Karin Quaas

On the interval-bound problem for weighted timed automata

A weighted timed automaton is a timed automaton equipped with weights on transitions and weight rates on locations. These weights may be positive or negative, corresponding to the production and consumption of some resources. We consider the interval-bound problem: does there exist an infinite run such that the accumulated weight for each prefix of the run is within some given bounds? We show that this problem is undecidable if the weighted timed automaton has more than one clock and more than one weight variable. We further prove that the problem is PSPACE-complete if we restrict the time domain to the natural numbers.

MSO logics for weighted timed automata

We aim to generalize Büchi’s fundamental theorem on the coincidence of recognizable and MSO-definable languages to a weighted timed setting. For this, we investigate weighted timed automata and show how we can extend Wilke’s relative distance logic with weights taken from an arbitrary semiring. We show that every formula in our logic can effectively be transformed into a weighted timed automaton, and vice versa. The results indicate the robustness of weighted timed automata and may also be used for specification purposes.

Kleene algebras and semimodules for energy problems

With the purpose of unifying a number of approaches to energy problems found in the literature, we introduce generalized energy automata. These are finite automata whose edges are labeled with energy functions that define how energy levels evolve during transitions. Uncovering a close connection between energy problems and reachability and Buchi acceptance for semiring-weighted automata, we show that these generalized energy problems are decidable. We also provide complexity results for important special cases.

Zone-based universality analysis for single-clock timed automata

During the last years, timed automata have become a popular model for describing the behaviour of real-time systems. In particular, there has been much research on problems such as language inclusion and universality. It is well-known that the universality problem is undecidable for the class of timed automata with two or more clocks. Recently, it was shown that the problem becomes decidable if the automata are restricted to operate on a single clock variable. However, existing algorithms use a region-based constraint system and suffer from constraint explosion even for small examples. In this paper, we present a zone-based algorithm for solving the universality problem for single-clock timed automata. We apply the theory of better quasi-orderings, a refinement of the theory of well quasi-orderings, to prove termination of the algorithm. We have implemented a prototype based on our method, and checked universality for a number of timed automata. Comparisons with a region-based prototype confirm that zones are a more succinct representation, and hence allow a much more efficient implementation of the universality algorithm.

Path Checking for MTL and TPTL over Data Words

Metric temporal logic (MTL) and timed propositional temporal logic (TPTL) are quantitative extensions of linear temporal logic, which are prominent and widely used in the verification of real-timed systems. It was recently shown that the path checking problem for MTL, when evaluated over finite timed words, is in the parallel complexity class NC. In this paper, we derive precise complexity results for the path-checking problem for MTL and TPTL when evaluated over infinite data words over the non-negative integers. Such words may be seen as the behaviours of one-counter machines. For this setting, we give a complete analysis of the complexity of the path-checking problem depending on the number of register variables and the encoding of constraint numbers (unary or binary). As the two main results, we prove that the path-checking problem for MTL is P-complete, whereas the path-checking problem for TPTL is PSPACE-complete. The results yield the precise complexity of model checking deterministic one-counter machines against formulae of MTL and TPTL.

Parameterized model checking of weighted networks

Abstract We consider networks of weighted processes that consist of an arbitrary number of weighted automata equipped with weights taken from a monoid. We investigate parameterized model checking problems used to verify whether certain qualitative and quantitative properties hold independently of the number of processes. We prove that model checking properties expressed in a weighted extension of LTL is decidable if the monoid satisfies some simple properties. We further present decision procedures for checking global properties of weighted networks.

Satisfiability for MTL and TPTL over Non-monotonic Data Words

In the context of real-time systems, Metric Temporal Logic MTL and Timed Propositional Temporal Logic TPTL are prominent and widely used extensions of Linear Temporal Logic. In this paper, we examine the possibility of using MTL and TPTL to specify properties about classes of non-monotonic data languages over the natural numbers. Words in this class may model the behaviour of, e.g., one-counter machines. We proved, however, that the satisfiability problem for many reasonably expressive fragments of MTL and TPTL is undecidable, and thus the use of these logics is rather limited. On the positive side we prove that satisfiability for the existential fragment of TPTL is NP-complete.

MTL-Model Checking of One-Clock Parametric Timed Automata is Undecidable.

Parametric timed automata extend timed automata (Alur and Dill, 1991) in that they allow the specification of parametric bounds on the clock values. Since their introduction in 1993 by Alur, Henzinger, and Vardi, it is known that the emptiness problem for parametric timed automata with one clock is decidable, whereas it is undecidable if the automaton uses three or more parametric clocks. The problem is open for parametric timed automata with two parametric clocks. Metric temporal logic, MTL for short, is a widely used specification language for real-time systems. MTL-model checking of timed automata is decidable, no matter how many clocks are used in the timed automaton. In this paper, we prove that MTL-model checking for parametric timed automata is undecidable, even if the automaton uses only one clock and one parameter and is deterministic.

On the Supports of Recognizable Timed Series

Recently, the model of weighted timed automata has gained interest within the real-time community. In a previous work, we built a bridge to the theory of weighted automata and introduced a general model of weighted timed automata defined over a semiring and a family of cost functions. In this model, a weighted timed automaton recognizes a timed series , i.e., a function mapping to each timed word a weight taken from the semiring. Continuing in this spirit, the aim of this paper is to investigate the support and cut languages of recognizable timed series. We present results that lead to the decidability of weighted versions of classical decidability problems as e.g. the emptiness problem. Our results may also be used to check whether weighted timed systems satisfy specifications restricting the consumption of a resource.

Synchronizing Data Words for Register Automata

Register automata (RAs) are finite automata extended with a finite set of registers to store and compare data from an infinite domain. We study the concept of synchronizing data words in RAs: does there exist a data word that sends all states of the RA to a single state? For deterministic RAs with k registers (k-DRAs), we prove that inputting data words with 2k+1 distinct data from the infinite data domain is sufficient to synchronize. We show that the synchronization problem for DRAs is in general PSPACE-complete, and it is NLOGSPACE-complete for 1-DRAs. For nondeterministic RAs (NRAs), we show that Ackermann(n) distinct data (where n is the size of the RA) might be necessary to synchronize. The synchronization problem for NRAs is in general undecidable, however, we establish Ackermann-completeness of the problem for 1-NRAs. Another main result is the NEXPTIME-completeness of the length-bounded synchronization problem for NRAs, where a bound on the length of the synchronizing data word, written in binary, is given. A variant of this last construction allows to prove that the length-bounded universality problem for NRAs is co-NEXPTIME-complete.