The avoidability of binary patterns by binary cube-free words is investigated and the exact bound between unavoidable and avoidable patterns is found. All avoidable patterns are shown to be D0L-avoidable. For avoidable patterns, the growth rates of the avoiding languages are studied. All such languages, except for the overlap-free language, are proved to have exponential growth. The exact growth rates of languages avoiding minimal avoidable patterns are approximated through computer-assisted upper bounds. Finally, a new example of a pattern-avoiding language of polynomial growth is given.

We address the problem to know whether the relation induced by a one-rulelength-preserving rewrite system is rational. We partially answer to a conjecture of ÉricLilin who conjectured in 1991 that a one-rule length-preserving rewrite system is arational transduction if and only if the left-hand side u and theright-hand side v of the rule of the system are not quasi-conjugate orare equal, that means if u and v are distinct, there donot exist words x, y and z such thatu = xyz and v = zyx.We prove the only if part of this conjecture and identify two non trivialcases where the if part is satisfied.

The traveling salesman problem is one of the most important problems in operations research, especially when additional precedence constraints are considered. Here, we consider the well-known variant where a linear order on k special vertices is given that has to be preserved in any feasible Hamiltonian cycle. This problem is called Ordered TSP and we consider it on input instances where the edge-cost function satisfies a β-relaxed triangle inequality, i.e., where the length of a direct edge cannot exceed the cost of any detour via a third vertex by more than a factor of β> 1. We design two new polynomial-time approximation algorithms for this problem. The first algorithm essentially improves over the best previously known algorithm for almost all values of k and β< 1.087889. The second algorithm gives a further improvement for 2n ≥ 11k + 7 and β< √34/3 , where n is the number of vertices in the graph.

A Hamming compatible metric is an integer-valued metric on the words of a finite alphabet which agrees with the usual Hamming distance for words of equal length. We define a new Hamming compatible metric and show this metric is minimal in the class of all “well-behaved” Hamming compatible metrics. This gives a negative answer to a question stated by Echi in his paper [O. Echi, Appl. Math. Sci. (Ruse) 3 (2009) 813–824.].

We present a new incremental algorithm for minimising deterministic finite automata. Itruns in quadratic time for any practical application and may be halted at any point,returning a partially minimised automaton. Hence, the algorithm may be applied to a givenautomaton at the same time as it is processing a string for acceptance. We also includesome experimental comparative results.

Laminations are classic sets of disjoint and non-self-crossing curves on surfaces. Lamination languages are languages of two-way infinite words which code laminations by using associated labeled embedded graphs, and which are subshifts. Here, we characterize the possible exact affine factor complexities of these languages through bouquets of circles, i.e. graphs made of one vertex, as representative coding graphs. We also show how to build families of laminations together with corresponding lamination languages covering all the possible exact affine complexities.

Rauzy fractals are compact sets with fractal boundary that can be associated with any unimodular Pisot irreducible substitution. These fractals can be defined as the Hausdorff limit of a sequence of compact sets, where each set is a renormalized projection of a finite union of faces of unit cubes. We exploit this combinatorial definition to prove the connectedness of the Rauzy fractal associated with any finite product of three-letter Arnoux–Rauzy substitutions.

Several types of systems of parallel communicating restarting automata are introduced andstudied. The main result shows that, for all types of restarting automata X, centralizedsystems of restarting automata of type X have the same computational power asnon-centralized systems of restarting automata of the same type and the same number ofcomponents. This result is proved by a direct simulation. In addition, it is shown thatfor one-way restarting automata without auxiliary symbols, systems of degree at least twoare more powerful than the component automata themselves. Finally, a lower and an upperbound are given for the expressive power of systems of parallel communicating restartingautomata.

Some of the most interesting and important results concerning quantum finite automata arethose showing that they can recognize certain languages with (much) less resources thancorresponding classical finite automata. This paper shows three results of such a typethat are stronger in some sense than other ones because (a) they deal with models ofquantum finite automata with very little quantumness (so-called semi-quantum one- andtwo-way finite automata); (b) differences, even comparing with probabilistic classicalautomata, are bigger than expected; (c) a trade-off between the number of classical andquantum basis states needed is demonstrated in one case and (d) languages (or the promiseproblem) used to show main results are very simple and often explored ones in automatatheory or in communication complexity, with seemingly little structure that could beutilized.

We investigate the finite repetition threshold for k-letter alphabets, k ≥ 4, that is the smallest number r for which there exists an infinite r+-free word containing a finite number of r-powers. We show that there exists an infinite Dejean word on a 4-letter alphabet (i.e. a word without factors of exponent more than 7/5 ) containing only two 7/5 -powers. For a 5-letter alphabet, we show that there exists an infinite Dejean word containing only 60 5/4 -powers, and we conjecture that this number can be lowered to 45. Finally we show that the finite repetition threshold for k letters is equal to the repetition threshold for k letters, for every k ≥ 6.

Six kinds of both of primitivity and periodicity of words, introduced by Ito and Lischke [M. Ito and G. Lischke, Math. Log. Quart. 53 (2007) 91–106; Corrigendum in Math. Log. Quart. 53 (2007) 642–643], give rise to defining six kinds of roots of a nonempty word. For 1 ≤ k ≤ 6, a k-root word is a word which has exactly k different roots, and a k-cluster is a set of k-root words u where the roots of u fulfil a given prefix relationship. We show that out of the 89 different clusters that can be considered at all, in fact only 30 exist, and we give their quasi-lexicographically smallest elements. Also we give a sufficient condition for words to belong to the only existing 6-cluster. These words are also called Lohmann words. Further we show that, with the exception of a single cluster, each of the existing clusters contains either only periodic words, or only primitive words.

Based on the formal framework of reaction systems by Ehrenfeucht and Rozenberg[Fund. Inform. 75 (2007) 263–280], reaction automata (RAs)have been introduced by Okubo et al. [Theoret. Comput. Sci.429 (2012) 247–257], as language acceptors with multiset rewritingmechanism. In this paper, we continue the investigation of RAs with a focus on the twomanners of rule application: maximally parallel and sequential. Considering restrictionson the workspace and the λ-input mode, we introduce the correspondingvariants of RAs and investigate their computation powers. In order to explore Turingmachines (TMs) that correspond to RAs, we also introduce a new variant of TMs withrestricted workspace, called s(n)-restricted TMs. Themain results include the following: (i) for a language L and a functions(n), L is accepted by ans(n)-bounded RA with λ-input mode insequential manner if and only if L is accepted by alog s(n)-bounded one-way TM; (ii) if a languageL is accepted by a linear-bounded RA in sequential manner, thenL is also accepted by a P automaton [Csuhaj−Varju and Vaszil, vol. 2597of Lect. Notes Comput. Sci. Springer (2003) 219–233.] in sequentialmanner; (iii) the class of languages accepted by linear-bounded RAs in maximally parallelmanner is incomparable to the class of languages accepted by RAs in sequential manner.

We present a new model of a two-dimensional computing device called Sgraffito automaton. In general, the model is quite simple, which allows a clear design of computations. When restricted to one-dimensional inputs, that is, strings, the Sgraffito automaton does not exceed the power of finite-state automata. On the other hand, for two-dimensional inputs, it yields a family of picture languages with good closure properties that strictly includes the class REC  of recognizable picture languages. The deterministic Sgraffito automata define a class of picture languages that includes the class of deterministic recognizable picture languages DREC, the class of picture languages that are accepted by four-way alternating automata, those that are accepted by deterministic one-marker automata, and the sudoku-deterministically recognizable picture languages, but the membership problem for the accepted languages is still decidable in polynomial time. In addition, the deterministic Sgraffito automata accept some unary picture languages that are outside of the class REC.

The aim of this paper is to design a theoretical framework that allows us to perform the computation of regular expression derivatives through a space of generic structures. Thanks to this formalism, the main properties of regular expression derivation, such as the finiteness of the set of derivatives, need only be stated and proved one time, at the top level. Moreover, it is shown how to construct an alternating automaton associated with the derivation of a regular expression in this general framework. Finally, Brzozowski’s derivation and Antimirov’s derivation turn out to be a particular case of this general scheme and it is shown how to construct a DFA, a NFA and an AFA for both of these derivations.

We introduce and investigate several variants of a bidirectional string assemblingsystem, which is a computational model that generates strings from copies of assemblyunits. The underlying mechanism is based on two-sided piecewise assembly of adouble-stranded sequence of symbols, where the upper and lower strand have to match. Thegenerative capacities and the relative power of the variants are our main interest. Inparticular, we prove that bidirectional string assembling system generate languages notrepresented as any finite concatenation of one-sided string assembling systems. The latterbuild an infinite, strict and tight concatenation hierarchy. Moreover, it is shown thateven the strongest system in question can only generate NL languages, while there areunary regular languages that cannot be derived. Furthermore, a finite strict hierarchywith respect to the different variants considered is shown and closure properties of thelanguages generated are presented.

The minimum roman dominating problem (denoted by γR(G), the weight of minimum roman dominating function of graph G) is a variant of the very well known minimum dominating set problem (denoted by γ(G), the cardinality of minimum dominating set of graph G). Both problems remain NP-Complete when restricted to P5-free graph class [A.A. Bertossi, Inf. Process. Lett. 19 (1984) 37–40; E.J. Cockayne, et al. Discret. Math. 278 (2004) 11–22]. In this paper we study both problems restricted to some subclasses of P5-free graphs. We describe robust algorithms that solve both problems restricted to (P5,(s,t)-net)-free graphs in polynomial time. This result generalizes previous works for both problems, and improves existing algorithms when restricted to certain families such as (P5,bull)-free graphs. It turns out that the same approach also serves to solve problems for general graphs in polynomial time whenever γ(G) and γR(G) are fixed (more efficiently than naive algorithms). Moreover, the algorithms described are extremely simple which makes them useful for practical purposes, and as we show in the last section it allows to simplify algorithms for significant classes such as cographs.

We consider the languages of finite trees called tree-shift languages which are factorial extensible tree languages. These languages are sets of factors of subshifts of infinite trees. We give effective syntactic characterizations of two classes of regular tree-shift languages: the finite type tree languages and the tree languages which are almost of finite type. Each class corresponds to a class of subshifts of trees which is invariant by conjugacy. For this goal, we define a tree algebra which is finer than the classical syntactic tree algebra based on contexts. This allows us to capture the notion of constant tree which is essential in the framework of tree-shift languages.

We combine a new data model, where the random classification is subjected to rather weakrestrictions which in turn are based on the Mammen−Tsybakov [E. Mammen and A.B. Tsybakov,Ann. Statis. 27 (1999) 1808–1829; A.B. Tsybakov,Ann. Statis. 32 (2004) 135–166.] small margin conditions,and the statistical query (SQ) model due to Kearns [M.J. Kearns, J. ACM45 (1998) 983–1006] to what we refer to as PAC + SQ model. We generalize the classconditional constant noise (CCCN) model introduced by Decatur [S.E. Decatur, inICML ’97: Proc. of the Fourteenth Int. Conf. on Machine Learn. MorganKaufmann Publishers Inc. San Francisco, CA, USA (1997) 83–91] to the noise modelorthogonal to a set of query functions. We show that every polynomial time PAC + SQ learning algorithm can beefficiently simulated provided that the random noise rate is orthogonal to the queryfunctions used by the algorithm given the target concept. Furthermore, we extend theconstant-partition classification noise (CPCN) model due to Decatur [S.E. Decatur, inICML ’97: Proc. of the Fourteenth Int. Conf. on Machine Learn. MorganKaufmann Publishers Inc. San Francisco, CA, USA (1997) 83–91] to what we call theconstant-partition piecewise orthogonal (CPPO) noise model. We show how statisticalqueries can be simulated in the CPPO scenario, given the partition is known to thelearner. We show how to practically use PAC +SQ simulators in the noise model orthogonal to the query space bypresenting two examples from bioinformatics and software engineering. This way, wedemonstrate that our new noise model is realistic.