between the problem to decide if an automaton is never-minimal and the “syntactic monoid problem”.

When generalizing the projection-valued measurements to the positive operator-valued measurements, the notion of the quantum logic generalizes from the sharp quantum logic to the unsharp quantum logic. It is known that: (i) the distributive law is one of the main differences between the sharp quantum logic and the boolean logic, and the block or the center of the sharp quantum structures are boolean algebras; (ii) the unsharp quantum logic does not satisfy the non-contradiction law, which forces the block or the center of unsharp quantum structures to be multiple valued algebras, rather than boolean algebras. Multiple valued algebras, as special quantum structures, are the algebraic semantics of multiple valued logic. Interestingly, we recently discovered that the difference between some unsharp quantum structures and multiple valued algebras is also some kind of distributive law.Choosing an orthomodular lattice (an algebraic model of a sharp quantum logic) to be the truth valued lattice, Ying et al. have systematically developed automata theory based on sharp quantum logic. In this paper, choosing a lattice ordered quantum multiple valued algebra E (an extended lattice ordered effect algebra E, respectively) to be the truth valued lattice, we also systematically develop an automata theory based on unsharp quantum logic. We introduce E-valued finite-state automata and E-valued pushdown automata in the framework of unsharp quantum logic. We study the classes of languages accepted by these automata and re-examine their various properties in the framework of unsharp quantum logic. The study includes the equivalence between finite-state automata and regular expressions, as well as the equivalence between pushdown automata and context-free grammars. It is also demonstrated that the universal validity of some important properties (such as some closure properties of languages and Kleene theorem etc.) depends heavily on the aforementioned distributive law. More precisely, when the underlying model degenerates into an MV algebra, then all the counterparts of properties in classical automata are valid. This is the main difference between automata theory based on unsharp quantum logic and automata theory based on sharp quantum logic.

Asynchronous automata are a model of communication processes with a control structure distributed on a set P of processes, global initializations and global accepting conditions. The well-known theorem of Zielonka states that they recognize exactly the class of regular Mazurkiewicz trace languages. The corresponding synthesis problem is, given a global specification A of any regular trace language L, to build an asynchronous automaton that recognizes L, automatically. Yet, all such existing constructions are quite involved and yield an explosion of the number of states in each process, which is exponential in both the sizes of A and P. In this paper, we introduce the particular case of distributed asynchronous automata, which require that the initializations and the accepting conditions are distributed as well. We present an original technique based on simple compositions/decompositions of these distributed asynchronous automata that results in the construction of smaller non-deterministic asynchronous automata: now, the number of states in each process is only polynomial in the size of A, but is still exponential in the size of P.

This paper investigates the expressiveness of Propositional Projection Temporal Logic with Star (PPTL*). To this end, Büchi automata and ω-regular expressions are first extended as Stutter Büchi Automata (SBA) and Extended Regular Expressions (ERE) to include both finite and infinite strings. Further, by equivalent transformations among PPTL* formulas, SBAs and EREs, PPTL* is proved to represent exactly the full regular language. Moreover, some fragments of PPTL* are characterized, and finally, PPTL* and its fragments are classified into five different language classes.

We reformulate the theory of ludics introduced by J.-Y. Girard from a computational point of view. We introduce a handy term syntax for designs, the main objects of ludics. Our syntax also incorporates explicit cuts for attaining computational expressivity. In addition, we consider design generators that allow for finite representation of some infinite designs. A normalization procedure in the style of Krivine’s abstract machine directly works on generators, giving rise to an effective means of computation over infinite designs.The acceptance relation between machines and words, a basic concept in computability theory, is well expressed in ludics by the orthogonality relation between designs. Fundamental properties of ludics are then discussed in this concrete context. We prove three characterization results that clarify the computational powers of three classes of designs. (i) Arbitrary designs may capture arbitrary sets of finite data. (ii) When restricted to finitely generated ones, designs exactly capture the recursively enumerable languages. (iii) When further restricted to cut-free ones as in Girard’s original definition, designs exactly capture the regular languages.We finally describe a way of defining data sets by means of logical connectives, where the internal completeness theorem plays an essential role.

Deterministic finite automata (DFAs) are constructed for various purposes in computational biology. Little attention, however, has been given to the efficient construction of minimal DFAs. In this article, we define simple non-deterministic finite automata (NFAs) and prove that the standard subset construction transforms NFAs of this type into minimal DFAs. Furthermore, we show how simple NFAs can be constructed from two types of pattern popular in bioinformatics, namely (sets of) generalized strings and (generalized) strings with a Hamming neighborhood.

During the last years, weighted timed automata have received much interest in the real-time community. Weighted timed automata form an extension of timed automata and allow us to assign weights (costs) to both locations and edges. This model, introduced by Alur et al. (2001) and Behrmann et al. (2001), permits the treatment of continuous consumption of resources and has led to much research on scheduling problems, optimal reachability and model checking. Also, several authors have derived Kleene-type characterizations of (unweighted) timed automata and their accepted timed languages. The goal of this paper is to provide a characterization of the behaviours of weighted timed automata by rational power series. We define weighted timed automata with weights taken in an arbitrary semiring, resulting in a model that subsumes several weighted timed automata concepts of the literature. For our main result, we combine the methods of Schützenberger, a recent approach for a Kleene-type theorem for unweighted timed automata by Bouyer and Petit as well as new techniques. Our main result also implies Kleene-type theorems for several subclasses of weighted timed automata investigated before, e.g., for timed automata and timed automata with stopwatch observers.