The Monadic Theory of Morphic Infinite Words and Generalizations

On square-free arithmetic progressions in infinite words

The monadic second-order theory of term rewritings is considered. It is shown that the monadic theory of the rewriting (or the suffix rewriting) of a ground rewrite system is undecidable. Furthermore, the first-order theory is undecidable for the prefix derivation according to a linear context-free grammar on linear terms. Nevertheless, a new notion on terms with variables is introduced: a term is entire if each of its subterms either is a variable, or is without variable or has the same variables as the term. It is shown that the monadic theory is decidable (respectively undecidable) for the prefix rewriting according to a rewrite system on entire terms, with an axiom (respectively without axiom). >

We deal with the monadic (second-order) theory of order. We prove all known results in a unified way, show a general way of reduction, prove more results and show the limitation on extending them. We prove (CH) that the monadic theory of the real order is undecidable. Our methods are modeltheoretic, and we do not use automaton theory.

The palindromic length $\text{PL}(v)$ of a finite word $v$ is the minimal number of palindromes whose concatenation is equal to $v$. In 2013, Frid, Puzynina, and Zamboni conjectured that: If $w$ is an infinite word and $k$ is an integer such that $\text{PL}(u)\leq k$ for every factor $u$ of $w$ then $w$ is ultimately periodic. Suppose that $w$ is an infinite word and $k$ is an integer such $\text{PL}(u)\leq k$ for every factor $u$ of $w$. Let $\Omega(w,k)$ be the set of all factors $u$ of $w$ that have more than $\sqrt[k]{k^{-1}\vert u\vert}$ palindromic prefixes. We show that $\Omega(w,k)$ is an infinite set and we show that for each positive integer $j$ there are palindromes $a,b$ and a word $u\in \Omega(w,k)$ such that $(ab)^j$ is a factor of $u$ and $b$ is nonempty. Note that $(ab)^j$ is a periodic word and $(ab)^ia$ is a palindrome for each $i\leq j$. These results justify the following question: What is the palindromic length of a concatenation of a suffix of $b$ and a periodic word $(ab)^j$ with "many" periodic palindromes? It is known that $\lvert\text{PL}(uv)-\text{PL}(u)\rvert\leq \text{PL}(v)$, where $u$ and $v$ are nonempty words. The main result of our article shows that if $a,b$ are palindromes, $b$ is nonempty, $u$ is a nonempty suffix of $b$, $\vert ab\vert$ is the minimal period of $aba$, and $j$ is a positive integer with $j\geq3\text{PL}(u)$ then $\text{PL}(u(ab)^j)-\text{PL}(u)\geq 0$.

Monads of sets

17 Automata-theoretic techniques for temporal reasoning

The article surveys some decidability results for DPDAs on infinite words\n(omega-DPDA). We summarize some recent results on the decidability of the\nregularity and the equivalence problem for the class of weak omega-DPDAs.\nFurthermore, we present some new results on the parity index problem for\nomega-DPDAs. For the specification of a parity condition, the states of the\nomega-DPDA are assigned priorities (natural numbers), and a run is accepting if\nthe highest priority that appears infinitely often during a run is even. The\nbasic simplification question asks whether one can determine the minimal number\nof priorities that are needed to accept the language of a given omega-DPDA. We\nprovide some decidability results on variations of this question for some\nclasses of omega-DPDAs.\n

In the automata-theoretic approach to verification, we model programs and specifications by automata on infinite words. Correctness of a program with respect to a specification can then be reduced to the language-containment problem. In a concurrent setting, the program is typically a parallel composition of many coordinating processes, and the language-containment problem that corresponds to verification is $$(\dag )\mathcal{L}(P_1 ) \cap \mathcal{L}(P_2 ) \cap ... \cap \mathcal{L}(P_n ) \subseteq \mathcal{L}(T),$$ where P 1, P 2, ..., P n are automata that model the underlying coordinating processes, and T is the task they should perform. In 1994, Kurshan suggested the heuristic of Reduction Hierarchies for circumventing the exponential blow-up introduced by conventional methods that solve the problem (†). In the reduction-hierarchy heuristic, we solve the problem (†) by solving a sequence of easier problems, which involve only automata of tractable sizes. Complexity-theoretic conjectures (NP ≠ PSPACE) imply that there are settings in which the heuristic cannot circumvent the exponential blow-up. In this paper, we demonstrate the strength of the heuristic, study its properties, characterize settings in which it performs effectively, and suggest a method for searching for reduction hierarchies. In particular, we prove, independently of the NP ≠ PSPACE question, that reduction hierarchies of tractable sizes do not always exist.KeywordsProof SystemAtomic PropositionAcceptance ConditionTree HierarchyDeterministic AutomatonThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We study an automata-theoretic approach to planning for temporally extended goals. Specifically, we devise techniques based on nonemptiness of Buchi automata on infinite words, to synthesize sequential and conditional plans in a generalized setting in which we have that: goals are general temporal properties of desired execution; dynamic systems are represented by finite transition systems; incomplete information on the initial situation is allowed; and states are only partially observable. We prove that the techniques proposed are optimal wrt the worst case complexity of the problem. Thanks to the scalability of the nonemptiness algorithms, the techniques presented here promise to be applicable to fairly large systems, notwithstanding the intrinsic complexity of the problem.

The aim of this chapter is to present some recent research work in the combinatorial aspects of the theory of semigroups which are of great interest both for Algebra and Theoretical Computer Science. This research mainly concerns that part of combinatorics of finite and infinite words over a finite alphabet, which is usually called the theory of “unavoidable” regularities. The book by Lothaire [77] can be considered an excellent introduction to this theory.

Given an infinite word with values in a finite alphabet that admit frequencies for all its factors, the smallest frequency function associates with a given positive integer the smallest frequency for factors of this length, and the recurrence function measures the gaps between successive occurrences of factors. We develop in this paper the relations between the growth orders of the smallest frequency and of the recurrence functions. As proved by M. Boshernitzan, linearly recurrent words are characterized in terms of the smallest frequency function. In this paper, we see how to extend this result beyond the case of linearly recurrent words. We first establish a general relation between the smallest frequency and the recurrence functions. Given a lower bound for the smallest frequency function, this relation provides an upper bound for the recurrence function which involves the primitive of this lower bound. We then see how to improve this relation in the case of Sturmian words where the product of the smallest frequency and the recurrence function is bounded for most of the lengths of factors. We also indicate how to construct Sturmian words having a prescribed behaviour for the smallest frequency function, and for the product of the smallest frequency and the recurrence function.

Infinite words and biprefix codes

Let r and s be multiplicatively independent positive integers. We establish that the r-ary expansion and the s-ary expansion of an irrational real number, viewed as infinite words on {0,1,...,r-1} and {0,1,...,s-1}, respectively, cannot have simultaneously a low block complexity. In particular, they cannot be both Sturmian words.

Many important parallel computer programs exhibit ongoing behaviour that is characterized naturally in terms of infinite execution traces, which can be organized into “branching” trees, and which reflect the high degree of nondeterminism inherent in parallel computation. In this paper, we give a systematic account of Branching Time Temporal Logics, which provide a formal system for describing and reasoning about the correct behaviour of such programs. Several systems of branching time temporal logic that have appeared in the literature are presented, and significant related issues such as their axiomatizations, and decision procedures for their satisfiability and model checking problems are discussed. The applicability of their axiomatizations to formulate deductive systems of these temporal logics to reason about the correctness of concurrent programs is then described as is that of their decision procedures to the tasks of mechanical synthesis and verification. A comparison of the relative expressive power of these systems of branching time temporal logic is also presented, and their ability to specify important correctness properties of programs, including those that involve fairness, is discussed. Moreover, their expressiveness is related to both, the expressiveness of corresponding linear time temporal logics, as well as that of other standard formalisms such as the monadic second-order theory of many successors and finite-state tree automata. Finally, a comparison is undertaken between branching time temporal logics and linear time ones, particularly with respect to their adequacy for such applications as specifying and reasoning about the correctness of concurrent programs.

The word problem for inverse monoids generated by a set Γ subject to relations of the form e = f, where e and f are both idempotents in the free inverse monoid generated by Γ, is investigated. It is shown that for every fixed monoid of this form the word problem can be solved in polynomial time which solves an open problem of Margolis and Meakin. For the uniform word problem, where the presentation is part of the input, EXPTIME-completeness is shown. For the Cayley-graphs of these monoids, it is shown that the first-order theory with regular path predicates is decidable. Regular path predicates allow to state that there is a path from a node x to a node y that is labeled with a word from some regular language. As a corollary, the decidability of the generalized word problem is deduced. Finally, it is shown that the Cayley-graph of the free inverse monoid has an undecidable monadic second-order theory.