Showing papers in "Journal of the ACM in 1984"
TL;DR: In this article, a mathematical model for communicating sequential processes is given, and a number of its interesting and useful properties are stated and proved, and the possibilities of nondetermimsm are fully taken into account.
Abstract: A mathematical model for communicating sequential processes is given, and a number of its interesting and useful properties are stated and proved. The possibilities of nondetermimsm are fully taken into account.
1,193 citations
TL;DR: There are precise conditions that should be satisfied in a semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on tables with “null values” of various kinds allowed.
Abstract: This paper concerns the semantics of Codd's relational model of data. Formulated are precise conditions that should be satisfied in a semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on tables with “null values” of various kinds allowed. These conditions require that the system be safe in the sense that no incorrect conclusion is derivable by using a specified subset Ω of the relational operators; and that it be complete in the sense that all valid conclusions expressible by relational expressions using operators in Ω are in fact derivable in this system. Two such systems of practical interest are shown. The first, based on the usual Codd's null values, supports projection and selection. The second, based on many different (“marked”) null values or variables allowed to appear in a table, is shown to correctly support projection, positive selection (with no negation occurring in the selection condition), union, and renaming of attributes, which allows for processing arbitrary conjunctive queries. A very desirable property enjoyed by this system is that all relational operators on tables are performed in exactly the same way as in the case of the usual relations. A third system, mainly of theoretical interest, supporting projection, selection, union, join, and renaming, is also discussed. Under a so-called closed world assumption, it can also handle the operator of difference. It is based on a device called a conditional table and is crucial to the proof of the correctness of the second system. All systems considered allow for relational expressions containing arbitrarily many different relation symbols, and no form of the universal relation assumption is required. Categories and Subject Descriptors: H.2.3 [Database Management]: Languages— query languages; H.2.4 [Database Management]: Systems— query processing General Terms: Theory
1,130 citations
TL;DR: A data structure for representing a set of n items from a universe of m items, which uses space n+o(n) and accommodates membership queries in constant time and is easy to implement.
Abstract: A data structure for representing a set of n items from a umverse of m items, which uses space n + o(n) and accommodates membership queries m constant time is described. Both the data structure and the query algorithm are easy to ~mplement.
943 citations
TL;DR: In this paper, it was shown that the linear programming problem in d variables and n constraints can be solved in O(n) time when d is fixed and bounded by a slowly growing function of n.
Abstract: It is demonstrated that the linear programming problem in d variables and n constraints can be solved in O(n) time when d is fixed. This bound follows from a multidimensional search technique which is applicable for quadratic programming as well. There is also developed an algorithm that is polynomial in both n and d provided d is bounded by a certain slowly growing function of n.
660 citations
TL;DR: It is shown that two one-pass methods proposed by van Leeuwen and van der Weide are asymptotically optimal, whereas several other methods, including one proposed by Rein and advocated by Dijkstra, are slower than the best methods.
Abstract: This paper analyzes the asymptotic worst-case running time of a number of variants of the well-known method of path compression for maintaining a collection of disjoint sets under union. We show that two one-pass methods proposed by van Leeuwen and van der Weide are asymptotically optimal, whereas several other methods, including one proposed by Rein and advocated by Dijkstra, are slower than the best methods.
475 citations
TL;DR: Definition d'une classe de dependances, dependances generant l'equalite and les triples and generalisation du processus de poursuite pour traiter ces dependances.
Abstract: Definition d'une classe de dependances, dependances generant l'equalite et les triples et generalisation du processus de poursuite pour traiter ces dependances. On montre quelques cas pour lesquels il y a une procedure de decision. Redondance de l'egalite pour decider de l'implication des dependances generant les triples et «presque redondance» pour decider de l'implication des dependances generant l'egalite
418 citations
TL;DR: A method is given for deciding formulas in combinations of unquatified first-order theories that makes use of a single, uniform procedure that minimizes the code needed to accommodate each additional theory.
Abstract: A method is given for deciding formulas in combinations of unquatified first-order theories. Rather than coupling separate decision procedures for the contributing theories, it makes use of a single, uniform procedure that minimizes the code needed to accommodate each additional theory. The method is applicable to theories whose semantics can be encoded within a certain class of purely equational canonical form theories that is closed under combination. Examples are given from the equational theories of integer and real arithmetic, a subtheory of monadic set theory, the theory of cons, car, and cdr, and others. A discussion of the speed performance of the procedure and a proof of the theorem that underlies its completeness are also given. The procedure has been used extensively as the deductive core of a system for program specification and verification.
359 citations
TL;DR: The solution uses resolution-proof techmques over connection graphs to derive a program of relational database operations that gives all the answers to a query and has a welldefined termmatton condiUon.
Abstract: A first-order database ~s defined as a function-free first-order theory in which the ground units serve as the extensional database and the proper noniogical axioms serve as the intensional database. The following problem is addressed: \"Given a recurswe nonlogical axiom and the form of a potential query, can one describe a set of database retrieval requests that gives the correct answers and is guaranteed to terminate.\" The solution uses resolution-proof techmques over connection graphs to derive a program of relational database operations that gives all the answers to a query and has a welldefined termmatton condiUon.
311 citations
Journal Article•
245 citations
TL;DR: It is shown that the time complexity of finding an Armstrong relation, a relation that satisfies each FD implied by the set but no FD that is not implied by it, is precisely exponential in the number of attributes.
Abstract: An Armstrong relation for a set of functional dependencies (FDs) is a relation that satisfies each FD implied by the set but no FD that is not implied by it. The structure and size (number of tuples) of Armstrong relatsons are investigated. Upper and lower bounds on the size of minimal-sized Armstrong relations are derived, and upper and lower bounds on the number of distinct entries that must appear m an Armstrong relation are given. It is shown that the time complexity of finding an Armstrong relation, gwen a set of functional dependencies, is precisely exponential in the number of attributes. Also shown ,s the falsity of a natural conjecture which says that almost all relations obeying a given set of FDs are Armstrong relations for that set of FDs. Finally, Armstrong relations are used to generahze a result, obtained by Demetrovics using quite complicated methods, about the possible sets of keys for a relauon.
205 citations
TL;DR: L'atelier d'ecoulement de permutation de la machine 3 n'ayant pas d'attente dans le processus est montre etre NP dur au sens fort.
Abstract: L'atelier d'ecoulement de permutation de la machine 3 n'ayant pas d'attente dans le processus est montre etre NP dur au sens fort. Ceci etablit une question ouverte bien connue en theorie de l'ordonnancement
TL;DR: The authors show that finding a minimum complement of a given view is NP-complete and define the explicit functional dependencies, a new kind of dependency that intuitively states that some part of the database information can be computed from the rest.
Abstract: The problem of translating updates of database views is studied. View updates are disambi- guated by requiring that a specified view complement (i.e., a second view that contains all the information omitted from the given view) remain constant during the translation. Some of the computational problems related to the apphcafion of this general methodology in the context of relational databases are studied. Projective views of databases that consist of a single relation and satisfy funcuonal dependencies are emphasized. After characterizing complementary views, the authors show that finding a minimum complement of a given view is NP-complete. The problem of translating the insertion of a tuple into a view is then studied in detail, and the results are extended to the cases of deletion and replacement of a tuple. Finally, the explicit functional dependencies, a new kind of dependency that intuitively states that some part of the database information can be computed from the rest, are defined and studied.
TL;DR: It is proved that this network topology enables a fully distributed probabilistic algorithm to execute a variety of communication requests efficiently and suggests an efficient solution to a fundamental problem in the design of parallel computers.
Abstract: A family of balanced communication schemes for connecting N processors with only a constant number of lines entering or leaving each processor is defined. It is proved that this network topology enables a fully distributed probabilistic algorithm to execute a variety of communication requests efficiently. In particular it enables implementation of an arbitrary permutation, that is, a set of N packets initially located in distinct processors and destined for distinct destinations in O(log/sub 2/N) steps. Similar results are proved for randomly generated communication requests. These results suggest an efficient solution to a fundamental problem in the design of parallel computers.
TL;DR: Theformat model provides a new approach for the formal study of the construction of "user views" and other data manipulations indatabases by obtaining several nontrivial results concerning notions of relative information capacity and restructuring of data sets.
Abstract: A mathematical theory for the study of data representation in databases is introduced and developed. The theory focuses on three data constructs (collection, composition and classification). "Formats" with semantically rich yet tractable structure are built recursively using these constructs. Using formats, we obtain several nontrivial results concerning notions of relative information capacity and restructuring of data sets. As such, the format model provides a new approach for the formal study of the construction of "user views" and other data manipulations in databases.
TL;DR: Analyse du cas particulier des matrices stochastiques et on donne un exemple numerique de matrices irreductibles non reparties.
Abstract: Cet article traite des vecteurs propres positifs des matrices irreductibles non reparties qui sont simplement caracterisees par une limite superieure donnee λ sur leur rayon spectral et par une matrice L donnee de limites inferieures pour leurs elements. Analyse du cas particulier des matrices stochastiques et on donne un exemple numerique
TL;DR: Modelisation de l'effet des conflits sur les transactions dans une base de donnees et on obtient des formules exactes pour les mesures de performances a l'equilibre et des algorithmes efficaces pour leur calcul
Abstract: Modelisation de l'effet des conflits sur les transactions dans une base de donnees et on obtient des formules exactes pour les mesures de performances a l'equilibre et des algorithmes efficaces pour leur calcul
TL;DR: On donne des algorithmes fonctionnant par etapes de O(n) pour resoudre un certain nombre de problemes de graphes sur un reseau n×n de processeurs.
Abstract: On donne des algorithmes fonctionnant par etapes de O(n) pour resoudre un certain nombre de problemes de graphes sur un reseau n×n de processeurs. Le probleme considere comprend: la recherche des ponts et points d'articulation d'un graphe non oriente, la recherche de la longueur du cycle le plus court, la recherche d'un arbre maximal, minimal et d'autres problemes
TL;DR: On montre que le probleme de savoir si un cas du probleme du commis voyageur a une solution uniquement optimale est complet pour Δ 2 p.
Abstract: On montre que le probleme de savoir si un cas du probleme du commis voyageur a une solution uniquement optimale est complet pour Δ 2 p
TL;DR: Analyse du retard bout a bout contracte par les objets lorsqu'ils traversent un tel systeme, comprenant le retard de declassement, le retard introduit par l'algorithme de resequencement and le retard cause par le serveur de sortie au canal de reception.
Abstract: Le probleme de serialisation ou de resequencement est d'un interet important dans les systemes repartis et les systemes informatiques de communication. Analyse du retard bout a bout contracte par les objets lorsqu'ils traversent un tel systeme, comprenant le retard de declassement, le retard introduit par l'algorithme de resequencement et le retard cause par le serveur de sortie au canal de reception
TL;DR: Presentation d'un algorithme de recherche lineaire qui reconnait le probleme de sac a dos a n-dimensions par etapes de 2n 4 Logn+O(n 3 )
Abstract: Presentation d'un algorithme de recherche lineaire qui reconnait le probleme de sac a dos a n-dimensions par etapes de 2n 4 Logn+O(n 3 )
TL;DR: Algorithms are presented which modify logical networks of bounded fan-in to obtain functionally equivalent networks of constrained fan-m and fan-out, so that both size and depth are not increased by more than constant factors.
Abstract: Algorithms are presented which modify logical networks of bounded fan-in to obtain functionally equivalent networks of bounded fan-m and fan-out, so that both size and depth are not increased by more than constant factors.
TL;DR: Soit G=(V, E) un graphe non planaire using the techniques de 1ere profondeur pour extraire sur un sous-graphe de Kuratowsky en un temps O(IVI).
Abstract: Soit G=(V, E) un graphe non planaire. On montre la methode utilisant les techniques de 1ere profondeur pour extraire sur un sous-graphe de Kuratowsky en un temps O(IVI)
TL;DR: Evidence was presented that the asymmetric traveling salesman problem can be solved exactly in time O(n31n(n) on the average, and the best-bound search strategy is shown to be optimal in both time and space.
Abstract: Branch and bound procedures are the most efficient known means for solving many NP-hard problems A special class of branch and bound procedures called relaxation-gutded procedures ig presented. While for some branch and bound procedures a worst-case complexity bound is known, the average case complexity is usually unknown, despite the fact that it may g~ve more useful information about the performance of the algorithm. A random process which generates labeled trees is introduced as a model of the kind of trees that a relaxatlon-gutded procedure generates over random instances of a problem Results concerning the expected time and space complexity of searching these random trees are derived with respect to several search strategies. The best-bound search strategy is shown to be optimal in both time and space. These results are illustrated by data from random traveling salesman instances Evidence ~s presented that the asymmetric traveling salesman problem can be solved exactly in time O(n31n(n)) on the average. Categories and Subject Descnptors: F.2.2 [Analysis of Algorithms and Problem Complexity]: Nonnumencal Algorithms and Problems, G.2.1 [Discrete Mathematics]: Combinatorics; G.I.6 [Numerical Analysis]. OpUmlzatlon General Terms: Algorithms, Theory Addlttonal
TL;DR: Cet article etablit quelques limites inferieures de la forme Ω(n log n) pour le nombre de messages necessaires afin de trouver l'etiquette maximale dans une configuration circulaire de n processus etiquettes sans aucun controleur central.
Abstract: Cet article etablit quelques limites inferieures de la forme Ω(n log n) pour le nombre de messages necessaires afin de trouver l'etiquette maximale dans une configuration circulaire de n processus etiquettes sans aucun controleur central
TL;DR: An intuitive presentation of the trace method for the abstract specification of software contains sample specifications, syntactic and semantic definitions of consistency and totalness, methods for proving specifications consistent and total, and a comparison of the method with the algebraic approach to specification.
Abstract: An intuitive presentation of the trace method for the abstract specification of software contains sample specifications, syntactic and semantic definitions of consistency and totalness, methods for proving specifications consistent and total, and a comparison of the method with the algebraic approach to specification. This intuitive presentation is underpinned by a formal syntax, semantics, and derivation system for the method. Completeness and soundness theorems establish the correctness of the derivation system with respect to the semantics, the coextensiveness of the syntactic definitions of consistency and totalness with their semantic counterparts, and the correctness of the proof methods presented. Areas for future research are discussed.
TL;DR: La methode est basee sur des expansions asymptotiques en puissance de N −1 pour les moments qui nous interessent.
Abstract: Presentation d'une methode pour calculer les moments d'ordre 1 et d'ordre superieur des longueurs de file d'attente pour chaque classe de client, a chaque nœud, dans un reseau de file d'attente de Markov. La methode est basee sur des expansions asymptotiques en puissance de N −1 pour les moments qui nous interessent
TL;DR: A priority queuing system which a customer can feed back and change priority after being served is analyzed and it is shown that bulk arrivals and branching can be included m the model without substantially changing the form of the solution.
Abstract: A priority queuing system m which a customer can feed back and change priority after being served is analyzed. More exactly, there are c customer types. Customer type i feeds back after service N(I) 1 times. The kth time a type / customer is m the queue it has priority level f(t, k) and requires a service time with arbitrary distribution G,k Type ~ customers enter from outside as a Poisson process w~th rate X, and can be e~ther preemptive or nonpreemptlve. A customer who has been preempted must start serwce over from the beginning w~th a new (independently chosen) service Ume. The analysis gwes mean waiting times for each customer type at each stage m its itinerary, the mean number of each customer type m the system, and other quantities of interest. It ~s also shown that bulk arrivals and branching can be included m the model without substantially changing the form of the solution.
TL;DR: The author shows that it is possible to determine efficiently whether the transactions in a given set can be permitted to run safely by themselves without the need of any control while ensuring viewserializability, although the problem is np-complete in the case of state serializability.
Abstract: The power of locking as a primitive for controlling concurrency in database systems is examined. It is accepted that the concurrent execution (or schedule) of different transactions must be serializable; that is, it must behave like a serial schedule, one in which the transactions run one at a time. It is shown that locking cannot achieve the full power of serializability. An exact characterization of the schedules that can be produced if locking is used to control concurrency is given for two versions of serializability. In the first one, state serializability, only the effect of the schedule on the database is taken into account. In the second one, view serializability, the view of the data received by the transactions is also taken into account. The author shows that it is possible to determine efficiently whether the transactions in a given set can be permitted to run safely by themselves without the need of any control while ensuring view serializability, although the problem is np-complete in the case of state serializability. 20 references.
TL;DR: A proof by a computer program of the unsolvability of the halting problem is described, believed to be the first instance of a machine proving that a given problem is not solvable by machine.
Abstract: : The authors describe a proof by a computer program of the unsolvability of the halting problem. The halting problem is posed in a constructive, formal language. The computational paradigm formalized is Pure LISP, not Turing machines. They believe this is the first instance of a machine proving that a given problem is not solvable by machine.
TL;DR: Description d'une technique generale pour resoudre certains problemes de graphes NP-durs en un temps qui est exponentiel dans un parametre k defini comme le maximum.
Abstract: Description d'une technique generale pour resoudre certains problemes de graphes NP-durs en un temps qui est exponentiel dans un parametre k defini comme le maximum, sur tous les composants C non separables du graphe, d'un nombre de bords qui peut etre ajoute a l'arbre pour produire C; on donne un algorithme de temps de complexite O(|E|•(6r) rh/27 )