Abstract: A parallel machine scheduling problem is considered in which each job has a processing time and a delivery time. The objective is to find a schedule which minimizes the time by which all jobs are delivered. For a single machine this problem is easily solved in polynomial time, form≥2 machines it becomes NP-hard. Several heuristics using list scheduling as a subroutine are proposed and a tight worst-case analysis is given. The best one of our heuristics has a worst-case performance guarantee of 2−2/(m+1). For the on-line case we give a heuristic with the (best possible) worst-case performance of two.

Abstract: We deal with the generation of code selectors in compiler backends. The fundamental concepts are systematically derived from the theory of regular tree grammars and finite tree automata. We use this general approach to construct algorithms that generalize and improve existing methods.

Abstract: An extension of Myhill's theorem of automata theory, due to Ehrenfeucht et al. [4] shows that a subsetX of a semigroupsS is recognizable if and only ifX is closed with respect to a monotone well quasi-order onS. In this paper we prove that a similar extension of Nerode's theorem is not possible by showing that there exist non-regular languages on a binary alphabet which are closed with respect to a right-monotone well quasi-order. We give then some additional conditions under which a setX S closed with respect to a right-monotone well quasi-order becomes recognizable. We prove the following main proposition: A subsetX ofS is recognizable if and only ifX is closed with respect to two well quasi-orders<=1 and<=2 which are right-monotone and left-monotone, respectively. Some corollaries and applications are given. Moreover, we consider the family ℱ of all languages which are closed with respect to a right-monotone well quasi-order on a finitely generated free monoid. We prove that ℱ is closed under rational operations, intersection, inverse morphisms and direct non-erasing morphisms. This implies that ℱ is closed under faithful rational transductions. Finally we prove that the languages in ℱ satisfy a suitable ‘pumping’ lemma and that ℱ contains languages which are not recursively enumerable.

Abstract: This paper shows the equivalence between the family of recognizable languages over infinite traces and the family of languages which are recognized by deterministic asynchronous cellular Muller automata. We thus give a proper generalization of McNaughton's Theorem from infinite words to infinite traces. Thereby we solve one of the main open problems in this field. As a special case we obtain that every closed (w.r.t. the independence relation) word language is accepted by someI-diamond deterministic Muller automaton.

Abstract: We consider the problem of sorting a multiset of sizen containingm distinct elements, where theith distinct element appearsn i times. Under the assumption that our model of computation allows only the operations of comparing elements and moving elements in the memory,Ω(n logn ? ? i=1 m n i logn i +n) is known to be a lower bound for the computational complexity of the sorting problem. In this paper we present aminimum space algorithm that sortsstably a multiset in asymptoticallyOptimal worst-case time. A Quicksort type approach is used, where at each recursive step the median is chosen as the partitioning element. To obtain a stable minimum space implementation, we develop linear-time in-place algorithms for the following problems, which have interest of their own: Stable unpartitioning: Assume that ann-element arrayA is stably partitioned into two subarraysA 0 andA 1. The problem is to recoverA from its constitutentsA 0 andA 1. The information available is the partitioning element used and a bit array of sizen indicating whether an element ofA 0 orA 1 was originally in the corresponding position ofA. Stable selection: The task is to find thekth smallest element in a multiset ofn elements such that the relative order of identical elements is retained.

Abstract: The relation between an operational interleaving semantics for TSCP based on a transition system and a compositional true concurrency semantics based on event structures is studied. In particular we extend the consistency result of U. Goltz and R. Loogen [Ann. Soc. Math. Pol., Ser. IV, Fundam. Inf. 14, 39-73 (1991; Zbl 0717.68028)] for TCSP processes without recursion to the general case. Thus, we obtain for every TCSP process P that its operational meaning O(P) and the interleaving behaviour O(M[[P]]) which is derived from the event structure M[[P]] associated with P are bisimilar. (aus: Zentralblatt MATH)

Abstract: We consider sets of grammars (calledteams) which process strings by cooperating together; a single derivation step in a team is done in such a way that each grammar in the set rewrites a symbol in the string. A cooperating grammar system with prescribed teams (CPT) consists of a finite number of teams. In a maximal rewriting mode, a team in a CPT can take a string for rewriting whenever it can make a derivation step on it; it keeps it and rewrites as long as it can, and once a string is obtained that cannot be rewritten by the team anymore it is returned and becomes available to all other teams. In this paper we investigate the power of CPT in the maximal and other rewriting modes. We establish the relationships of CPT with other models of grammars cooperating together and with various kinds of controlled grammars. We also solve some open problems from [6] and provide alternative proofs for some results from [6].

Abstract: In this paper we introduce a new formal model for the concept of syntax-directed semantics, called macro attributed tree transducer (for skort: mat tree transducer). This model is based on (noncircular) attributed tree transducers and on macro tree transducers. In the first type of transducer, semantic values are computed by means of meaning names called synthesized attributes, and by means of context names called inherited attributes. Both, synthesized and inherited attributes represent basic semantic values. In the second type of transducer, semantic values are computed by meaning names only which are called states. However, in order to have a means of handling context information, states represent functions over semantic values

Abstract: We propose a method based on geometrical tools to map problems onto regular and synchronous processor arrays. The problems we consider are defined by systems of affine recurrence equations (SARE). From such a problem specification we extract the data dependencies in terms of two classes of vectors: the utilization vectors and the dependence vectors. We use these vectors to express constraints on the timing or the allocation functions. We differentiate two classes of constraints. The causal ones are intrinsic timing constraints induced by the system of equations defining the problem. A given choice of target architecture may impose new constraints on the timing or the allocation. We call them the architecture-related constraints. We use these constraints to determine first an affine timing function and next an allocation by projection. We finally illustrate the method with three examples: the matrix multiplication, the recursive convolution and the LLt Cholesky factorization.

Abstract: The apex graph grammars generate precisely the context-free graph languages of bounded degree, independently of whether one considers hyperedge replacement systems or (boundary or confluent) NLC or edNCE graph grammars. The main feature of apex graph grammars is that nodes cannot be "passed" from nonterminal to nonterminal. The proof is based on a normal form result for arbitrary hyperedge replacement systems that forbids "passing chains". This generalizes Greibach Normal Form.

Abstract: In this paper we present information theoretic approximations for theM/G/1 queue with retrials. Various approximations for this model are obtained according to the available information about the service time probability density and the steady-state distribution of the system state. The results are well-suited for numerical computation.

Abstract: A categorial semantic domain for objects is presented in order to clarify both aggregation and specialization. Three kinds of specialization are discussed: (1) subtyping (specialization with no side effects and no non-monotonic overriding); (2) monotonic specialization (possibly with side effects but still only with monotonic overriding); and (3) non-monotonic specialization (possibly with side effects and non-monotonic overriding). A sequence of three categories of objects differing only in the morphisms is presented. The first one is used to explain object aggregation (respecting locality through a frame constraint) and the strictest form of specialization (subtyping). The second category is shown to be adequate for explaining specialization with side effects (by relaxing the frame constraint). Finally, the third category supports also non-monotonic overriding, by adopting as morphisms suitable partial morphisms of the second one. All these categories are complete and cocomplete. Colimits are used for explaining the semantics of aggregation and multiple inheritance.

Abstract: In this paper we introduce a new formal model for the concept of syntaxdirected semantics, calledmacro attributed tree transducer (for short: mat tree transducer). This model is based on (noncircular) attributed tree transducers and on macro tree transducers. In the first type of transducer, semantic values are computed by means of meaning names called synthesized attributes, and by means of context names called inherited attributes. Both, synthesized and inherited attributes represent basic semantic values. In the second type of transducer, semantic values are computed by meaning names only which are called states. However, in order to have a means of handling context information, states represent functions over semantic values. The new model integrates attributed tree transducers and macro tree transducers by allowing both, meaning names and context names to represent functions over semantic values. In analogy to the terminology of attributed tree transducers, we call such meaning names and context names also synthesized functions and inherited functions, respectively. We present an inductive characterization of the tree transformation computed by an mat tree transducer. We prove that mat tree transducers are more powerful than both, attributed tree transducers and macro tree transducers. We characterize mat tree transducers by the two-fold composition of attributed tree transducers. This characterization has three consequences: (1) the height of output trees of mat tree transducers is bounded exponentially in the size of the input tree, (2) the composition hierarchy of mat tree transducers is strict, and (3) mat tree transducers are closed under right-composition with top-down tree transducers, but not under left-composition. Moreover, we prove that the addition of inherited attributes does not increase the computational power of macro tree transducers.

Abstract: Theoretical results on the scope and limits of first order algebraic specifications can be used to show that certain natural algebras have no recursively enumerable equational specification under first order initial algebra semantics. A well known example is the algebraP? of primitive recursive functions over the natural numbers. In this paper we show thatP? has a recursive equational specification under second order initial algebra semantics. It follows that higher order initial algebra specifications are strictly more powerful than first order initial algebra specifications.

Abstract: An alphabetic cone is a family of forests stable under alphabetic tree transductions, whereas we call REC-closed each family of forests closed under the operations of union, top-catenation,a-product anda-star; the sheaves are families having both the above properties.

Abstract: This paper focuses on observability issues in the framework of loose algebraic specifications. The main purpose of observability is to extend the model class of some given algebraic specification in order to consider not only the algebras that satisfy the axioms of the specification in order to consider not only the algebras that satisfy the axioms of the specification, but as well other ones, provided that the differences between the properties satisfied by these algebras and the properties required by the specification (i.e., the axioms) are not “observable”. We compare various behavioural approaches developed so far. We point out their respective advantages and limitations. Expressive power is our main criterion for the discussion.

Abstract: We present a semantic method to check the finiteness of CCS terms. The method is interpretative, i.e. it is based on a non-standard CCS operational semantics. According to this semantics it is always possible, given a processp, to build a finite state transition system which, if a condition holds, is a finite representation ofp; otherwise it is a suitable approximation of its semantic behaviour. The method is able to decide the finiteness of a CCS term in a larger number of cases than those captured by known syntactic criteria.

Abstract: We consider languages in NP whose certificate size is bounded by a fixed, slowly growing function (sayf (n)) of the input size The classesf (n)-NP, which are related to classes of Kintala and Fischer, are defined in order to classify such languages We show that several natural problems, involving Boolean satisfiability, graph colouring and Hamiltonian circuits, are complete forf (n)-NP Each of our problems is obtained by taking a known NP-complete problem and introducing an ingredient we callforcing, whereby a partial structure is enlarged by a sequence of local improvements As special cases of these results we obtain some new logspace completeness results for P

Abstract: We continue the study of communication-bounded synchronized alternating finite automata (SAFA), first considered by Hromkovia? et al. We show that to accept a nonregular language, an SAFA needs to generate at least Ω(log logn) communication symbols infinitely often; furthermore, a synchronized alternating finite automaton without nondeterminism (SUFA) needs to generate at leastΩ(log logn) communication symbols infinitely often for some constantk?1. We also show that these bounds are tight. Next, we establish dense hierarchies of these machines on the function bounding the number of communication symbols. Finally, we give a characterization of NP in terms of communication-bounded multihead synchronized alternating finite automata, namely, NP = ? k?1 L(SAFA(k-heads,n k -com)). This result recasts the relationships between P, NP, and PSPACE in terms of multihead synchronized alternating finite automata.

Abstract: Image generation with DOL-systems is discussed. It is shown that, if either the vector or the turtle geometry interpretation is used, DOL-systems can produce step-by-step all images that can be generated by regular languages (or by equivalent Iterative Matrix Homomorphisms of Shallit and Stolfi). An extension of turtle geometry interpretation is introduced that enables L-systems to generate gray-tone images. It is shown that with our extension every Weighted Finite Automaton can be simulated step-by-step by a DOL-system.

Abstract: Hierarchical design represents a natural solution for managing the growing complexity of computing systems. This leads to systems that are hierarchically structured from a logical or physical viewpoint. Because of the inherent complexity of such systems, the incorporation of fault-tolerance features is highly recommended, to achieve the required dependability level. Hence, it is important to develop evaluation tools that allow to analyse the effectiveness of different fault-tolerance mechanisms. We present a systematic procedure for the construction and evaluation of Markov models for transient dependability analysis, which fully exploits the hierarchical structure of the system under evaluation. A thorough time/space complexity analysis of the proposed procedure shows that it allows a considerable time and space saving with respect to a “naive” approach to the same problem.

Abstract: We present a new class of context-free grammars whose sentences are parsable in linear time and space. The class calledboundedly LR(k)-conflictable (BLRC(k)) grammars includes all LR(k) grammars, some non-LR unambiguous grammars and some boundedly ambiguous grammars. A context-free grammar is said to beBLRC(k) if the number of conflict occurences during LR(k) parsing for every sentence of the grammar is inherently bounded. A BLRC(k) grammar can be considered as a natural extension of an LR(k) grammar whose sentences can be parsed by an LR(k) manner with multiple stacks. We show that it is a decidable problem whether a context-free grammar is BLRC(k) for a givenk, whereas it is undecidable for arbitraryk. The result is derived from anLR(k) machine description grammar which describes the behavior of a give LR(k) parser in terms of the grammar symbols. The relationship between the class of BLRC(k) grammars (and languages) and those of other associated grammars (and languages), is also discussed.

Abstract: Multiclass M/G/1 systems in steady-state with work-conserving scheduling strategies are studied. Restricting a system's scheduling strategy to making no direct use of the required service times, every time the server becomes idle its memory is cleared, and service may only be interrupted by newly arriving customers, a conservation law is developed by means of inequalities. The conservation law states that if a response time vector composed of the expected response times of the different classes of a system in steady-state is achievable, then it must belong to a well-defined convex polytope (a set bounded by hyperplanes). Furthermore, on each hyperplane bounding the relevant polytope there lies at least one vertex of the convex set of achievable response time vectors. Therefore, this polytope is the least one including the set of all achievable response time vectors. — Author's Abstract

Abstract: A simple semantic or object-based data model is considered, which includes objects and object identifiers, classes and class hierarchies, attributes ranging over atomic values. Transactions are composed from five, basic operators manipulating objects. Preservation of functional and acyclic inclusion dependencies by transactions is studied in such a context of semantic databases and update transactions. It is shown to be decidable whether a given transaction preserves a given set of functional dependencies, or acyclic inclusion dependencies, or both functional and acyclic inclusion dependencies. Time complexity (with respect to the sizes of transactions and database schemas) for testing preservation is also discussed. It turns out that the problem is co-NP-complete in the simplest cases where there is only one nontrivial dependency and transactions consist of only creations and deletions of objects. It implies that the problem is at least co-NP-hard in general.

Abstract: We study the problem of updating intensional relations in the framework of deductive databases on which integrity constraints (specifically functional dependencies) are defined. First, a formalization of a model-theoretic semantics of updates is provided: the notions ofrepresentability, consistency anddeterminism are introduced to characterize the various cases. Then, a proof-theoretic approach, based on a variant of resolution integrated with the chase procedure, is defined, showing that the method exactly captures the above notions. It turns out that using functional dependencies it is possible to resolve potential ambiguities in several practical cases. Also, precomputations can be performed at definition time to execute update requests more efficiently.