1,721,112 research outputs found
The Bernays-Schönfinkel-Ramsey class for set theory: decidability.
No full textAs proved recently, the satisfiability problem for all prenex formulae in the set-theoretic Bernays-Schönfinkel-Ramsey class is semi-decidable over von Neumann's cumulative hierarchy. Here that semi-decidability result is strengthened into a decidability result for the same collection of formulae
Discrete Semantics for Hybrid Automata
Full text linkMany natural systems exhibit a hybrid behavior characterized by a set of continuous laws which are switched by discrete events. Such behaviors can be described in a very natural way by a class of automata called hybrid automata. Their evolution are represented by both dynamical systems on dense domains and discrete transitions. Once a real system is modeled in a such framework, one may want to analyze it by applying automatic techniques, such as Model Checking or Abstract Interpretation. Unfortunately, the discrete/continuous evolutions not only provide hybrid automata of great flexibility, but they are also at the root of many undecidability phenomena. This paper addresses issues regarding the decidability of the reachability problem for hybrid automata (i.e., "can the system reach a state a from a state b?") by proposing an "inaccurate" semantics. In particular, after observing that dense sets are often abstractions of real world domains, we suggest, especially in the context of biological simulation, to avoid the ability of distinguishing between values whose distance is less than a fixed ε. On the ground of the above considerations, we propose a new semantics for first-order formulæ which guarantees the decidability of reachability. We conclude providing a paradigmatic biological example showing that the new semantics mimics the real world behavior better than the precise one
Banishing Ultrafilters from Our Consciousness
No full textThe way in which Martin Davis conceived the first chapter of his book “Applied nonstandard analysis ” is a brilliant example of information hiding as a guiding principle for the design of widely applicable constructions and methods of proof. We discuss here a common trait that we see between that book and another writing of the year 1977, “Metamathematical extensibility for theorem provers and proof-checkers”, which Martin coauthored with Jacob T. Schwartz . To tie the said part of Martin’s study on nonstandard analysis to proof technology, we undertake a verification, by means of a proof-checker based on set theory, of key results of the non-standard approach to analysis
Average Linear Time and Compressed Space Construction of the Burrows-Wheeler Transform
No full textThe Burrows-Wheeler Transform is a text permutation that has revolutionized the fields of pattern matching and text compression, bridging the gap existing between the two. In this paper we approach the BWT-construction problem generalizing a well-known algorithm—based on backward search and dynamic strings manipulation—to work in a context-wise fashion, using automata on words. Let n , σ , and Hk be the text length, the alphabet size, and the k -th order empirical entropy of the text, respectively. Moreover, let H∗k=min{Hk+1,⌈logσ⌉} . Under the word RAM model with word size w∈Θ(logn) , our algorithm builds the BWT in average O(nH∗k) time using nH∗k+o(nH∗k) bits of space, where k=log_σ(n/log2n)−1 . We experimentally show that our algorithm has very good performances (essentially linear time) on DNA sequences, using about 2.6 bits per input symbol in RAM
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A Deductive Data Model for Representing and Querying Semistructured Data
No full textWe show how the mechanisms of an active/deductive object-oriented model are capable of modeling the information available on the World Wide Web, the paradigmatic example of a large, distributed collection of poorly structured, heterogeneous information, consisting of documents connected by hypertext links. We exhibit the basic features of the object model, including the schema definition language, the query language with multiple roles, the basic update operations, and a form of active rules, and a compilation of the mentioned features into a fragment of LDL++, which is essentially Datalog extended with non-determinism and a form of stratified negation. The proposed compilation has a twofold aim. On one side, it should provide an abstract implementation level, where the object model is declaratively reconstructed and its semantics clarified. On the other side, the proposed compilation should form the basis of a realistic implementation, as LDL++ can be efficiently executed, and suppor..
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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